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RICHTER'S ORGANIC CHEMISTRY
i^i***"! •
..
*-*
VOLUME II. OF THIS WORK INCLUDES THE CARBOCYCLIC AND HETEROCYCLIC SERIES
hem .
ORGANIC CHEMISTRY
OR
CHEMISTRY OF THE CARBON COMPOUNDS
BY
VICTOR VON RICHTER
EDITED BY PROF. R. ANSCHUTZ AND PROF. G. SCHRQETER
VOLUME I CHEMISTRY OF THE ALIPHATIC SERIES
NEWLY TRANSLATED AND REVISED FROM THE GERMAN EDITION (AFTER PROF. "EDGAR F. SMITH'S THIRD AMERICAN EDITION)
BY PERCY E. SPIELMANN, PH.D., B.Sc., F.I.C, A.R.C.SC.
SECOND (REVISED) EDITION
PHILADELPHIA \
P. BLAKISTON'S SON & CO.
1012 WALNUT STREET 1919
Printed in Great Britain by Butler & Tanner. Frome and London.
PREFACE TO THE FIRST ENGLISH EDITION
A COMPARISON between the present work, the latest edition of the German original, and the last American translation, will show that while the German text-book has been faithfully followed, modifications have been introduced which will be regarded, it is hoped, in the light of solid improvement. Certain statements have been corrected or modified, changes which have usually been indicated, and a great number of minor alterations have been made in the marshalling of facts and the setting out of formulae with the object of a more logical sequence and a clearer emphasis of the point under discussion.
References to German literature have been retained with the object of preserving to the student the advantages of the origin of the book; the English references will be otherwise readily obtainable by him.
I take great pleasure in expressing my gratitude to Mr. W. P. Skertchly, F.I.C., not only for assistance in the more mechmical part of the translation, but also for the careful way in which he has read through the proofs.
Furthermore, to Mr. A. J. Greenaway, Sub-Editor of the Journal of the Chemical Society, I offer my most cordial thanks for his valued advice on certain doubtful points of nomenclature.
PERCY E. SPIELMANN. LONDON, 1915.
N.B. — The Publishers beg to explain that a year's delay has occurred in the production of this volume (announced for the autumn of 1914), owing to Dr. Spielmann's employment on important work connected with explosives for the Government.
K. P. T. T. & Co., LTD.
PREFACE TO THE SECOND ENGLISH EDITION
NOTWITHSTANDING the depletion of students from the many Technical Institutions as a result of the late war, a second edition of the first volume of this text-book has been called for — a gratifying recognition of its continued and increasing usefulness.
As inevitable to the first production of a book of this character, with its innumerable formulae and figures, a certain number of mis- prints had crept in, and a careful search for these has been made.
The need of such rectifications must not deter me from paying a tribute to the printers, Messrs. Clowes and Sons, for the care and success with which they have carried through so complicated a piece of type setting ; while to the Publishers is due acknowledgment for much that was of assistance in my share of the work of production.
It is believed that, in its revised form, this volume will be found to meet all the requirements of the daily expanding class of chemical students, on whose services will depend so important a share in the scientific foundation of the firm establishment and success of British Industry.
PERCY E. SPIELMANN. LONDON, 1919.
PREFACE TO THE THIRD AMERICAN
EDITION
IN presenting this translation of the eighth German edition of v. Richter's " Organic Chemistry " the writer has little to add to what has previously been expressed in the prefaces to the preceding American editions of this most successful book. The student of the present edition will, however, very quickly discover that the subject-matter, so ably edited by Professor Anschiitz, is vastly different from that given in the earlier editions. Indeed, the book has sustained very radical changes in many particulars, and certainly to its decided advantage. The marvellous advances in the various lines of synthetic organic chemistry have made many of the changes in the text abso- lutely necessary, and for practical reasons it has seemed best to issue this new edition in two volumes.
Eminent authorities, such as Profs, v. Baeyer, E. Fischer, AVaitz, Claisen, and others, have given the editor the benefit of their super- vision of chapters relating to special fields of investigation in which they are the recognized authorities.
The translator here acknowledges his great indebtedness to his publishers, P. Blakiston's Son & Co., for their constant aid in his work, as well as to Messrs. Wm. F. Fell & Co., for the care they have taken and the skill they have displayed in the composition of what will generally be admitted to be a difficult piece of typography.
E. F. SMITH.
PREFACE TO THE SECOND AMERICAN
EDITION
THE present American edition of v. Richter's " Organic Chemistry " will be found to differ very considerably, in its arrangement and size, from the first edition. The introduction contains new and valuable additions upon analysis, the determination of molecular weights, recent theories on chemical structure, electric conductivity, etc. The section devoted to the carbohydrates has been entirely rewritten, and presents the most recent views in regard to the constitution of
viii PREFACES
this interesting group of compounds. The sections relating to the trimethylene, tetramethylene, and pentamethylene series, the fur- furane, pyrrol, and thiophene derivatives, have been greatly enlarged, while the subsequent chapters, devoted to the discussion of the aromatic compounds, are quite exhaustive in their treatment of special and important groups. Such eminent authorities as Profs. Ostwald, von Baeyer, and Emil Fischer have kindly supervised the author's presentation of the material drawn from their special fields of investigation.
The characteristic features of the first edition have been retained, so that the work will continue to be available as a text-book for general class purposes, useful and reliable as a guide in the preparation of organic compounds, and well arranged and satisfactory as a refer- ence volume for the advanced student as well as for the practical chemist.
The translator would here express his sincere thanks to Prof. v. Richter, whose hearty co-operation has made it possible for him to issue this translation so soon after the appearance of the sixth German edition.
E. F. SMITH.
PREFACE TO THE FIRST AMERICAN EDITION
THE favourable reception of the American translation of Prof, von Richter's " Inorganic Chemistry " has led to this translation of the " Chemistry of the Compounds of Carbon/' by the same author. In it will be found an unusually large amount of material, necessitated by the rapid advances in this department of chemical science. The portions of the work which suffice for an outline of the science are presented in large type, while in the smaller print is given equally important matter for the advanced student. Frequent supplementary references are made to the various journals containing original articles, in which details in methods and fuller descriptions of properties, etc., may be found. The volume thus arranged will answer not only as a text -book, and indeed as a reference volume, but also as a guide in carrying out work in the organic laboratory. To this end numerous methods are given for the preparation of the most important and the most characteristic derivatives of the different classes of bodies.
E. F. SMITH.
ABBREVIATIONS
A.
A. chim. phys. Am. Anorg. Ch.
Arch. exp. Path. Arch. ges. Phys.
BP.
Bull. soc. chim.
C.
Ch. Ztg.
C.r.
D.
A1, A2, A3, etc.
D. R. P.
Gaz. chim. ital. F.Hdw.
J. Chem. Soc.
J. pr. Ck., or J. pr. Ch.
N. F. ' . L.Hdw.
M. .... Pharm. Centr. Phil. Mag. . Pogg. A., or Wied. A.
R.
R. Meyer's J. Wied. A. Wien. Monatsh. Z.
Z. anal. Ch. . Z. angeiv. Ch. Z. anorg. Ch. Z. Electroch. . Z. Kryst. Z.fhys. Ch. . Z. physiol. Ch.
Liebig's Annalen der Chemie. Spl. — Supplementband.
Annales de chemie et de physique.
American Chemical Journal.
Richter-Klinger, Lehrbuch der anorganischen Chemie.
Richter-Smich, Text-book of Inorganic Chemistry.
Archiv fur experimentelle Pathologic und Pharmakologie.
Archiv fur die gesammte Physiologic.
Specific optical rotation.
Berichte der deatschen chemischen Gesellschaft.
R = Referate.
Boiling point. Bp10 = Boiling point at 10 mm. pressure of
mercury.
Bulletin de la societe chimique de Paris. Chemisches Centralblatt. Chemiker-Zeitung.
Comptes rendus des stances de 1' Academic des science?. Density, specific gravity, D20= Sp. gr. at 20° C. Denotes the position of a double linkage in a carbon chain,
reckoned from the C-atom I, 2, 3, etc. to the next
higher member. Deutsches Reichspatent. Gazetta chimica italiana. Fehling's Hand wort erbuch fur Chemie. Jahresbericht fur die Fortschritte der Chemie. Journal of the Chemical Society.
Journal fur praktische Chemie. Neue Folge.
Ladenburg's Handworterbuch fur Chemie.
Monatshefte fiir Chemie.
Pharmaceutische Centralhalle.
Philosophical Magazine.
Annalen der Physik und Chemie, published by Poggendorf j
or new series, published by Wiedemann. See B.
Richard Meyer's Jahrbuch der Chemie. See Pogg. A.
Monatsheft fiir chemie (Vienna). Zeitschrift fur Chemie. Zeitschrift fiir analytische Chemie. Zeitschrift fiir angewandte Chemie. Zeitschrift fiir anorganische Chemie. Zeitschri^t fiir Electrochemie. Zeitschrift fiir Krystallographie und Mineralogie. Zeitschrift fiir physicalische Chemie. Hoppe-Seyler's Zeitschrift fiir physiologische Chemie.
CONTENTS
INTRODUCTION
Determination of the Composition of Carbon Compounds
Determination of the Molecular Formula .
The Chemical Constitution of the Carbon Compounds
The Nomenclature of the Carbon Compounds .
Physical Properties of the Carbon Compounds . .
Heat of Combustion of Carbon Compounds
Action of Heat, Light, and Electricity upon Carbon Compounds
The Direct Combination of Carbon with other Elements
Classification of the Carbon Compounds . . .
PAG-
2
9
18 42
43 60 61
65
68
I. FATTY COMPOUNDS, ALIPHATIC SUBSTANCES OR METHANE DERIVATIVES, CHAIN OR ACYCLIC CARBON DERIVATIVES
69
I. HYDROCARBONS .... 69
A. Saturated or Limit Hydrocarbons, Paraffins, Alkanes, Marsh Gas or Methane
Hydrocarbons ........... 69
B. Unsaturated Hydrocarbons. I. Olefines or Alkylenes, 79 ; 2. Acetylenes or
Alkines, 85 ; 3. Diolefines, 90; 4. Olefine Acetylenes, 91 ; 5. Diacetylenes,
91 ; 6. Triolefines . . . . . . . . . .91
II. HALOGEN DERIVATIVES OF THE HYDROCARBONS 91
OXYGEN DERIVATIVES OF THE METHANE HYDROCARBONS
98
III. THE MONOHYDRIC ALCOHOLS AND THEIR
OXIDATION PRODUCTS . . . . 100
Monohydric Alcohols, 100. A. Saturated Alcohols, Paraffin Alcohols . . 109 B. Unsaturated Alcohols, 123. I. Olefine Alcohols, 123 j 2. Acetylene
Alcohols, 125 ; 3. Diolefine Alcohols ...... 125
Alcohol Derivatives. I. Simple and Mixed Ethers, 125 ; 2. Esters of the Mineral Acids, 130; 3. Sulphur Derivatives of the Alcohol
Radicals . 142
4. Selenium and Tellurium Compounds ...... 148
5. Nitrogen Derivatives of the Alcohol Radicals ..... 148
6. Phosphorus Derivatives of the Alcohol Radicals . . . . • *73
CONTENTS
7. Alkyl Derivatives of Arsenic, 175 ; 8. Antimony, 179 ; 9. of Bismuth 179; 10. of Boron, 180 ; n. of Silicon, 180 ; 12. of Germanium
13. Tin Alkyl Compounds .
14. Metallo-organic Compounds .......
2. Aldehydes, and 3. Ketones
2A. Aldehydes of the Saturated Series
1. Halogen Substitution Products of the Saturated Aldehydes Peroxides of the Aldehydes
2. Ethers and Esters of Methylene and Ethylidene Glycols
3. Sulphur Derivatives of the Saturated Aldehydes
181 182
183 189 191
201 203
2O4 208
4. Nitrogen Derivatives of the Aldehydes . ' . . . .210
2B. Olefine Aldehydes 214
2C. Acetylene Aldehydes . . . . . . . . .215
3 A. Ketones of the Saturated Series 216
1. Halogen Substitution Products of the Ketones .... 224
2. Alkyl Ethers of the Ortho-ketones 225
3. Ketone Halides 225
4. Ketone Bisulphites and Sulphoxylates . . . . .225 .5. Sulphur Derivatives of the Saturated Ketones .... 225
6. Nitrogen Derivatives of the Ketones ..... 226
36. Olefine and Diolefine Ketones ....... 228
3C. Acetylene Ketones 232
Monobasic Carboxylic Acids ......... 232
A. Monobasic Saturated Acids . . . . . ..... 235
Derivatives of the Fatty Adds 265
1. Esters of the Fatty Acids 265
2. Acid Halides of the Fatty Acids 269
3. Acid Anhydrides . . . . . . . .271
4. Acid Peroxides ......... 273
5. Thio-Acids . . . . 273
6. Acid Amides ......... 274
7. Acid Hydrazides 278
8. Acid Andes 278
9. Fatty Acid Nitriles 278
10. Amide Chlorides 281
11. Imide Chlorides ......... 281
12. Imido-Ethers 281
13. Thiamides 281
14. '1 hio-imido-Ethers . . . . . . . . 282
15. Amidines . . . . . . ' . . . 282
16. Hydroxamic Acids ........ 282
17. Hydroximic Acid Chlorides ...... 283
18. Nitrolic Acids 283
19. Amidoximes or Oxamidines ....... 283
20,21. Hydroxamic Oxime ; Nitrosoximes ..... 284
22, 23. Hydrazidine and Hydrazo-oxime ..... 284
24. Ortho-fatty Acid Derivatives ...... 284
Halogen Substitution Products of the Fatty Acids . . . 284
B. Oleic Acids, Olefine Monocarboxylic Acids ..... 290
C. Acetylene Carboxylic Acids ........ 302
D. Diolefine Carboxylic Acids 305
IV. DIHYDRIC ALCOHOLS OR GLYCOLS, AND
THEIR OXIDATION PRODUCTS . . 306
I. Dihydric Alcohols or Glycols 307
Glycol Derivatives . . . . . . . . . .316
1. Alcohol Ethers of the Glycols ...... 316
2. Esters of the Dihydric Alcohols ...... 319
3. 1 hio-Compounds of Ethylene Glycols 324
4. Nitrogen Derivatives of the Glycols ...... 327
CONTENTS xiii
PAGE
2. Aldehyde-Alcohols, 337 ; Nitrogen-containing Derivatives of the Aldehyde-
Alcohols ............ 339
3. Ketone- Alcohols or Ketols, 340 ; Nitrogen-containing Derivatives of the
Ketone-Alcohols ........... 344
4. Dialdehydes ............ 346
5. Ketone-Aldehydes, or Aldehyde-Ketones . . . . . . . 348
6. Diketones, 348 ; Nitrogen-containing Derivatives of the Dialdehydes, Alde-
hyde-Ketones and Diketones ........ 353
7. Alcohol- or Hydroxy-acids ......... 356
A. Saturated Hydroxymonocarboxylic Acids, 362 ; o-Hydroxy-acids, 362 ;
/8-Hydroxycarboxylic Acids, 369; 7- and 0-Hydroxy-acids, 371; Sulphur Derivatives of the Hydroxy-acids, 376 j Nitrogen Derivatives of the Hydroxy-acids, 378 ; Amino-Fatty Acids, 385 ; Dipeptides and Polypeptides ......... 390
B. Unsaturated Hydroxy-acids, Hydroxy-olefine Carboxylic Acids . . 397
8. Aldehyde-acids, 400 ; Nitrogen Derivatives of the Aldehyde-acids . . . 402
9. Ketonic Carboxylic Acids .......... 406
A, Saturated Ketone Carboxylic Acids. I. o-Ketonic Acids, 407 ; Nitrogen
Derivatives of the o-Ketonic Acids, 409. II. ^-Ketonic Acids, 410 ; Acetoacetic Acid, 410 ; Nitrogen Derivatives of j8-Ketonic Acid, 419 ; Halogen Substitution Products of the £-Ketonic Esters, 420. III. 7-Ketonic Acids, 421 ; Nitrogen Derivatives of the 7-Ketonic Acids, 423. IV. 5-Ketonic Acids 424
B. Unsaturated Ketonic Acids ; Olefine Ketonic Acids .... 425
CARBONIC ACID AND ITS DERIVATIVES . . 425
Chlorides of Carbonic Acid, 430 ; Sulphur Derivatives of Ordinary Carbonic Acid . 431
Amide Derivatives of Carbonic Acid, 435 ; Carbamide Urea, 4^8 ; Ureides, 441 ; Hydrazine-, Azine-, and Azido-Derivatives of Carbonic Acid, 446 ; Sulphur-containing Derivatives of Carbamic Acid and of Urea . . . 448
Guanidine and its Derivatives ......... 454
Nitriles and Imides of Carbonic and Thiocarbonic Acids, 459 ; Oxygen Deriva- tives of Cyanogen, their Isomerides and Polymerides, 460 ; Halogen Com- pounds of Cyanogen and its Polymers, 465 ; Sulphur Compounds of Cyanogen, their Isomers and Polymers, 466 j Cyanamide and the Amides of Cyanuric Acid, 471 ; Ketenes ........ 474
10. Dibasic Acid, Dicarboxylic Acids ........ 476
A. Paraffin Dicarboxylic Acids, 476 ; Oxalic Acid and its Derivatives, 480 ;
Nitriles of Oxalic Acid, 484 ; the Malonic Acid Group, 487 ; Carbon Suboxide, 488 ; Ethyl ene Succinic Acid Group, 491 ; Nitrogen- containing Derivatives of the Ethylene Succinic Acid Group, 496 ; Halogen Substitution Products of the Succinic Acid Group, 499 ; Glutaric Acid Group, 501 ; Group of Adipic Acid and Higher Normal Paraffin Dicarboxylic Acids ...... 504
B. Olefine Dicarboxylic Acids, 507 ; Fumaric Acid, 509 ; Maleic Acid,
510; The Isomerism of Fumaric and Maleic Acids, 512; Itaconic Acid, 515 ; Citraconic Acid, 516; Mesaconic Acid . . . 516
V. TRIHYDRIC ALCOHOLS: GLYCEROLS AND
THEIR OXIDATION PRODUCTS . . 523
1. Trihydric Alcohols, 524. A. Glycerol Esters of Inorganic Acids, 529. B.
Glycerol Fatty Acid Esters, Glycerides, 530 ; Glycerol Ethers, 531 ;
Nitrogen Derivatives of the Glycerols ... ... 533
2. Dihydroxy- Aldehydes 533
3. Dihydroxy-Ketones (Oxetones) • • 534
4. Hydroxy-Dialdehydes • 535
5. Hydroxy- Aldehyde Ketones S36
xiv CONTENTS
PAGE
6. Hydroxy-Ketones ....... 8 ... 536
7. Dialdrhyde K clones .......... 537
8. Aldehyde Diketones .......... 537
9. Triketones ............ 537
10. Dihydroxy-monocarboxylic Acids, 538 ; Monoamino-hydroxy-carboxylic Acids,
540; Monoamino-thio-carboxylic Acids, 541; Diamino-monocarboxylic Acids, 542 ; Dihydroxy-olefine Monocarboxylic Acids .... 543
11, 12. Aldo-hydroxy-carboxylic Acids, and Hydroxy-keto-carboxylic Acids . 543
13. Aldehydo-ketone Carboxylic Acids ...... . 545
14. Diketo-carboxylic Acids 546
15. Monohydroxy-dicarboxylic Acids.
A. Monohydroxy- Paraffin D carboxylic Acids • 548 Hydroxymalonic Acid Group ........ 549
Hydroxysuccinic Acid Group . . . . . . . . 551
Aminosuccinic Acids . . . . . . . . -553
Hydroxyglutaric Acid Group ........ 558
B. and C. Hydroxy-olefine Carboxylic Acids and Hydroxy-olefine Dicar-
boxylic Acids .......... 560
16. Aldodicarboxylic Acids. A. j8-Aldodicarboxylic Acids, 561. B. 7-Aldodi-
carboxylic Acids . . . . . . . . . . .561
17. Ketone-dicarboxylic Acids, 562 ; Ketomalonic Acid Group, 562 ; Nitrogen
Derivatives of Mesoxalic Acid, 563 ; Ketosuccinic Acid Group, 564 ; Nitrogen Derivatives of Oxalacetic Acid, 567 ; Ketoglutaric Acid Group, 568 ; Olefine- and Di-olefine-Ketone Dicarboxylic Acids . . . -571 Uric Acid Group: Urei'des or Carbamides of Aldehyd- and Keto-Mono- carboxylic Acids, 572 ; Urei'des or Carbamides of Dicarboxylic Acids, 575 ; Diureides, 580 ; Oxidation of Uric Acid, 584 ; Synthesis of Uric Acid, 585 ; Conversion of Uric Acid into Xan thine, Guanine, Hypoxan thine and Adenine, 587 ; Synthesis of Heteroxanthine, Theobromine, and Paraxan- thine 590
18. Tricarboxylic Acids : A. Saturated Tricarboxylic Acids, 592 ; B. Olefine
Tricarboxylic Acids 594
VI. TETRAHYDRIC ALCOHOLS AND THEIR
OXIDATION PRODUCTS ... 595
1 Tetrahydric Alcohols 596
2 Trihydroxyaldehydes ; 3. Trihydroxyketones . ... . . • 597
4 Hydroxytriketones ........... 597
Tetraketones ............ 597
Trihydroxy-monocarboxylic Acids . . . . . . . . 598
Dihydroxyketo-monocarboxylic Acids 598
Hvdroxydiketo-carboxylic Acids 598
9. Triketo-monocarboxylic Acids 598
10. Dihydroxy-dicarboxylic Acids : A. Malonic Acid Derivatives, 599 ; B.
Succinic Acid Derivatives, 599 ; Synthesis of Racemic Acid, 601 ; C.
Glutaric Acid Derivatives, 605 ; D. Adipic Acid Derivatives and Higher v
Homologues ........... 606
11. Hydro xy-keto-dicarboxylic Acids 607
12. Diketone Dicarboxylic Acids 607
13. Hydroxytricarboxylic Acids ......... 610
14. Ketone Tricarboxylic Acids 612
15. Tetracarboxylic Acids: A. Paraffin Tetracarboxylic Acids, 613; B. Olefine
Tctracarboxylic Acids 615
CONTENTS
VII. THE PENTAHYDRIC ALCOHOLS OR PEN- TITOLS AND THEIR OXIDATION PRODUCTS. 615
1. Pentahydric Alcohols, Pentitols .
2. Tetrahydroxyaldehydes, Aldopentoses
3. Tetrahydroxymonocarboxylic Acids
4. Trihydroxydicarboxylic Acids .
5. Dihydroxy-ketone Dicarboxylic Acids
6. Triketone Dicarboxylic Acids
7. Dihydroxytricarboxylic Acids .
8. Pentacarboxylic Acids
615 616 619 621 621 621 621 622
VIII. HEXA- AND POLY-HYDRIC ALCOHOLS
AND THEIR OXIDATION PRODUCTS . . 622
I A. Hexhydric Alcohols, Hexahydroxyparaffins, Hexitols - ,. . . . 622
I B. Heptahydric Alcohols .......... 624
i C. Octahydric Alcohols . *',*.. 625
1 D. Nonohydric Alcohols 625
2, 3. Penta-, Hexa-, Hepta-, and Octo-Hydroxyaldehydes and Ketones . . 625
2 A. Pentahydroxyaldehydes, and 3 A. Pentahydroxyketones, Hexoses, Dextroses
(Glucoses), Monoses . . . . . . . . 626
2 A. Aldohexoses . . . v . . . , . ./. . . . 631
3 A. Ketohexoses 635
2 B. Aldoheptoses j 2 C. Aldo-octoses ; 2 D. Aldononoses .... 637
The synthesis of Grape-sugar or d-Dextrose, and of Fruit-sugar or d-Fructose. 637
A. The Space-Isomerism of the Pentitols and Pentoses, the Hexitols and
Hexoses ........... 639
B. The Space-Isomerism of the Simplest Hexitols and the Sugar-Acids, the
Aldohexoses and the Gluconic Acids . . . . . .641
Derivation of the Space-formula for d-Dextrose or Grape-sugar . . 643
Derivation of the Configuration of d-Tartaric Acid . . . . 646
4. Hexaketones ............ 647
5. PolyhydroxymoRocarboxylic Acids . . . . . . . 647
A. Pentahydroxycarboxylic Acids 647
B. Hexose Carboxylic Acids, Hexahydroxymonocarboxylic Acids . . 651
C. Aldoheptose Carboxylic Acids, Heptahydroxycarboxylic Acids . .651
D. Aldo-octose Carboxylic Acids, Octohydroxycarboxylic Acids . . 652
6. Tetrahydroxy- and Pentahydroxy-Aldehyde Acids « 652
7. Monoketotetrahydroxycarboxylic Acids . . . . . . . 652
8. Pol> hydro xydi Carboxylic Acids: A. Tetrahydroxydicarboxylic Acids, 652 ;
B. Pentahydroxydicarboxylic Acids . ... 655
9. Tetraketodicarboxylic Acids . . ' ;' ' . . . 655
10. Triketo-tricarboxylic Acids ... . . 655
11. Hydroxyketotetracarboxylic Acids
12. Diketotetracarboxylic Acids . ^ „_
Appendix : Higher Polycarboxylic Ethyl Esters . . - » - * . . 656
CARBOHYDRATES
656
A. Disaccharides ; Saccharobioses .......•• 657
B. Trisaccharides j Saccharotrioses 66l
C. Polysaccharides, 66 1 ; Nitrocelluloses ....... 664
xvi CONTENTS
ANIMAL SUBSTANCES OF UNKNOWN CONSTITUTION 665
Proteins, Albumins, 666 ; a Monamino-monocarboxylic Acids, 666 ; b Mon- amino-dicarboxylic Acids, 666 ; c Hydroxamino-, Thioamino, Diamiuo-,
Imino- Acids . . . . . . .-.*..'. . . ' . 667
A. Glucoproteins ............. 071
B. Phosphoprote'ins ... . 672
C. Gelatin (Derivatives of Intercellular Materials) ••;"•• • . . . 673
D. Haemoglobins, 674 ; Chlorophyll . ' . . . . . . . 675
E. Biliary Substances . . . .- . . . . . . 676
F. Unorganized Ferments or Enzymes ........ 677
INDEX «... . ' '. . . 679
A TEXT-BOOK
OF
ORGANIC CHEMISTRY
INTRODUCTION
WHILST inorganic chemistry was developed primarily through the investigation of minerals, and was in consequence termed mineral chemistry, it may be said that the development of organic chemistry was due to the study of products resulting from the alteration of plant and animal substances. About the close of the eighteenth century Lavoisier demonstrated that, when the organic substances present in vegetable and animal organisms were burned, carbon dioxide and water were always formed. It was this chemist also who showed that the component elements of these bodies, so different in properties, were generally carbon, hydrogen, oxygen, and, especially in animal substances, nitrogen. Lavoisier further gave utterance to the opinion that peculiarly constituted atomic groups, or radicals, were to be accepted as present in organic substances ; whilst the mineral sub- stances were regarded by him as the direct combinations of single elements.
As it seemed impossible, for a long time, to prepare organic bodies synthetically from the elements, the opinion prevailed that there existed an essential difference between organic and inorganic sub- stances, which led to the use of the names Organic Chemistry and Inorganic Chemistry. The prevalent opinion was, that the chemical elements in the living bodies were subject to other laws than those in the so-called inanimate nature, and that the organic substances were formed in the organism only by the intervention of a peculiar vital force, and that they could not possibly be prepared in an artificial way.
One fact sufficed to prove these rather restricted views to be un- founded. The first organic substance artificially prepared was urea (Wohler, 1828). By this synthesis chiefly, to which others were soon added, the idea of a peculiar force necessary to the formation of organic compounds was contradicted. All further attempts to separate organic substances from the inorganic (the chemistry of the simple and the chemistry of the compound radicals, p. 18) were futile. At present we know that these do not differ essentially from each other ;
VOL. I. B
2 ORGANIC CHEMISTRY
that the peculiarities of organic compounds are dependent solely on the nature of their essential constituent, Carbon ; and that many sub- stances belonging to plants and animals can be prepared artificially from the elements. Organic Chemistry is, therefore, the chemistry oj the carbon compounds. Its separation from the chemistry of the other elements is necessitated only by practical considerations, on account of the very great number of carbon compounds (about 120,000 : see M. M. Richter's Lexikon der Kohlenstoffverbindungen), which far exceeds those of all other elements put together. No other possesses in the same degree the ability of the carbon atoms to unite with one another to form open and closed rings or chains. The numerous existing carbon nuclei in which atoms or atomic groups of other elements have entered in the formation of organic derivatives have arisen in this manner.
The impetus given to the study of the compounds of carbon has not only brought new industries into existence, but it has caused the rapid development of others of like importance to the growth and welfare of the nation.*
The advances of organic chemistry are equally important to the investigation of the chemical processes in vegetable and animal organisms, a section of the subject known as Physiological Chemistry.
DETERMINATION OF THE COMPOSITION OF CARBON COMPOUNDS
ELEMENTARY ORGANIC ANALYSIS
Most carbon compounds occurring in the animal and vegetable kingdoms consist of carbon, hydrogen, and oxygen, as was demonstrated by Lavoisier, the founder of organic elementary analysis. Many, also, contain nitrogen, and on this account these elements are termed OrganogensJ whilst sulphur and phosphorus are often present. Almost all the elements, non-metals and metals, may be artificially introduced as constituents of carbon compounds in direct union with carbon. The number of known carbon compounds is exceedingly great (see above). The general procedure, therefore, of isolating the several compounds of a mixture, as is done in inorganic chemistry in the separation of bases from acids, is impracticable, and special methods have to be devised. The task of elementary organic analysis is to determine, qualitatively and quantitatively, the elements of a carbon compound after it has been obtained in a pure state and characterized by definite physical properties, such as crystalline form, specific gravity, melting point, and boiling point. Simple practical methods for the direct determination of oxygen do not exist ; its quantity is usually calculated by difference, after the other constituents have been found.
* Wirthschaftliche Bedeutung chemischer Arbeit, von H. Wichelhaus, 1893. f This word is retained here from the German, but is not in general use in Euj lish chemical language. (Translator's note.)
DETERMINATION OF CARBON AND HYDROGEN 3
DETERMINATION OF CARBON AND HYDROGEN
The presence of carbon in a substance is shown by its charring when ignited out of contact with air. In general its quantity, as also that of the hydrogen, is ascertained by combustion. The substance is mixed in a glass tube with copper oxide and heated, or the vapour of the substance is passed over red-hot copper oxide. The cupric oxide gives up its oxygen and is reduced to metallic copper, whilst the carbon burns to carbon dioxide, and the hydrogen to water. In quantitative analysis, these products are collected separately in special apparatus, and the increase in the weight of the latter determined. Carbon and hydrogen are always simultaneously determined in one operation. The details of the quantitative analysis are fully described in the text- books of analytical chemistry.* It is only necessary here, therefore, to outline the methods employed. Liebig's name is especially associ- ated with the elaboration of these methods (Pogg. A. 1831, 21, i).
Usually the combustion is effected by the aid of copper oxide or fused and granulated lead chromate in a tube of hard glass, fifty to seventy centimetres long (depending upon the greater or less volatility of the organic body). Substances which burn with difficulty should be mixed with finely divided cupric oxide, finely divided lead chromate, or with cupric oxide to which potassium bichromate has been added.
The combustion tube is drawn into a point, and the contracted end given a bayonet-shape (Liebig), or it is open at both ends (Glaser, A. Suppl. 7, 213). Chez has also suggested the use of an iron tube (Z. anal. Ch. 2, 413).
The tube is placed in a suitable furnace, which formerly was heated by a char- coal fire, but at present gas is usually employed (A. W. Hofmann, A. 90, 235 ; 107, 37 ; Erlenmeyer, ST., A. 139, 70 ; Glaser, I.e. ; Anschiitz and Kekule, A. 228, 301 ; Fuchs, B. 25, 2723). Recently electric heating has been adopted with success (comp. B. 39, 2263).
When the tube has been charged, the open end is attached to an apparatus designed to collect the water produced in the combustion. The substances used to retain the moisture are :
1. A U-tube filled with carefully purified calcium chloride, which has been dried at 180° C.
2. Pure, concentrated sulphuric acid contained in a specially designed tube, or pumice fragments, dipped in the acid, and placed in a U-tube (Mathesius, Z. anal. Ch. 23, 345).
3. Pellets of glacial phosphoric acid, contained in a U-tube. The vessel intended to receive the water is in air-tight connection with the apparatus designed to absorb the carbon dioxide. For the latter purpose a Liebig potash bulb was formerly employed, but later that of Geissler came into use ; and very many other forms have been recommended (B. 24, 271 ; C. 1900," 1, 1240). U -tubes, filled with granulated soda-lime, are substituted for the customary bulbs (Mulder, Z. anal. Ch. 1, 2).
When the combustion is finished, oxygen free from carbon dioxide is forced into or drawn through the combustion-tube, air being substituted for it later, with the precaution that the pieces of apparatus serving to dry the oxygen and air are filled with the same material which was used for absorbing the water produced by the combustion. As soon as the entire system is filled with air, the pieces of apparatus employed for absorbing the water and carbon dioxide are disconnected and weighed separately. The increase in weight of the apparatus in which the water is collected represents the water resulting from the combustion of the
* Anleitung zur Analyse organischer Korjjpr, J. Liebig. 2. Aufl. 1853. Quantitative chemische Analyse, R. Fresenius. 6. Aufl., Bd. 2. Chemische Analyse organischer Stoffe, von Vortmann. Die Entwicklung der organischen Elementaranalyse, M. Dennstedt, 1899.
4 ORGANIC CHEMISTRY
weighed substance, and the increase in the other the quantity of carbon dioxide. Knowing the composition of water and carbon dioxide the quantity of carbon and hydrogen contained in the burnt substance can readily be calculated in percentage.
Fig. i represents one end of a combustion furnace of the type devised by Kekult and Anschfitz (A. 228, 301). In it lies the combustion tube V. This is connected with a Klinger calcium chloride tube, A ; B is a Geissler potash-bulb, joined to a U-tube, C, one limb of which is filled with pieces of stick potash, and the other with calcium chloride. G represents mica plates, which permit of a careful observation of the flame. £ is a section of the iron tube (Modification, C. 1903, 1, 609) in which the combustion tube V rests; T a side clay cover placed over the mica strips ; D a clay cover for the top. R is the gutter into which the gas-pipe, bearing the burners, is placed, and from which it can be removed for repair, etc.
FIG. i.
Fig. l also shows, above the combustion tube, the anterior portion of a similar tube V1. provided with a Bredt and Posth (A. 285, 385) calcium chloride tube A1, in which the movement of a drop of water enables the analyst to determine the rapidity of the combustion. B1 is a U-tube filled with soda-lime and provided with ground-glass stoppers. C1 is a similar tube, filled one-half with soda-lime and one-half with calcium chloride.
Instead of oxidizing the organic substance with the combined oxygen of cupric oxide or lead chromate, the method of Kopfer may be employed, in which platinum black is made to carry free oxygen to the vapours of the substance. A simpler combustion furnace may then be employed.
This method has been perfected by Dennstedt * and his co-workers. In his " rapid combustion method " the substance is introduced into a small tube and vapourized therefrom into a slow stream of oxygen. At the same time a more rapid current of the gas is sent round the small containing tube and over the heated contact substance (thin strips of platinum foil), so that the vapour of the compound to be combusted is always in the presence of a large excess of oxygen. The accompanying illustration (Fig. 2) indicates clearly the arrangement (B. 38, 3729; 39, 1623). p^
* Dennstedt, Anleitung zur vereinfachten Elementar-analyse, 2. Aufl. Hamburg, 1906.
DETERMINATION OF CARBON AND HYDROGEtf 5
Dudley recommends that the substance be placed in a boat and burned in a platinum tube containing granular manganese dioxide in the anterior part (B. 21, ^172). Or the substance may be combusted in a drawn-out copper tube (C. 1898, 2,305).
Methods for the complete combustion of solid carbon compounds have been worked out by W. Hempel, Krockey, as well as by Zuntz and Frentzel (B. 30, 202, 380, 605), by which the substance is completely burned in oxygen under pressure in an autoclave.
Gaseous bodies can be analysed according to the usual gas analysis methods, either with Bunsen's * apparatus, or with Hempel' s,f when great accuracy is not required. The volume of the gas or mixture of gases is measured after each successive reaction with potassium hydroxide solution, fuming sulphuric acid, alkaline pyrogallic acid and ammoniacal cuprous chloride. These reagents absorb respectively carbon dioxide, the so-called heavy hydrocarbons (defines, acetylene, aromatic hydrocarbons of the CnH2;l_6 series), oxygen and carbon monoxide. The gaseous residue, which may consist of nitrogen, hydrogen and methane, is either exploded with oxygen and the contraction in volume measured both before and after absorption of the carbon dioxide formed ; or else the two combustible gases may be separately dealt with, the hydrogen being absorbed by paladium
FIG. 2.
black and the methane being led over incandescent platinum. A complete separation of the ethylene hydrocarbons from those of the benzene series has often been attempted, but the results have not been satisfactory.
When nitrogen is present in the substances burned, its oxides are sometimes produced, which have to be reduced to nitrogen. This may be effected by con- ducting the gases of the combustion over a layer of metallic copper filings, or a roll of copper gauze placed in the front portion of the combustion tube. The latter, in such cases, should be a little longer than usual. The copper, which has been previously reduced in a current of hydrogen, often includes some of the gas which, on subsequent combustion, would yield water. To remedy this, the copper after reduction is heated in an air-bath or, better, in a current of carbon dioxide or to 200° in a vacuum. Its reduction by the vapours of formic acid or methyl alcohol is more advantageous ; this may be done by pouring a small quantity of these liquids into a dry test tube and then suspending in them the roll of copper heated to redness ; copper thus reduced is perfectly free from hydrogen.
It is generally unnecessary to use a copper spiral when the combustions are carried out in open tubes.
If the substance contains chlorine, bromine or iodine, copper halides are formed, which, being volatile, would pass into the calcium chloride tube. In order to avoid this a spiral of thin copper, or better, silver foil is introduced into the front
* Bunsen, Gasometrische Methoden, 2. Aufl, Braunschweig, 1877. t Hempel, Gasometrische Methoden, Braunschweig, 1900. Winkler, Gas- analyze, Freiberg, 1901.
6 ORGANIC CHEMISTRY
part of the tube. When the organic compound contains sulphur a portion of the latter will be converted into sulphur dioxide (during the combustion with cupric oxide), which may be prevented from escaping by introducing a layer of lead peroxide (Z. anal. Ch. 17, i). Or lead chromate may be substituted for the cupric 'oxide, which would convert the sulphur into non-volatile lead sulphate. In the combustion of organic salts of the alkalies or alkaline earths, a portion of the carbon dioxide is retained by the base. To prevent this and to expel the CO2, the substance in the boat is mixed with potassium bichromate or chromic oxide (B. 13, 1641).
An organic substance, containing nitrogen, sulphur, chlorine or bromine, can be analysed by Dennstedt's method (see above, Fig. i). It is mixed with pure lead peroxide and placed in a boat of special shape in the front part of the tube. The temperature is then raised to about 320°. The nitrogen, sulphur, and halogens are held back in the form of lead compounds, whilst the carbon and hydrogen pass away as carbon dioxide and water, and are estimated in the usual way.
When carbon alone is to be determined this can be effected, in many instances, in the wet way, by oxidation with chromic acid and sulphuric acid (Messinger, B. 21, 2910 ; compare A. 273, 151).
DETERMINATION OF NITROGEN
In many instances, the presence of nitrogen is disclosed by the odour of burnt feathers when the compounds under examination are heated. Many nitrogenous substances yield ammonia when heated with alkalies (or, better still, with soda-lime) . A simple and very delicate test for the detection of nitrogen is the following : the substance is heated in a test tube with a small piece of sodium or potassium, or, when the substance is explosive, with the addition of dry soda. Potassium cyanide is produced, accompanied perhaps by a slight detonation. The residue is treated with water ; to the nitrate, ferrous sulphate containing a ferric salt is added, and then a few drops of potassium hydroxide ; the mixture is then heated, and finally an excess of hydro- chloric acid is added. An undissolved, blue-coloured precipitate (Prussian blue), or a bluish-green coloration, indicates the presence of nitrogen in the substance examined.
Nitrogen is determined quantitatively : (i) as nitrogen, by the method of Dumas ; (20) as ammonia, by the ignition of the material with soda-lime (method of Will and Varr entrap) ; (26) as ammonia, by heating the substance with sulphuric acid according to the direc- tions of Kjeldahl.
i. Dumas' Method. — The substance, mixed with cupric oxide, is burned in a tube of hard glass in the anterior end of which is a layer of metallic copper which serves for the reduction of the oxides of nitrogen. The tube is filled with carbon dioxide, obtained by heating either dry, primary sodium carbonate or magnesite, contained in the posterior and closed end of the tube. It can also be filled from a carbon dioxide apparatus of the type recommended by Kreusler (Z. anal. Ch, 24, 440), in which case an open tube is used. A more practicable method of procedure consists in evacuating the tube, previous to the combustion, by means of an air-pump, and filling each time with carbon dioxide (A. 233, 330, note) ; or the air may be removed by means of a mercury pump (Z. anal. Ch. 17, 409).
When the combustion is ended, excess of carbon dioxide is employed to sweep all the nitrogen from the combustion tube into the graduated tube or azotomeier, which may have one of a variety of forms (Zulkowsky, A. 182, 296 ; B. 13, 1099 ; Schwarz, B. 13, 771 ; Ludwig, B. 13, 883 ; H. Schiff, B. 13, 885 ; Staedel, B. 13, 2243 ; Groves, B. 13, 1341 ; Ilinski, B. 17, 1348). The potassium hydroxide in the graduated vessel absorbs all the disengaged carbon dioxide, and only pure nitrogen remains.
DETERMINATION OF NITROGEN >
Given the volume V^ of the gas, the barometric pressure p and the vapour- pressure s of the potassium hydroxide (Wullm-r, Pogg. A. 103, 529; 110, 564) at the temperature t of the surrounding air, the volume V0 at o° and 760 mm. may be easily deduced :
v
760 (1+0-0036650
Multiply V0 by 0-0012507, the weight of i c.c. of nitrogen at o° and 760 mm., and the product will represent the weight in grams of the observed volume of nitrogen :
760 (
from which the percentage of nitrogen in the substance analysed can easily be calculated.
Instead of reducing the observed gas volume V, from the observed barometric pressure and the temperature at the time of the experiment, to the normal pressure of 760 mm. and the temperature of o° (" N.T.P."), the reduction may be more readily effected by comparing the observed volume of gas or vapour with the expansion of a normal gas- volume (100) measured at 760 mm. and o°. For this
purpose the equation V0=V.-^ is employed, in which v represents the changed
normal volume (100). The gas-volumometer recommended by Kreusler (B. 17, 30) and Winkler (B. 18, 2534), or the Lunge nitrometer (B. 18, 2030 ; 23, 440 ; 24, 1656, 3491 l J.A. Muller, B. 26, R. 388) will answer very well for this purpose. Or the nitrogen may be collected in a gas-baroscope, and its weight calculated from the pressure of a known constant volume of nitrogen (B. 27, 2263).
Frankland and Armstrong conduct the combustion in a vacuum, and dispense with the layer of metallic copper in the anterior portion of the tube. If any nitric oxide is formed it is collected together with the nitrogen, and is subsequently removed by absorption (B. 22, 3065).
Consult Hempel (Z. anal. Ch. 17, 409) ; E. Pfluger (ibid., 18, 296) ; and Jannasch&nd V. Meyer (A. 233, 375), for methods by which carbon, hydrogen, and nitrogen are determined simultaneously.
See Gehrenbeck (B. 22, 1694) when nitrogen and hydrogen are to be estimated simultaneously, in cases where the carbon was determined in the wet way, as by Messinger's method.
For the simultaneous determination of carbon and nitrogen, see Klingemann (A. 275, 92).
2. Will and Varrentrap's Method. — When most nitrogenous organic com- pounds (nitro-derivatives excepted) are ignited with alkalies, all the nitrogen is eliminated in the form of ammonia gas. The weighed, finely pulverised sub- stance is mixed with about 10 parts soda-lime, and placed in a combustion tube about 30 cm. in length, which is then filled with soda-lime. At the open end of the tube there is connected a bulb apparatus, containing dilute hydrochloric acid. The anterior portion of the tube in the furnace is first heated, then that containing the mixture. In order to carry all the ammonia into the bulb, air is passed through the tube, after the fused-up end has been broken. The ammonium chloride in the hydrochloric acid is precipitated with platinic chloride, as ammonium-platinum chloride (PtCl4 ,2NH4C1); the precipitate is then ignited, and the residual Pt weighed ; i atom of Pt corresponds to 2 molecules of NH3 or 2 atoms of nitrogen.
Or, having employed a definite volume of acid in the apparatus, the excess after the ammonia absorption may be determined volumetrically, using fluorescein or methyl orange as an indicator.
Generally, too little nitrogen is obtained by this method, because a portion of the ammonia undergoes decomposition. This is avoided by adding sugar to the mixture of substance and soda-lime, and by avoiding heating the tube too strongly (Z. anal. Ch. 19, 91). Further, the tube must be filled with soda-lime as com- pletely as possible (Z. anal. Ch. 21, 278).
The method of Will and Varrentrap is made more widely applicable by the addition of reducing substances to the soda-lime. Goldberg (B. 16, 2549) recom- mends a mixture of soda-lime (100 parts), stannous sulphide (100 parts), and sulphur (20 parts) ; this he considers especially advantageous in estimating the nitrogen of nitro- and azo-compounds. For nitrates, Arnold (B. 18, 806) employs a mixture of soda-lime (2 parts), sodium thiosulphate (i part), and sodium formate (i part).
8 ORGANIC CHEMISTRY
3. Kjeldahl's Method. — The substance is dissolved by heating it with con- centrated sulphuric acid. This decomposes the organic matter and converts the nitrogen into ammonia. After the liquid has been diluted with water and cooled, and a small quantity of potassium permanganate has been added, the ammonia is expelled from it by boiling with sodium hydroxide (Z. anal. Ch. 22, 366). This method is well adapted for the determination of the nitrogen of plants and animal substances (compare urea) . When the ni trogen in nitro- and cyanogen compounds is to be estimated, sugar must be added ; and in the case of nitrates, benzoic acid. The addition of mercury or mercuric oxide is highly advantageous (B. 18, R. 199, 297 ; 29, R. 146). Pyridine and quinoline cannot be analysed by this method (B. 19, R. 367, 368).
The Kjeldahl method for the determination of nitrogen has rapidly come into favour on account of the simplicity ot the operation and of the apparatus, and of the possibility to carry out a number of determinations simultaneously. A large number of modifications of the method have been proposed to render it generally applicable (B. 27, 1633, 28, R. 937 ; C. 1898, 2, 312).
NOTE. — The nitrogen of nitro- and nitroso-compounds can be determined indirectly with a standardized solution of stannous chloride. The latter converts the groups NO2 and NO into the amide group, and is itself converted into an equivalent quantity of stannic chloride. This can be determined by titrating the excess of stannous salt with an iodine solution (Limpricht, B. 11, 40).
DETERMINATION OF THE HALOGENS, SULPHUR, AND PHOSPHORUS
Qualitative Analysis : Substances containing chlorine, bromine and iodine burn with a flame having a green-tinged border. The following reaction is exceedingly delicate. A little cupric oxide is first ignited on a platinum wire, then some of the substance to be examined is placed upon it, and the whole is heated in the non-luminous gas flame, which is coloured an intense greenish-blue if a halogen is present. A more definite test is to ignite the substance in a test tube with burnt lime (free from halogens), dissolve the mass in nitric acid, and then to add silver nitrate to the filtered solution.
The presence of sulphur can frequently be detected by fusing the substance with potassium hydroxide ; potassium sulphide results, which produces a black stain of silver sulphide on a clean piece of silver ; or by heating the substance with metallic sodium and testing the aqueous filtrate for sodium sulphide with sodium nitroprusside : if present, a purple-violet coloration is produced. When testing for sulphur and phosphorus, the subctance is oxidized with a mixture of potassium nitrate and potassium carbonate ; the resulting sulphuric and phosphoric acids are sought for by the usual methods.
Quantitative Analysis : A hard glass tube, closed at one end, and about 33 cm. in length, containing a mixture of the substance with ch!o.rinc-free lime, is heated. After cooling, its conter*s are dissolved in dilute nitiic acid, the solution is filtered and silver titrate is added to precipitate the Lalogen.
The deccirpcsi-l'on is carrier if irste?d cf lime a mixture of lime with \ part sodium carbonate, or i part sod? urn cr.rbo2i3.ta -with 2 parts potassium nitrate is employed ; and in the case of sutcw^nc^s vo'-ati'K.'ng with difficulty, a platinum or porcelain crucille, heated over a gas lamp, can be used (Volhard, A. IfiO, 40; Scheff, A. 195, 293). With compounds crn-'air^rg iedme, iodic acid may form, which, after solution of the ir^ss, may be reduced by sulphurous acid. The volumetric method of Volhard (A. 190, i) for estimating halogens, employing ammonium thiocyanate as indicator, is strongly to be recommended in place of the customary gravimetric method.
DETERMINATION OF THE MOLECULAR FORMULA 9
The same decomposition can also be effected by ignition with iron, ferric oxide, and sodium carbonate (E. Kopp, B. 10, 290).
The substances containing the halogens may also be burned in oxygen. The gases are conducted ever platinized quartz sand, and the products collected in suitable solutions (Zulkowsky, B. 18, R. 648).
The substances may be burned in a current of oxygen, and the products con- ducted through a layer of pure granular lime (or soda-lime) raised to a red heat. Later, the lime is dissolved in dilute nitric acid, and the halogens, the sulphuric acid and the phosphoric acid may then be estimated. Arsenic may be determined similarly (Brugelmann, Z. anal. Ch. 15, 1 ; 16, i). Sauer recommends collecting the sulphur dioxide, formed in the combustion of the substance, in hydrochloric acid containing bromine (ibid. 12, 178). See also the simultaneous estimation of halogens and sulphur in the presence of carbon and hydrogen, by Dennstedt's method (p. 4).
To determine sulphur and the halogens by the method suggested by Klason (B. 19, 1910), the substance is oxidized in a current of oxygen charged with nitrous vapours, and the products of combustion are conducted over rolls of platinum foil. Consult Poleck (Z. anal. Ch. 22, 171) for the estimation of the sulphur contained in coal gas.
A method of frequent use for the determination of the halogens, sulphur, and phosphorus in organic bodies is that of Carius (Z. anal. Ch. 1, 240 ; 4, 451 ; 10, 103) ; Linnemann (ibid. 11, 325) ; Obermeyer (B. 20, 2928).
The substance, weighed out in a small glass tube, is heated together with concentrated nitric acid and silver nitrate to 150-300° C. in a sealed tube, and the quantity of the resulting silver haloid (B. 28, R. 478, 864), sulphuric acid, and phosphoric acid determined. The furnace of Babo (B. 13, 1219) is especially adapted for heating the tubes. The results by this method are not always reliable (A. 223, 184).
The following method is more generally applicable for the estima- tion of sulphur and the halogens : the substance is carefully heated in a nickel crucible with a mixture of sodium and potassium carbonates and sodium peroxide. After having been melted, the product of re- action is dissolved in water and acidified with hydrochloric acid con- taining bromine ; the sulphur is then precipitated as barium sulphate (B. 28, 427 ; C. 1904, 2, 1622, etc.).
In many instances, the halogens may be separated by the action of sodium amalgam on the aqueous solution of the substance, or by that of sodium on the alcoholic solution. The quantity of the resulting salt is determined in the filtered liquid (KekuU, A. Suppl. 1, 340 ; comp. C. 1905, if 1273 ; B. 39, 4056).
Sulphur and phosphorus can often be estimated by the wet method. The oxidation is effected by means of potassium permanganate and alkali hydroxide, or with potassium bichromate and hydrochloric acid (Messinger, B. 21, 2914).
I
DETERMINATION OF THE MOLECULAR FORMULA*
The results of elementary analysis are expressed as the percentage composition of the substance thus examined ; then follows the deter- mination of the molecular formula.
We arrive at the simplest ratio in the number of elementary atoms
* Die Bestimrmmg des Molecularge\vichts in theoietischer und practischer Beziehung, von K. Windisch, 1892.
io ORGANIC CHEMISTRY
contained in a compound, by dividing the percentage numbers by the respective atomic weights of the elements.
Thus, the analysis of lactic acid gave the following percentage composition : — Carbon ........ 40-0 per cent.
Hydrogen ....... 6'6 ,,
Oxygen ....... 53-4 ,, (by difference)
lOO'O
Dividing these numbers by the corresponding weights (C = 12, H =i, O = iC). the following quotients are obtained : —
4^ ±5 = 6.6 50-3-3
12 I 16
Therefore, the ratio of the number of atoms of C, H, and O, in lactic acid, is as 3-3 : 6*6 : 3*3, or i : 2 : i. The simplest atomic formula, then, would be CH2O ; however, it remains undetermined what multiple, if any, of this formula expresses the true composition. The lowest formula of a compound, by which is expressed the ratio of the atoms of other elements to those of the carbon atoms, is an empirical formula. Indeed, we are acquainted with different substances having the empirical formula CH2O, for example, formaldehyde, CH2O; acetic acid, C2H4O2; lactic acid, C3H6O3; dextrose, C6H12O6, etc.
With compounds of complicated structure, the derivation of the simplest formula is, indeed, unreliable, because various formulae may be deduced from the percentage numbers on account of the possible errors of observation. The true molecular formula, therefore, can only be ascertained by some other means. Three courses of procedure are open to us. First, the study of the chemical reactions, and the derivatives of the substance under consideration ; second, the deter- mination of the vapour density of volatile substances ; and third, the examination of certain properties of the solutions of soluble substances.
(i) Determination of the Molecular Weight by the Chemical Method
This is applicable to all substances, but does not invariably lead to definite conclusions. It consists in preparing derivatives, analysing them and comparing their formulae with the supposed formula of the original compound. The problem becomes simpler when the sub- stance is either a base or an acid. Then it is only necessary to prepare a salt, determine the quantity of metal combined with the acid, or of the mineral acid in union with the base, and from this to calculate the equivalent formula. A few examples will serve to illustrate this.
The silver salt of lactic acid may be prepared (the silver salts are easily obtained pure, and generally crystallize without water) and*the quantity of silver in it determined ; 54-8 per cent, of silver will be found. As the atomic weight of silver = 107-7, the amount of the other constituent combined with one atom of Ag in silver lactate, may be calculated from the proportion — 54-8 : (100 - 54-8) : : 107-7 : x
x = 89-0.
Granting that lactic acid is monobasic, that in the silver salt one atom of hydrogen is replaced by silver, it follows that the molecular weight of the free (lactic) acid must = 89 + i = 90. Consequently the simplest empiric formula of the acid, CH8O = 30, must be tripled. Hence, the molecular formula of the free acid is
6 ... 67 48 . . . 53'3
90 loo-o
DETERMINATION OF THE MOLECULAR WEIGHT n
In studying a base, the platinum double salt is usually prepared. The con- stitution of these double salts is analogous to that of ammonium-platinum chloride — PtCl4 . 2(NH3HC1) — the ammonia being replaced by the base. The quantity of platinum in the double salt is determined by ignition, and calculating the quantity oi the constituent combined with one atom of Pt (195-2 parts). From the number found, six atoms of chlorine and two atoms of hydrogen are subtracted, and the result is then divided by two ; the final figure will be the equivalent or molecular weight of the base.
Or, the substance is subjected to reactions of various kinds, e.g. the substitu- tion of its hydrogen by chlorine. The simplest formula of acetic acid, as described above, is CH2O. By substitution three acids can be obtained from acetic acid. These, upon treatment with nascent hydrogen, revert to the original acetic acid. They are —
C2H3C1O2 — Monochloracetic Acid,
C2H2C12O2 — Dichloracetic Acid, and
C2HC13O2 — Trichloracetic Acid.
Consequently, there must be three replaceable hydrogen atoms in the acid. This would lead us to the formula C2H4O2 for it. (Comp. also Ladenburg : Die Theorie der aromatischen Verbindungen (1876), p. 10.)
Knowing the molecular value of an analysed compound, it will often be necessary to multiply its empirical formula to obtain one which will express the number of atoms contained in the molecule. This will be the empirical molecular formula.
(2) Determination of the Molecular Weight from the Vapour Density
This method is limited to those substances which can be volatilized without undergoing decomposition. It is based upon the law of Avogadro, according to which equal volumes of all gases and vapours at like temperature and like pressure contain an equal number of molecules. The molecular weights are, therefore, the same as the specific gravities. As the specific gravity is compared with H = i, and the molecular weights with H2 = 2, we ascertain the molecular weights by multiplying the specific gravity by 2. Should the specific gravity be referred to air = i, then the molecular weight is equal to the specific gravity multiplied by 28*86 (since air is 14*43 times heavier than hydrogen).
Molecular Weight. Specific Gravity.
Air — 14 43 i
Hydrogen . H2 = 2 i 0*0693
Oxygen . O, = 3i'74 15*87 1*1060
Water . . . . H2O = 17-87 8-93 0-622
Methane . . . CH4 == 15-97 7-98 °'553
Experience has shown that the results arrived at by the chemical method and those obtained from the vapour density — are almost always identical. If a deviation should occur, it is invariably in con- sequence of the substance undergoing decomposition, or dissociation, in its conversion into vapour.
Two essentially different methods are employed in determining the vapour density. According to one, by weighing a vessel of known capacity filled with vapour, the weight of the latter is ascertained — method of Dumas and of Bunsen; in accordance with the other, a weighed quantity of substance is vaporized and the volume of the resulting vapour determined. In the latter case the vapour volume may be directly
12
ORGANIC CHEMISTRY
measured — methods of Gay-Lussac and A. W. Hofmann / or it may be calculated from the equivalent quantity of a liquid expelled by the vapour — displacement methods. The first three methods, of which a fuller description may be found in more extended text-books,* are seldom employed at present in laboratories, because the method of V. Meyer, which is characterised by simplicity in execution, affords sufficiently accurate results for all ordinary purposes.
Method of Victor Meyer. — Determination of vapour density by displacement of air (B. 11, 1867, 2253). A weighed quantity of substance is vaporized in an enclosed space, and the volume of air which it displaces is measured. Fig. 3 represents the apparatus constructed for this purpose. It consists of a narrow glass tube, ending in a cylindrical vessel, A . The upper, somewhat enlarged opening, B, is closed with an india-rubber stopper. A short capillary side tube, C, conducts the displaced air into the water bath, D. The substance is weighed out in a small glass tube provided with a stopper, and is vaporized in A, the escaping air being collected in the eudiometer, E. The vapour-bath, used in heating A, consists of a wide glass cylinder, F (B. 19, 1862), f whose lower, some- what enlarged end, is closed and filled with a liquid of known boiling point. The liquid employed is determined by the substance under examination ; its boiling point must be above that of the latter. Some of the liquids in use are water (100°), xylene (about 140°), aniline (184°), ethyl benzoate (213°), amyl benzoate (261°), and.diphenylamine (310°).
The vapour density, S, equals the weight of the vapour, P (the same, naturally, as the weight of the substance employed), divided by the weight of an equal volume of air, P' —
i c.c. of air at o° and 760 mm. pressure weighs 0-001293 gram. The air volume V£, found at the observed temperature is under the pressure p— s, in which p indicates the barometric pressure and s the tension of the aqueous vapour at temperature t. The weight then would be —
P'=
r ' P~s
*' i+ 0-00367* 760 '
Consequently the vapour density sought is—
P(i -f 0*003670760 —fi^) '
FIG. 3.
0-001293.
The displaced air may be collected in the gas-baroscope
(compare p. 7). (B. 27, 2267.)
,V. Meyer's method yields results that are sufficiently
accurate in practice, because in deducing the molecular
weight from the vapour density, relatively large numbers a're considered and the little differences do not come into consideration. A greater inaccuracy may arise in the method of introducing the substances into the apparatus because air is apt to enter the vessel. L. Meyer (B. 13, 991), Piccard (B. 13, 1080), Mahlmann (B. 18, 1624), and V. Meyer and Biltz (B. 21, 688) have suggested various devices to avoid this source of error. To test the liability to decomposition of the substance at the temperature of the experiment, a small
* Consult Handwbrterbuch der Chemie, Ladenburg, Bd. 3, 244. •f It is simpler to make the reduction to 760 mm. o° by comparison with a normal volume (p. 7).
DETERMINATION OF THE MOLECULAR WEIGHT 13
portion of it may be heated in a glass bulb drawn out to a long point (B. 14, 1466).
Substances boiling above 300° are heated in a lead-bath (B. 11, 2255). Porce- lain vessels are used when the temperature required is so high as to melt glass, and the heating is then carried out in a Ferret's gas oven (B. 12, 1112). Where air affects the substances in vapour form, the apparatus is filled with pure nitrogen (B. 18, 2809 ; 21, 688). If the substances under investigation attack porcelain, tubes of platinum are substituted for the latter, which are enclosed in glazed porcelain tubes, and then heated in furnaces (B. 12, 2204 ; Z. phys. Ch. 1, 146 ; B. 21, 688). This form of apparatus allows of the simultaneous determination of temperature (B. 15, 141 ; Z. phys. Ch. 1, 153).
For modifications in displacement methods of determining the density of gases, consult V. Meyer (B. 15, 137, 1161, 2771); Langer and V. Meyer, Pyrotechnische Untersuchungen, 1885; Crafts (B. 13, 851; 14, 356; 16, 457). For air-baths and regulators see L. Meyer (B. 16, 1087 ; 17, 478).
Modifications of the displacement method, adapted for work under reduced pressure, have been proposed by La Costs (B. 18, 2122), Schall (B. 22, 140, with bibliography ; B. 27, R. 604), Eyckmann (B. 22, 2754), V. Meyer and Demuth (B. 23, 311) ; Richards (B. 23, 919, note), Neuberg (B. 24, 729, 2543).
For further methods see Nilson and Pettersson (B. 17, 987 ; 19, R. 88 ; J. pr. Ch. 33, i) ; Bill* (B. 21, 2767).
(3) Determination of the Molecular Weight of Substances when in
Solution
i. By Means of Osmotic Pressure. — According to the theory oi solutions developed by van 't Hoff (Z. phys. Ch. 1, 481 ; 3, 198 ; B. 27, 6),* chemical substances, when in dilute solution, behave as though they were in the form of a gas or vapour ; so that the laws of Boyle and Gay-Lussac, and the hypothesis of Avogadro, apply also to dilute solutions. We know that the gas particles exert pressure, and it is also true that the particles of compounds, when dissolved, exert a pres- sure, which is directly expressed or shown by osmotic phenomena, and hence it is termed osmotic pressure. This pressure is equal to that which would be exerted by an equal amount of the substance, if it were converted into a gas, and occupied the same volume, at the same temperature, as the solution. Solutions containing molecular quan- tities of different substances exert the same osmotic pressure. It is, therefore, possible, as in the case of gas pressure, to deduce directly the molecular weight of the substance in solution from its osmotic pressure.
Pfeffer has determined osmotic pressure by means of artificial cells having semi-permeable walls. If suitably modified, this method promises to be of wide applicability (Ladenburg, B. 22, 1225).
The plasmolytic method of de Vries for the determination of osmotic pressure, is based upon the use of living plant-cells, in place of which Hamburger employed red blood corpuscles (Z. physik. Ch. 2, 415 ; 14, 424).
The molecular weight is most simply calculated by the general formula for gases : pv = RT, in which R represents a constant, and T the absolute tempera- ture, calculated from — 273°. If this equation is also to include the hypothesis of Avogadro (that the molecular weights of gases or dissolved substances occupy the same volume at like temperature and pressure), then molecular quantities of the substances must always be taken into consideration. The constant equals 84000 for gram-molecular weights (2 grams hydrogen, or 31*74 grams oxygen)
* See Ostwald's Grundriss der allgemeinen Chemie, 2. Aufl. 1890; Lothat Meyer-Rimbach Grundzuge der theoretischen Chemie, 4. Aufl. 1907.
i4 ORGANIC CHEMISTRY
at the temperature o° (or 2-73°), and the pressure (gas or osmotic pressure) of 76 cm. of mercury.
p . v = 84000 . T.*
where v represents the volume corresponding to the gram-molecular weight
(v = — , in which a is the weight in grams of i c.c. of the gas, or dissolved sub-
a stance, contained in i c.c. of the solution). After substitution the formula reads :
P- I3-59 X-- = 84000 (273+0.
with the four variables p, M, a and t. If three of these be given the fourth can be calculated. Consequently, the molecular weight M is found from the formula —
M_ a. 84000(273+0 = a. 618(273+0
/>• 13*59 P
2. From the Lowering of the Vapour Pressure or the Raising of the Boiling Point. — The lowering of the vapour pressure of solutions is closely connected with osmotic pressure. Solutions at the same temperature have a lower vapour pressure (/') than the pure solvent (/), and consequently boil at a higher temperature than the latter. The lowering in pressure (/—/') is in pro- portion to the quantity of the substance dissolved (Wintrier), according to the
equation — ~- =k g, in which k represents the " relative lowering of the vapour
pressure " ( ^ ) *or I Per cent- solutions, and g their percentage content.
If the lowering be referred not to equal quantities, but to molecular quantities of the substances dissolved, it is found that equi-molecular solutions (those con- taining molecular quantities of the different substances in equal amounts in the same solvent) show equal lowering — the molecular vapour pressure lowering is constant : —
Again, on comparing the relative lowering of vapour pressure in different solvents, it will be found also that they are equal, if equal amounts of the sub- stances are dissolved in molecular quantities of the solvent. In its broadest sense the law would read : The lowering of vapour-pressure is to the vapour- pressure of the solvent (/) as the number of molecules of the dissolved body («) is to the total number of molecules (n + N) : —
/-/'_ *
G Substituting ^ and ^ (g and G represent the weight quantities of the sub-
stance and the solvent ; m and M are their molecular weights), for n and N, the molecular weights, can readily be calculated.
F. M. Raoult (1887) discovered these relationships and put them forward as being empirical. Soon after van 't Hoff (Z. phys. Ch. 3, 115) deduced them theoretically from the osmotic pressure. They are only of value for substances non- volatile as compared with the solvent, or for such as volatilize with difficulty, and show the same abnormalities as are observed with osmotic pressure and depression in the freezing point.
The methods for the determination of vapour pressure are yet too little known and primitive in their nature to be applied in the practical determination of molecular weights (B. 22, 1084 ; Z. phys. Ch. 4, 538). Far more simple and exact is the determination of the rise in the boiling point, which corresponds with this (Beckmann, Z. phys. Ch. 4, 539 ; 6, 437 ; 8, 223 ; 15, 656 ; B. 27, R. 727 ; 28, R. 432).
* R = -^-', p = 1033 = 76 x i3'59 (sp. gr. of mercury) ; v = 22196 = 3i'74 0-001430 (wt. of i c.c. of oxygen). R = -IO33X22196.
DETERMINATION OF THE MOLECULAR WEIGHT 15
Method of Beckmann.— A tube, A (Fig. 4), is employed as the boiling vessel, and is provided with two side tubes tt and tz. The substance under examination is introduced through tr ; a condenser, N, is attached to *, and a calcium chloride tube may be inserted at M. Garnets or fragments of platinum are introduced into the main tube, followed by the solvent, and finally the opening is closed by a differential ther- mometer (Beckmann, Z. physik. Ch. 51, 329), of which the bulb must be completely covered by the liquid. The boil- ing tube is surrounded with an air-bath consist- ing of a mica cylinder, g, and two glass-wool plugs, A! and hz. When dealing with liquids of high boiling point the air-bath may be re- placed by a vapour- bath made of glass or porcelain, which is charged with the same liquid as that which is employed as the solvent ; otherwise the boiling tube may be heated directly on an asbestos netting, LD, over a micro-burner. The boiling point of the pure solvent is first read, and then again after a known quantity of the solute has been introduced down the tube t. A rise of temperature is observed, and should be taken after each of several successive additions of weighed quantities of the solute.
A modification of the apparatus has been devised by Beckmann (Z. physik. Ch. 44, 161) based on that of Sakurai and Landsberger (B. 31, 458 ; 36, 1555). In this form, the temperature of the solution is raised by passing into it the vapour of the solvent, whereby continuous readings can be taken of the boiling point of the solution of a constant weight of solute in an increasing quantity of solvent. 5. Arrhenius has deduced a formula for the molecular rise in boiling
point, which is perfectly analogous to that of van 'tHoffiorihe molecular depression
T2
of the freezing point. The molecular rise is expressed by d=o~O2 . — , in which
T represents the absolute boiling point, and w the heat of evaporation of the solvent. Upon dissolving i gram-molecule of a substance, i.e. if the molecular weight of the body is m, then m grams of it in 100 grams of solvent, the boiling point will be raised d° ; upon dissolving p grams of the substance in 100 gr. of
solvent the rise will be df whence d^=d. — ; from which
where
p = the weight (in grams) of the substance, dissolved in 100 grams of the solvent,
d — molecular rise in boiling point ( =o'O2 . - j,
dj= observed rise in boiling point.
The molecular rise of the boiling point in the case of ether is 21*1°, of chloroform 36'6°, and of acetic acid 25 '3°.
3. From the Depression of the Freezing Point.— The mole- cular weights of dissolved substances are accurately and readily
16 ORGANIC CHEMISTRY
deduced from the depression of the freezing points of their solutions. Blagden in 1788, and Rn-dorff m. 1861, found that the depression of the freezing points of crystall.izable solvents, or substances (as water, benzene, and glacial acetic acid) is proportional to the quantity of substance dissolved by them. The later researches of Coppet (1871), and especially those of Raoult (1882), have established the fact that when molecular quantities of different substances are dissolved in the same amount of a solvent, they show the same depression in their freezing points (Law of Raoult). If t represents the depression pro- duced by p grams of a substance dissolved in 100 grams of the solvent,
the coefficient cf depression -- will be the depression for I gram of
substance in 100 grams of the solution.* The molecular depression is the product obtained by multiplying the depression coefficient by the molecular weight of the dissolved substances. This is a constant for all substances having the same solvent : —
M .*- = C. P
Raoult's experiments show the constant to have approximately the following values : for benzene, 49 ; for glacial acetic acid, 39 ; for water, 19. When the constant is known, the molecular weight is calcu- lated as follows : —
M = cr
A comparison of the constants found for different solvents will disclose the fact that they bear the same ratio to each other as the molecular weights — that consequently the quotient obtained from the molecular depressions and molecular weights is a constant value of about 0*62. It means, expressed differently, that the molecule of any one substance dissolved in 100 molecules of a liquid lowers the point of solidification very nearly o'62°.
These empirical laws, discovered by Coppet and Raoult, have been theoretically deduced byGuldberg (1870) and van '/ Hoff (1886) from the diminution of vapour pressure and of osmotic pressure. The constant C is obtained for the various
solvents, from the formula 0-02 — , where T indicates the absolute temperature
of solidification of the solvent, and w is its latent heat of fusion. In this way van 't Hoff calculated the constants for benzene (53), acetic acid (38-8), and water 18-9 (see above).
The laws just described can only be employed in their simple form in the case of indifferent or but slightly chemically active substances.
Salts, strong acids, and bases (all electrolytes) behave unexpectedly in that the depressions of freezing point, the change in osmotic pressure, and the lowering of vapour pressure as found experimentally are all greater than their calculated values ; the electrolytic dissociation theory of Arrhenius (Z. phys. Ch. 1, 577, 631 ; 2, 491 ; B. 27, R. 542) accounts for this by the assumption that the electrolytes have separated into their free ions. However, even the indifferent bodies exhibit many abnormalities — generally the very opposite of the ordinary. These seem to be due to the fact that the substances held in solution had not completely broken up into their individual molecules. The
* Arrhenius (Z. phys. Ch. 2, 493) expresses the content of solutions by the weight in grams ot the substances contained in 100 c.c. of the solution.
DETERMINATION OF THE MOLECULAR WEIGHT 17
most accurate results are obtained by operating with very dilute solutions, and by employing glacial acetic acid as solvent. This dis- sociates solids most readily.
Various forms of apparatus suitable for the above purpose, and methods of working have been proposed by Auwers (B. 21, 711), Holleman (B. 21, 860), Hentschel (Z. phys. Ch. 2, 307), Beck- mann (Z. phys. Ch. 2, 638), Eykmann (Z. phys. Ch. 2, 964), Klobu- kow (Z. phys. Ch. 4, 10), and Baumann and Fromm (B. 24, 1431).
Method of Beckmann.— A thick walled test tube, 2-3 cm. in diameter, to which a side tube has been fused (Fig. 5), is partially filled with 10-15 gm. of solvent, weighed to the nearest gram. A platinum stirrer is inserted, which terminates at its upper end in a platinized or enamelled iron ring. The freezing tube is then closed with a stopper carrying a Beckmann thermometer (p. 15). Above the iron ring of the stirrer is fixed a small electromagnet, which is energized by the accumulators A at periods determined by the metronome M. The stirrer is thus kept continu- ously in motion, whilst the injurious effect of the atmo- spheric moisture is avoided. The lower part of the freezing tube is fixed by means of a cork inside a wider tube in order to prevent a too rapid fall of temperature when the apparatus is plunged into a beaker containing a freezing mixture. When the solvent
chosen is acetic acid (solidifying about 16°) cold water may be employed ; for benzene (solidifying about 5°), ice- water is suitable. The freezing point of the solvent is then determined, by cooling it to 1-2° below its solidifying point and then starting crystallization by stirring, or by the introduction of scraps of platinum foil or by " inoculation " with a crystal of the substance forming the solute. The thermometer then suddenly rises a little, and the freezing point is taken to be that at which the mercury remains constant for a little while. After allowing the mass to thaw, a carefully weighed quantity of the solid to be examined is introduced down the side tube, and allowed to dissolve. The freezing point of the solution is then determined in a similar manner to that just described (B. 28, R. 412 ; C. 1910, I. 241 ; II. 361 ; Z. phys. Ch. 40, 192 ; 44, 169).
Eykmann's Method (A. 273, 98) requires phenol as the solvent (melting about 38°), whereby considerable simplification is possible. Its molecular depression is greater than that of benzene, and has been calculated theoretically as being 76 (p. 16). Fig. 6 represents the form of apparatus, which consists of a flask with two tubulures, in one of which a thermometer is fixed, and over the other is placed a ground-glass cap.
The investigations of Paterno and others show, contrary to earlier observations, that when benzene is employed as the solvent the carbon derivatives mostly yield normal results ; the exceptions being the alcohols, phenols, acids, oximes, and pyrrole (B, 22* 1430 and Z. phys. Ch. 5, 94 '• B. 27, R. 845; 28, R. 974) VOL, i, c
FIG. 0.
18 ORGANIC CHEMISTRY
Naphthalene may also be used for determinations of this kind ; van 't Hoff gives its depression constant as being about 70 (B. 22, 2501 ; 23, R. i ; 24,
Consult B. 28, 804 for a method of determining molecular weights from the decrease in solubility.
For the determination of molecular weight from molecular solution-volume, see P. 29, 1023.
THE CHEMICAL CONSTITUTION OF THE CARBON COMPOUNDS
Early Theories. — The opinion that the cause of chemical affinity resided in elec- trical forces was first expressed in the commencement of the last century, when the remarkable decompositions of chemical bodies through the agency of the electric current were discovered. It was assumed that the elementary atoms possessed different electrical polarities, and that the elements were arranged in a series accord- ing to their electrical behaviour. Chemical union depended on the equalization of different electricities. The dualistic idea of the constitution of compounds was a necessary consequence of this hypothesis. According to it, every chemical compound was composed of two groups, electrically different, and these were further made up of two different groups or elements. Thus, salts were viewed as combinations of electro-positive bases (metallic oxides), with electro-negative acids (acid anhydrides), and these, in turn, were held to be binary compounds of oxygen with metals and non-metals. With this as basis there was constructed the electro- chemical, dualistic theory of Berzelius, which almost exclusively domi- nated chemical science in Germany until the beginning of 1860.
The principles predominating in inorganic chemistry were also applied to organic substances.^ It was thought that in the latter complex groups (radicals) played the same role as that of the elements in inorganic chemistry. Organic chemistry was defined as the chemistry of the compound radicals (Liebig, 1832), and led to the chemical-radical theory, which flourished in Germany simultaneously with the electro-chemical theory. According to this view, the object of organic chemistry was the investigation and isolation of radicals, in the sense of the dualistic idea, as the more intimate components of the organic compounds, and by this means they sought to explain the constitution of the latter. (Liebig and Wbhler, Ueber das Radical der Benzoesaure, A. 3, 249 ; Bunsen, Ueber die Kakodylverbindungen, A. 31, 175 ; 37, i ; 42, 14 ; 46, i.)
In the meantime, about 1830, France contributed facts not in harmony with the electro-chemical, dualistic theory. It had been found that the hydrogen in organic compounds could be replaced (substituted) by chlorine and bromine, without any important change in the character of the compounds. To the electro- negative halogens was ascribed a chemical function similar to electro-positive hydrogen. This showed the electro-chemical hypothesis to be erroneous. The dualistic idea was superseded by a unitary theory. Laying aside all the primitive speculations on the nature of chemical affinity, the chemical compounds began to be looked upon as being constituted in accordance with definite funda- mental forms — types — in which the individual elements could be replaced by others (early type theory of Dumas, nucleus theory of Laurent) . Dumas, however, distinguished between chemical types and mechanical types. He considered substances to have the same chemical type, to be of the same species, when they possessed the same fundamental properties, e.g. acetic and chloracetic acids. Like Regnault, he considered that they were of the same mechanical type, belonged to the same natural family, when they were related in structure but showed a different chemical character, e.g. alcohol and acetic acid. At the same time, the dualistic view on the pre-existence of radicals was refuted.
The correct establishment of the ideas of equivalent, atom, and molecule (Laurent and Gerhardt) was an important consequence of the typical unitary idea of chemical compounds. By means of it a correct foundation was laid for further generalization. The molecule having been accepted as a chemical unit, the study of the grouping of atoms in the molecule became possible, and chemical constitution could again be more closely examined. The investigation of the reactions of double decomposition, whereby single atomic groups (radicals or residues) were preserved and could be exchanged (Gerhardt) ; the important discoveries of the amines or substituted ammonias by Wurtz (1849), and Hofmann
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 19
(1849) ; the epoch-making researches of Williamson and Chancel (1850), upon the composition of ethers ; and the discovery of acid-forming oxides by Gerhardt (1851), — led to a " type " explanation of the individual classes of compounds. Williamson referred the alcohols and ethers to the water type. A . W. Hofmann deduced the substituted ammonias from ammonia. The " type " idea found its culmination in the type theory of Gerhardt (1853), which was nothing more than an amalgamation of the early type or substitution theory of Dumas and Laurent with the radical theory of Berzelius and Liebig. The molecule was its basis, in which a further grouping of atoms was assumed. The conception of radicals became different ; they were no longer regarded as atomic groups that could be isolated and compared with elements, but as molecular residues which remained unaltered in certain reactions.
Comparing the carbon compounds with the simplest inorganic derivatives, Gerhardt referred them to the following principal fundamental forms or types : —
H\ C11 H\0 Hl
HJ H/ H/U H [N
Hydrogen. Hydrogen Water. H)
Chloride. Ammonia.
From these they could be obtained by substituting the compound radicals for hydrogen atoms. All compounds that could be viewed as consisting of two directly combined groups were referred to the hydrogen and hydrogen chloride types, e.g. :
CN} Cg,} C,H30
Ethyl Ethyl Cyanogen Ethyl Acetyl
Hydride. Chloride. Hydride. Cyanide. Chloride.
It was customary to refer all those bodies derivable from water by the replace- ment of hydrogen, to the water type :
C2H6)0 C2H3Oj C2H5j C2H3Oj
HJU H/U' C2H5(U C2H30/°
Alcohol. Acetic Acid. Ethyl Ether. Acetic Anhydride.
Associated types were included with the principal types. Thus, with the
fundamental type ~}1 were arranged, as subordinates, the types ?£l *!; with ti) rlj rlj
the water type g\O that of §\S, etc.
*»J **J
All derivatives of ammonia were referred to the ammonia type :
CH3) CH3) C2H30)
HN CH3 N HN
H) CH3| Hi
Methyl-amine. Trimethyl-amine. Acetamide. Cyanic Acid.
The types of Gerhardt were chemical types, as he himself expresses it : " Mes types sont des types de double decomposition." It is thus understood that he
included the type with that of
These types no longer possessed their early restricted meaning. Sometimes a compound was referred to different types, according to the transpositions the formula was intended to express. Thus aldehyde was referred to the hydrogen or water type ; cyanic acid to the water or ammonia type :
and C2H8a CNo and CON
The development of the idea of polyatomic radicals, the knowledge that the hydrogen of carbon radicals could be replaced by the groups OH and NHa, etc., contributed to the further establishment of multiple and mixed types (Williamson. Odling, Kekule) :
20 ORGANIC CHEMISTRY
Compound Types :
en H,O HSN
C2H4'
Cl Ethylene Chloride. tlj n2)
Ethylene Carbamide.
Glycol.
MMTypv:
IH } H N
(H {H}O
H2}°» HJO HJO
Cblorhydrin. Oxamic Acid. Amido-acetic Acid.
The presentation of these multiple and mixed types depended on the polyatomic radicals of two or more type-molecules, M one may so name them, becoming united into one whole— a molecule. Upon comparing these typical with the structural formulae employed at present, we observe that the first constitute the transitional state from the empirical, unitary formulae to those of the present day. The latter aim to express the kind of grouping of the atoms in the molecule.
The next step was the expansion of the Gerhardt type to the —
H
Marsh-gas type
H
C by Kekule, 1856 (A. 101, 204).
H
Recent Views. — A year later KekuU (1857) in a communication, "Ueber die sog. gepaarten Verbindungen und die Theorie der mehratomigen Radicale" (A. 104, 129), indicated the idea of types by the assumption of a peculiar function of the atoms — their atomicity or basicity (valence). This he supposed to be the cause of the types of Gerhardt.
As early as 1852 Frankland had enunciated similar views in regard to the elements of the nitrogen group (A. 85, 329 ; 101, 257 ; Frankland, Experimental Reseaches in Pure, Applied, and Physical Chemistry, London, 1871, p. 147). Kolbe concurred with these ideas (compare his derivation of the organic com- pounds from the radical carbonyl C2 and carbon dioxide C2O4 — Kolbe' s Lehrbuch der organischen Chemie, 1858, Bd. I. p. 567). The reason that they did not exert greater influence upon the development of theoretical chemistry is mainly due to the fact that the notions of the relations of equivalent weight and atomic weight were not clearly defined by either of these two investigators.
In his assumptions 'Kekule rather returned to Dumas' mechanical types than to the double decomposition types of Gerhardt. The distinction between the
type Hj and H| as drawn by Gerhardt did not exist for Kekule. The latter, in
1858, said, " It is necessary in explaining the properties of chemical compounds to go back to the elements which compose these compounds." He continues : " I do not regard it as the chief aim of our time to detect atomic groups which, owing to certain properties, may be considered radicals and thus to include the compounds under certain types, which in this way have scarcely any other signi- ficance than that of type or example formula. I am rather of the opinion that the generalization should be extended to the constitution of the radicals them- selves, to the determination of the relation of the elements among themselves, and thus to deduce from the nature of the elements both the nature of the radicals and that of their compounds " (A. 106, 136).
The recognition of the quadrivalence of the carbon atoms and the power they
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 21
possessed of combining with each other, accounted for the existence and the combining value of radicals ; also, for their constitution (Kekule, I.e., and Couper, A. ch. phys. [3] 53, 469). The type theory, consequently, is not, as sometimes declared, laid aside as erroneous ; it has only found generalization and ampli- fication in a broader principle — the extension of the valence theory of Kekule and Couper to the derivatives of carbon.
Whilst formerly it was"the custom to consider in addition to em/?mca/ formulae, representing mereby an atomic composition of the molecule, also rational formulae (Berzelius), which in reality were nothing more than reaction formulae adopted to explain to a certain degree the chemical behaviour of derivatives of carbon, Kekule now spoke of the manner of the union of the atoms in the molecule, by know- ledge of which the constitution of the carbon compounds may be determined (constitutional formula). Lothar Meyer next introduced the phrase "linking of the carbon atoms.'' The expression structure (structural formula) originated with Butlerow.
An application of the valency theory, which has been remarkably fruitful, is the Kekule benzene theory. Here for the first time there was assumed to be present in a carbon compound a closed carbon-chain, a ring consisting of six carbon atoms. The rather singular stability of the aromatic bodies is due to the presence of this " benzene ring." Korner applied these views to pyridine and deduced the pyridine ring ; and in recent years numerous other ring-systems have been suggested and substantiated.
Theory of Chemical Structure of Carbon Compounds. Theory of Atomic Linking, or the Structural Theory.
Constitutional or structural formulae are based upon the following principles, which have been deduced from experiment and repeatedly confirmed : —
1. The carbon atom is quadrivalent. The position of carbon in the periodic system gives expression to this fact. One carbon atom can combine at the most with four similar or dissimilar univalent atoms or atomic groups :
CH4 CF4 CC14
Methane. Carbon Tetrafluoride. Carbon Tetrachloride.
CH3C1 CH3NHa CH2C12 CHC1,
Methyl Chloride. Methyiamine. Dichloromethane. Chloroform.
In a few compounds, such as carbon monoxide CO, the isonitriles or carbyl- amines R'— N=C (A. 270, 267) ; and fulminic acid HO— N=C (A. 280, 303) carbon behaves as a bivalent element.
2. The four units of affinity of carbon are equal, i.e. no differences can be discovered in them when they form compounds , If one of the four hydrogen atoms in the simplest hydrocarbon, CH4, be replaced by a univalent atom or univalent atomic group, each mono-substitution product will appear in but one modification. The four hydrogen atoms are similarly combined, consequently it is immaterial which of them is replaced.
CHSC1 CHSOH CH,NH2
Chlorome thane. Methyl Alcohol. Mcthylamiue.
are known in but one modification each (p. 29).
3. The carbon atoms can unite with each other. When two carbon atoms combine the union can occur in three ways :
(a) The two carbon atoms unite with a single valence each, leaving the atomic group, ^C— C~, with six free valences.
11 ORGANIC CHEMISTRY
(b) The two carbon atoms unite with two valences each, constitu- ting an atomic group, =C=C=, with four free valences.
(c) Two carbon atoms are united by three valences. The residual group — C=C — has but two uncombined valences.
In the first case the union of the two carbon atoms is single, in the second case double, and in the third case triple. Carbon atoms can combine with themselves to a greater degree than the atoms of any other elements. This gives rise to carbon nuclei, and carbon skeletons, which form either open or closed chains or rings. The uncombined valences of the carbon nuclei can saturate or take up atoms of other elements or other atomic groups. This explains ths existence of the innumerable carbon compounds.
This mutual union is indicated, according to the recommendation of Couper, by lines. These formulae represent the internal construction of the compounds, and are known as structural formula :
OH
I I I H H H
Propylamine. H
H
Formaldehyde.
Such structural formulae have been deduced, by the help of the valency theory, from reactions which result in the building up and the breaking down of carbon compounds. They express clearly the relations between the bonds, which, in the main, determine the behaviour of the substance. Those atoms within the molecule which are bound most directly to each other exercise the greatest influence on one another. But it must not be supposed that atoms, unconnected directly by bonds, exert no mutual influence ; such structural formulas give no information of their relative distances apart in space. In the study of reactions where halogen atoms are substituted for hydrogen in the molecule, it is immediately apparent that such replacement takes place with varying facility. This is specially obvious in the case of the aromatic substances (see Volume II). Further, the carboxyl group reacts with different degrees of acidity varying with the individual acid. Reactions, in which the loss of some atoms causes a single bond to become a multiple one, or the formation of a ring complex, and where intra-molecular atomic migration (see p. 36) takes place, obviously depend on the mutual influence of atoms unconnected directly by bonds, as shown in the structural formulae.
Kekules valency theory explains clearly the function of the main bonds in our structural formulae, but does not deal with the subsidiary action of the various atoms on one another in the molecule. And yet one cannot go so far as to say that in each atomic constellation which constitutes a molecule, every atom exerts a chemical influence on every other. But so much can be asserted, that each atom contained in the molecule of a chemical compound is bound to each other atom in that molecule. To illustrate such attractions diagrammati- cally, it would be necessary to draw a network of interatomic bonds in every atomic formula. The greater or lesser strength of the bond could be indicated by a thicker or finer line. If such a diagram were examined at a certain distance, only the thick lines — Bonds of the First Order — would be seen clearly, i.e. practi- cally the same in appearance as the structural formula ordinarily represented.
In many cases it can be deduced from the behaviour of the substance that the Bonds of the Second Order exert an influence of negligible strength.
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 23
An external sign of the presence of such subsidiary valency — bonds of appreci- able influence — is found in the absence of such chemical reactions as might be expected to take place by analogy with others. Another exists in the relative ease with which a group of atoms can be split off, which indicates the pre- existence in the original molecule of such a group held together by these second-order bonds.
Saturated and Unsaturated Compounds. — Saturated carbon coin- pounds are those in which only singly bound carbon atoms occur. They cannot be united by more valences unless the carbon chain is broken up. Unsaturated compounds are those in which doubly or triply bound carbon atoms exist. As a single union is sufficient to link carbon atoms together, a pair of carbon atoms with double union can take up two additional valence units, if one of the double bonds becomes broken, for this purpose, leaving the other to avoid destruc- tion of the chain, e.g. :
H
Two carbon atoms, trebly linked, can take up four valences. The dissolution of the triple union may proceed step by step, whereby it may first be changed to a double linkage and then to a simple union :
|
C— H |
2H H— ( |
:— H i |
H 2H H— C— H |
|
H |
H— C— H |
^ H— C— H |
The Unsaturated compounds, by the breaking down of their double and triple unions and the addition of two or four univalent atoms, pass into saturated compounds.
This same behaviour is observed with many other compounds containing carbon and oxygen, doubly combined, =C=O (aldehydes and ketones) or double and triple union of carbon and nitrogenf =C=N— CsN (acid nitrites, imides, oximes). They are in the same sense unsaturated ; by the breaking down of their double or triple union they change to saturated compounds in which the polyvalent atoms are linked by a single bond to each other :
H H
H-C=O feN !
| H— C— OH H— C— NH-
H— C— H + 2H = H— C— H + 4H = |
H— C— H H— C— H
HI H |
H H
Acetaidehyde. Ethyl Alcohol. Acetonitrile. Ethylamine.
A second class of unsaturated carbon compounds exists, where the carbon atom itself and alone must be looked on as being unsaturated. (A. 288, 202.) For example :
=C=O =C=N.CaH6 =C=N.OH
Carb»n Ethyl Carbylaraine Fu'rainic Acid.
Monoxide. and homologues.
24 ORGANIC CHEMISTRY
Kadioals, Kesidues, Groups. — The assumption of the existence of radicals, capable of existing alone and playing a special rdle in mole- cules; has long been abandoned (B. 35, 1196). The structural formulae assign no especially favourable position to one atom over another in the molecule. Radicals are atomic groups, chiefly those containing Carbon, which in many reactions remain unaltered and pass from one compound into another without change. In this category must also be included the uni-, di-, tri-, and polyvalent atomic complexes, which remain when atoms or atomic groups are imagined to be removed from saturated bodies. By such gradual abstraction of hydrogen, methane yields the following radicals, having different valences : —
CH< — CH8 =CH, =CH
Methane, Methyl, Methylene, Methenyl or Methine,
saturated. univalent radical. divalent radical. trlvalent radical.
If such radicals are isolated from existing compounds, e.g. the halogen derivatives, then two of them unite to form a molecule :
CHSI CH,
+ 2Na = | '+2NaI CH,I CH3
CH2I, CH2
CH2I2 CHa
CHC13 CH
+ 6Na = HI -f 6NaCl CHC1, CH
Or, an atomic rearrangement may occur with the production of a molecule of the same number of carbon atoms :
CHC12 CH2 CH
|| CH2
2Na = || + 2NaCl and not |
CHj
The expressions residue and group are similar to radical. They are chiefly applied to inorganic radicals, e.g. :
— OH water residue or Hydroxyl group,
— SH hydrogen sulphide residue or Hydrosulphide group,
— NH2 ammonia residue or Amido group, =NH Imido group,
— NO2 Nitro group,
— NO Nitroso group.
Homologous and Isologous Series. — Schiel, in 1842 (A. 43, 107 ; 110, 141), directed attention to the phenomenon of homology, giving as evidence the alcohol radicals, and was followed shortly after by Dumas, who observed it in the fatty acids.^ Gerhardt introduced the terms homologous and isologous series, and showed the role these series played in the classification of the carbon derivatives. It was the theory of atomic linking that first disclosed the cause of homology.
The different kinds of linkages between the carbon atoms shows itself most plainly among the hydrocarbons. By removing one atom of hydrogen from the simplest hydrocarbon, methane, CH*, the remaining univalent group, CH3, can combine with another, yielding CH3 — CH3, or C2H«, ethane or dimethyl. Here, again, a hydrogen atom may be
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 25
replaced by the group CH8, resulting in the compound CH3 — CH2— CH3, propane. The structure of these derivatives may be more clearly represented graphically :
H H H
I I I
H— C— H H— C— C— H
I I I
H H H
CH4 C,H8
By continuing this chain -like union of the carbon atoms, there arises an entire series of hydrocarbons :
CH8— CHt— CHa— CH, CH3-CH2— CHa— CH2— CH8, etc.
C4H10 C6Hia
Such a series of bodies of similar chemical structure and corre- sponding in chemical characters is known as a homologous series.
The composition of such an homologous series can be expressed by a general empirical or rational formula. The series formula for the marsh gas or methane hydrocarbons is C«H3M4.2.
Each member differs from the one immediately preceding and the one following by CH2. The phenomenon of homology is therefore due to the linking power of the quadrivalent carbon atoms.
On the configuration of the carbon chain, see C. 1900, II. 28, 664, 1256, and Volume II., Cycloparaffins.
In addition to the homologous series of the saturated marsh-gas type, there are a large number of other such series, of which the simplest are those of the monohydroxy-alcohols, the aldehydes and mono- car boxylic acids.
CnH2n+20 CMHanO CMH2nO,
CH4O Methyl Alcohol CHaO Formaldehyde CH2O2 Formic Acid
CaH,O Ethyl Alcohol C2H4O Acetaldehyde C2H4O2 Acetic Acid
CgHjjO Propyl Alcohol C3HaO Propionaldehyde C8H8O2 Propionic Acid
C4H10O Butyl Alcohol C4H8O Butyraldehyde C4H8Oa Butyric Acid etc. etc. etc.
Carbon compounds, chemically similar, but differing from each other in com- position by a difference other than wCH8, e.g. the saturated and unsaturated hydrocarbons, form isologous series, according to Gerhardt :
CzH6 ...... C2H4 ...... C2H2
C3H8 ...... C3H6 ...... C8H4
Isomerism; Polymerism ; Metamerism; Chain or Nucleus Iso- merism ; Position or Place Isomerism. — The view once prevailed that bodies of different properties must necessarily possess a different composition. The first hydrocarbons showing that this opinion was. erroneous were discovered in 1820.
Liebig, in 1823, demonstrated that silver cyanate and fulminate were identical- In 1828 Wohler changed ammonium cyanate to urea, and in 1830 Berzelius estab~ lished the similarity of tartar ic acid and racemic acid.
Berzelius, in 1830, designated as isomers (10-0^^, composed of similar parts) bodies of similar composition but different in properties. A year later he distinguished two kinds of isomerism, viz. : isomerism of
26 ORGANIC CHEMISTRY
bodies of different molecular mass — fiolymerism ; and bodies of like molecular mass — metamerism.
Numerous isomeric carbon derivatives were discovered in rapid succession ; hence, an answer to the question as to what causes iso- meric phenomena acquired importance for the development of organic chemistry. The deeper insight into the structure of carbon compounds, which was gradually attained, gave rise in consequence to a further division of metameric phenomena.
The expression metamerism was employed to designate that kind of isomerism which is due to the homology ofcradicals held in combina- tion by atoms of higher valence. If the homologous radicals are joined by polyvalent elements, then those compounds are metameric, in which the sum of the elements contained in the radicals is the same (H may be viewed as the simplest radical) :
8 TjfO is metameric with ^ti3[O
LH8J
|
Ethyl Alcohol. |
Methyl Ether. |
||
|
C3H7) |
CtH6 |
||
|
0 is |
metameric with |
o |
|
|
H| Propyl Alcohol. |
CH, Bthyl-M |
ethyl |
|
|
Ether. |
|||
|
C2H6) |
CH, |
||
|
H N is |
metameric with |
CH |
N |
|
Hi |
H |
||
|
Ethylamine. |
Dime thylamine. |
||
|
C8H7 |
C2H6 |
CH,| |
|
|
H N is metameric |
with CH3 N |
and CH8JN |
|
|
H |
H |
CH3| |
|
|
Propylamine. |
Ethyl Methyl- araine. |
Trimethyl- amine. |
The constitution of the radicals in this division was disregarded, the type formulae were sufficiently explanatory. We have recognized the power of the quadrivalent carbon atoms to unite in a chain-like manner as the cause of homology, and to this cause may be attributed other phenomena of isomerism, which are not properly included under metamerism.
In deducing the formulae of the five simplest hydrocarbons of the homologous series Q^H^g, the formula for ethane, CH3.CH3, was developed from that of methane, CH4, and that of propane CH3.CH2.CH3 from the formula of ethane C2H?. In the case of propane intermediate and terminal carbon atoms are distinguished. The former are attached on either side to two other carbon atoms, still possessing two valency units which are saturated by two hydrogen atoms. The terminal carbon atoms of the chain are linked to three hydrogen atoms.
With the next member of the series we observe a difference. Above (p. 24), the fact that a hydrogen of the terminal methyl group of propane was replaced by methyl was the only condition considered. This led to the formula CH3.CH2.CH2.CH3. However, the CH3-group might replace a hydrogen atom of the intermediate CH2-group, and
CH..CH.CH, then the result would be the formula . In this hydro-
CH, carton there is a branched carbon chain. The hydrocarbon with a
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 27
continuous chain is termed normal butane ; its isomer is isobutane, i.e. isomeric butane.
Theoretically, by a similar deduction, the two butanes
CH8— CH2— CH2— CH8 CH3CH(CH3)8
Normal Butane. Isobutane.
yield three isomeric pentanes which are actually known.
'
CH3.CH2.CH2.CHa.CH8 CH3.CH.CHa.CH, H8C— C— CH,
Normal Pentane.
CH3 CH3
Isopentane. Pseudopentane
Tetrametbyl Methane.
The number of possible isomers increases rapidly with the increase in carbon atoms (B. 27, R. 725 ; 33, 2131).
The origin of isomerism in the homologous paraffins, as in so many other cases, is the different constitution of the carbon chain. The isomerism caused by a difference in linking, by the different structure of the carbon nucleus or the carbon chain, is termed nucleus or chain isomerism.
The investigation of the substitution products of the paraffin hydro- carbons brings to light another kind of isomerism. The principle of similarity of the four valences of a carbon atom (p. 21) renders logical and possible but one monochloro-substitution product of methane and ethane. The same consideration which heretofore recognized the possibility of two methyl substitution products of propane (the two butanes possible by theory) leads to the possibility of two monochloro- propanes, dependent upon whether the chlorine atom has replaced the hydrogen of a terminal or intermediate carbon atom :
CH3.CH2.CH2C1 CH3.CHC1.CH8
Normal Propyl Chloride. Isopropyl Chloride.
If two hydrogen atoms of one of the carbon atoms of propane be replaced by an oxygen atom, the following case of isomerism arises :
CH3.CH2.CHO CH8.CO.CH8
Propyl Aldehyde. Acetone.
In the case of the two known chloropropanes, and also in the case of propyl aldehyde and acetone, the cause of the isomerism is not due to difference in constitution of the carbon chain, but to the different position of the chlorine atoms with reference to the oxygen atoms of the same carbon chain. Isomerism, induced by the different arrange- ment or position of the substituting elements in the same carbon chain, is designated isomerism of place or position.
The intimate relationship of the two varieties of isomerism is appa- rent from the derivation of the ideas of nucleus or chain isomerism and place or position isomerism.
Recent Views on the Structural Theory, — The theory of atomic linking not only revealed an insight into the causes of the innumerable isomeric phenomena, but predicted unknown instances
28 ORGANIC CHEMISTRY
and determined their number in a very definite manner. In many cases isomeric modifications, possible by theory, were discovered at a later period. For certain isomers, however, at first few in number, the structural formulas deduced from their synthetic and analytical reactions were insufficient, inasmuch as different compounds were known, to which the same structural formula could be given. The greatest similarity in reactions indicative of the structure was com- bined with complete difference in physical properties of the com- pounds belonging in this class. The tendency at first was to designate such bodies physical isomers, meaning thereby an aggregation of varying complexes of chemically similar molecules.
The following groups of such isomers have been well investigated :
HO.HC.COaH
1. The four symmetrical dihydroxysuccinic acids : , the
HO.HC.CO,H
ordinary or dextro-tartaric acid, and racemic acid, which were proved to be isomeric in 1830 by Berzelius (see p. 25), and laevo-tartaric and the inactive or meso-tartaric acids which were added later, through Pasteur's classic researches.
CH.CO,H
2. The two symmetrical ethylene-dicarboxylic acids : \\ , fu- maric and maleic acid. CH.CO8H
3. The three a-hydroxypropionic acids : CH8.CH(OH).CO2H — inactive lactic acid of fermentation, sarcolactic acid, and laevo-lactic acid, which was added later.
Substances are included among these compounds, which when liquefied, either by fusion or solution, rotate the plane of polarization either to the right or left. The direction of deviation is indicated by prefixing " dextro " or " laevo " to the name of the bodies thus acting. Such carbon compounds are " optically active " (p. 54), in contra- distinction to the other almost innumerable derivatives which exert no influence on polarized light and are " optically inactive " or "inactive."
A direct synthesis of optically active carbon compounds has not yet been achieved (see asymmetric synthesis, p. 55), although optically inactive bodies have been synthesized. Pasteur discovered methods by means of which the latter can be resolved into their components, which rotate the plane of polarization to an equal degree but in opposite directions. Upon splitting sodium-ammonium racemate into sodium-ammonium laevo- and dextro-tartrates, Pasteur observed that the crystals of these salts showed hemihedrism ; that they were as an object to its mirror-image ; and that equally long columns of equally concentrated solutions of these salts, at the same temperature, deviated the plane of polarized light to an equal degree in opposite directions.
In 1860 Pasteur expressed himself as follows upon the cause of these phenomena — upon molecular asymmetry : " Are the atoms of the dextro-acid grouped in the form of a right-handed spiral, or are they arranged at the angles of an irregular tetrahedron, or are they distributed according to some other asymmetric arrange- ment ? We know not. Undoubtedly, however, we have to do with an asymmetric arrangement, the images of which cannot mutually cover each other. It is not less certain that the atoms of the laevo-acid are arranged in opposite order." In l873 /• Wislieenus added the following comment to the evidence of similar structure in the optically inactive lactic acid of fermentation and the optically active sarcolactic acid : " Facts compel us to explain the difference of isomeric
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 29
molecules of like structural formula by a difference in arrangement of the atoms in space." How the space configuration of the molecules of carbon compounds was to be represented was answered almost simultaneously and independently of each other by van 't Hoff and Le Bel (1874) (B. 26, R. 36), by the introduction of the hypothesis of the asymmetric carbon atom. This hypothesis is the basis of the chemistry of space or stereo-chemistry of the carbon atom.
The hypothesis of an asymmetric carbon atom * is designed to explain optical activity and the isomerism of optically active carbon compounds.
Whilst the theory of atomic linkage abstains from any representa- tion of the spacial arrangement of the atoms in a molecule, experience gathered from the investigation of simple carbon compounds shows that definite spacial relations do not harmonize with actual facts. Assuming that the four valences of a carbon atom act in a plane and in perpendicular directions upon each other, the following possible isomers for methane are evident : —
No isomers of the types CHgR1 and
Two „ „ „ CH^R1)* CHgRiR2, CHR2(R1)2,
Three „
„ „
Methylene iodide, for example, should appear in two isomeric modifications H H
H— O— I
I — C — I and
&
i
However, two isomers of a single disubstitution product of methane have never been found ; consequently, it is very improbable that the four affinities of a carbon atom are disposed in the manner indicated above. The carbon atom models of Kekule represent the carbon atom as a black sphere and the quadiivalence of it by four needles of equal length and firmly attached to the sphere, which Baeyer has called axes. These needles are not perpendicular to each other, nor do they lie in the same plane, but are so arranged that planes placed about their terminals produce a regular tetrahedron (Z. f . Ch. (1867) N. F. 3, 216). Van 't Hoff's generalizations are based upon this model, about which fundamental considerations will be more fully developed in the following pages.
On the assumption that the affinities of a carbon atom are directed towards the summits of a regular tetrahedron, in the centre of which is the carbon atom, there would be no imaginable isomers coinciding
* Pasteur : Recherches sur la dissymetrie moleculaires des produits organiques naturels. Le9ons de chimie professees en 1860. Paris, 1861. Vgl. Ostwald's Klassiker der exacten Wissenschaften, Nr. 28 : Ueber die Asymmetric bei natiirlich vorkommenden organischen Verbindungen, von Pasteur. Uebersetzt und herausgegeben von M. und A. Ladenburg. J. H. van 't Hoff : Dix annees dans 1'histoire d'une theorie, 1887. K. Auwers : Die Entwickelung der Stereo- chemie, Heidelberg, 1890. A. Hantzsch : Grundriss der Stereochemie, Breslau, 1893. C. A. Bischoff : Handbuch der Stereochemie, 1803, together with, Material ien der Stereochemie, 1904. Werner : Lehrbuch der Stereochemie, 1904.
30 ORGANIC CHEMISTRY
, CHR^R^but a case such asCHRiR'R3 or the more general CR1R2R3R4 — an isomeric phenomenon of peculiar nature — might be predicted. A carbon atom of this description — one that is connected with four different univalent atoms or atomic groups — van 't Hoff has designated an asymmetric carbon atom, proposing to represent it by an italic C. It is often indicated by a small star.
If a compound contains an asymmetric carbon atom we can conceive of its existence in two isomeric modifications, the one being an image of the other :
These spacial arrangements are more fully understood by the aid of the models suggested by Kekule, van 't Hoff, and others, than by their projection upon the flat surface of paper. Van 't Hoff introduced tetrahedron models in which the solid angles were coloured ; this was to represent and indicate different radicals. They lack this advantage, possessed by the Kekule model, that the carbon atom has entirely disappeared from the model. It must be imagined as being in the centre of the tetrahedron, and in projections of these models (see above) the radicals are united to each other by lines, the latter, however, not in any sense representing a chemical union.
In the left tetrahedron the successive series RXR2R3 proceeds in a direction directly opposite to that of the hand of a watch, whilst in the right tetrahedron the course coincides with that of the hand. The two figures cannot, by rotation, be by any means brought into the same position, — that is, in a position to cover each other completely, — any more than the left hand can be made to cover the right, or a picture its image or reflection.
The Isomerism of Optically Active Carbon Compounds. — The cause of optical activity, in the opinion of van 't Hoff and of Le Bel, is the presence of one or several asymmetric carbon atoms in the molecule of every optically active body. It is obvious that two mole- cules which only differ in that the series of atoms or atomic groups attached to an asymmetric carbon atom differ successively in order of arrangement, which therefore are identical in chemical structure, must be very similar in chemical properties. However, those physical properties, upon which the opposite successive series of atoms or atomic groups in union with asymmetric carbon exerts an influence, e.g. the power of deviating the plane of polarized light, must be equal in value, but opposite. The union of two molecules identical in structure, having equal but opposite rotatory power, gives rise to a molecule of an optically inactive polymeric compound.
Compounds containing an Asymmetric Carbon Atom. — fl-Hydroxy- propionic acid, CH3 — *CHOH.CO2H, is an example of a compound containing one asymmetric carbon atom. It exists in two optically
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 31
active, structurally identical, but physically isomeric modifications, and one optically inactive, structurally identical polymeric form :
Dextro-lactic Acid. Lasvo-lactic Aeid.
(Sarcolactic Acid.)
OH OH
C— H + H— C
/\ /\
H3C CO2H HO2C CH3
( ( + ) ^-Lactic Acid ( -) /-Lactic Acid = } LaTctic . AcidT of Fermentation or \v / Inactive Lactic Acid.
The following compounds also contain one asymmetric carbon atom : —
Leucine ........ C4H9*CH(NH2)CO2H
Malic Acid ....... CO2H.CH2.*CH(OH)CO2H
Asparagine ....... CONH2CH2.*CH(NH2)CO2H
MandelicAcid ...... CflH6.*CHOH.CO2H
Each of the preceding bodies is known in two optically active and one optically inactive modifications.
Compounds containing Two Asymmetric Carbon Atoms. — The relations are more complicated when two asymmetric carbon atoms are present.
The simplest case would be that in which similar groups are in union with the two asymmetric carbon atoms. The one half of the molecule would then be constructed chemically exactly like the other half. The four isomeric dihydroxysuccinic acids belong in this group. This group of tartaric acids has become of the greatest importance in the development of the chemistry of optically active carbon derivatives.
They were the first to be most carefully investigated chemically, optically, and crystallographically, and were employed by Pasteur in the development of methods for resolving the optically inactive com- pounds into their optically active components (p. 56). Their im- portance was further increased by the fact that they were brought into an intimate genetic relation with fumaric and maleic acids — two isomeric bodies which will be considered in the next section (p. 34).
When a carbon compound contains two asymmetric carbon atoms, united to similar groups, then a fourth compound becomes possible in addition to the three isomeric modifications which a compound con- taining only one asymmetric carbon atom is capable of forming. If the groups linked to one asymmetric carbon atom, viewed from the axis of union of the two asymmetric carbon atoms, show an opposite successive arrangement to that of the other asymmetric carbon atom,
ORGANIC CHEMISTRY
an inactive compound results, due to an intramolecular or internal compensation ; the action due to the one asymmetric atom upon polarized light will be cancelled by an equal but opposite action caused by the other asymmetric carbon atom.
The hypothesis of the asymmetric carbon atom gave the first and, indeed, the only satisfactory explanation for the occurrence of four isomeric symmetrical dihydroxysuccinic acids, which are represented as follows : —
'OK
HO
H
C02H
H C— OH
C02H
COfH
C02H -C— OH
H C— OH
C02H
(i) Dextro-tartaric Acid. (2) Laevo-tartaric Acid. (3) Inactive or Meso-tartaric Acid.
Dextro-tartaric Acid + Lfflvo-tartaric Acid=(4) Racemic Acid.
It is seen that the two independent rotating systems are in contact with one another at one angle of the tetrahedrons through a single carbon bond.
An excellent example of the formation of a meso-form during the purification of two optical antipodes, is supplied by laevo-alanyl- dextro-alanine. It is itself optically active, but loses water, giving rise to the meso-form of alanine anhydride (C. 1906, II. 59) :
NH\ ^^HN
i i JXT i
HOOC — C -> C— CO OC — C
H CH3 H8C H H CH, H3C H
J-Alanyl-r-alanine. Meso-alanine Anhydride.
The possibilities of isomerism in carbon compounds containing more than two asymmetric carbon atoms — a condition observable with the polyhydric alcohols, their corresponding aldehyde alcohols, and ketone alcohols (the simplest sugar varieties), as well as with their oxidation products, will be more elaborately discussed under these several groups of compounds.
Geometrical Isomerism, Stereoisomerism in the Ethylene Deriva- tives (Alloisomerism). — Two carbon atoms, singly linked to each other, whose valences are not required for mutual union, and which are united to other atoms or atomic groups, may be considered as being able to rotate independently of each other about their axis of union
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 33
/. Wislicenus assumes, however, that the atoms or atomic groups combined with these two carbon atoms exercise a "directing influence" upon each other until finally the entire system has passed into the "favourable configuration" or the " preferred position." It follows from this assumption that, in ethane derivatives in which asym- metric carbon atoms are not present, structurally identical isomers cannot occur. When the van 't Hoff tetrahedron models are employed for demonstration the two systems, capable of independent rotation about a common axis, are found to touch one another through a single carbon bond situated at one of the angles (comp. the projection- formula of the tartaric acids, p. 32).
A different state prevails where the carbon atoms are doubly linked. The double union, according to van 't Hoff, prevents a free and inde- pendent rotation of the two systems and space-isomers are possible. The tetrahedron models represent this double union in such a manner that two tetrahedra have two angles in common and are in contact along a common edge. The frequent and notable differ- ences in chemical behaviour of this class of isomers are to be attri- buted to the greater or less spacial distance of the atomic groups, which determine the chemical character.
Compounds having the general formulae abC=Cab or abC=Cac, may exist in two isomeric modifications. In one instance groups of like name are directed towards the same side — according to /. Wislicenus the " plane symmetrical configuration" — or they are directed towards opposite sides — then they have according to the same author the central or axially symmetrical configuration. Baeyer suggests for this form of asymmetry the term " relative asymmetry " in contradistinction to the kind of asymmetry which substances with asymmetric carbon atoms show ; the latter he prefers to call " absolute asymmetry."
The structurally symmetrical ethylene-dicarboxylic acid is the most striking example of this class of isomerism. It exists in two isomeric modifications, known as fumaric and maleic acids, both of which have been very carefully investigated. Maleic acid readily passes into an anhydride, hence the plane symmetrical configuration is ascribed to it ; fumaric acid does not form an anhydride, so that the axial symmetrical configuration is given to it, in which the two carboxyl groups are as widely removed from each other as possible. In projec- tion formulae and in structural formulae, to which there is given a spacial meaning, the configuration of these two acids would be represented in the following way : —
H C02ll
HC.C02H \ X / HO'C-CH
H * r~/\ ~ HCOH
HC.C02H / '</\
#02C
Maleic Acid. Fumaric Acid.
Plane Symmetrical Configuration. Central or Axially Symmetrical Configuration.
VOL. I. D
34 ORGANIC CHEMISTRY
The isomerism of mesaconic and citraconic acids, (CH3)(CO2H) C=CH(CO2H), is of the same class; the first acid corresponds to fumaric acid and the second to malei'c acid. Further examples of the class are :
Crotonic and Isocrotonic Acids .
Angelic and Tiglic Acids
Oleic and Elaidic Acids .
Erucic and Brassidic Acids .
The two a-Chlorocrotonic Acids ,, ,, /3-Chlorocro tonic Acids ., ,, Tolane Dichlorides „ ,, ,, Dibromides
„ ,, o-Dinitrostilbenes .
Cinnamic and Allocinnamic Acids
The two a-Bromocinnainic Acids ,, „ ^3-Bromocmnamic Acids
CH3CH : CHCO2H. CHS.CH : C(CH3)CO2H. C8H17CH : CH.CTH14.CO2H. C8HUCH : CH.C^Haj.COjH CH3.CH : CCl.COgH CH8.CC1 : CH.CCXH. « C8H,CC1 : CC1C.HB. C,H5CBr : CBrC,H5.
l^f.L.1. gV/JJi. . V/JJlV^jZTj.
NOa[2]€,H4[i]CH : CH[i}C6H4[2]NO.. C.H..CH : CHCOaH. C.Hj.CH : CBrCOjH. C,H6.CBr : CHCO2H. Coumaric Acids HO[2]C,H4[i]CH : CH.COaH, etc.
Isomeric phenomena of this kind Michael designates as allo- isomerism, without suggestion as to its cause. When a body passes into a more stable modification upon the application of heat, Michael prefixes " allo " to the name of the more stable form ; thus, fumaric acid is allomaleic acid (B. 19, 1384).
Fumaric and malei'c acids are placed at the head of this class of isomeric phenomena not only because they have been most thoroughly investigated, but chiefly because the two optically inactive dihydroxy- tartaric acids bear to them an intimate genetic relation (p. 31) . Kekule and Anschiitz showed that fumaric acid was converted into racemic acid, and malei'c acid into mesotartaric acid by potassium permanganate. This conversion harmonizes entirely with the van 't Hoff-Le Bel conception of these four acids ; indeed, it might have been predicted. These relations will be more fully elaborated in the discussion on the acids. In studying maleic and the alkyl-maleic acids, it will be further discussed whether or not it is required by configuration that maleic acid and its homologues should have a structure quite different from that of fumaric acid. The relations are similar in the case of the cou- maric acids (Vol. II.).
Baeyer considers that the isomerism of the saturated or carbocyclic compounds bears a definite relation to the stereoisomerism of the ethylene derivatives, as will be more fully explained when the hexahydroxyphthalic acids (Vol. II.) are described. The same author maintains that the simple ring-union of carbon atoms viewed from a stereochemical standpoint has the same signification as the double union in open chains. Therefore, stereoisomerism in the carbon compounds with double union would appear merely as a special case of isomerism in simple ring-unions. Baumann applied this idea to saturated heterocyclic compounds — to the polymeric thioaldehydes (q.v.).
Baeyer suggested the introduction of a common symbol for all geometrical isomers, such as the Greek letter r. ' ' The addition of an index will assist the ready expression of the kind of isomerism. In the case of compounds which
contain "absolute asymmetric carbon atoms, the signs H can be employed.
Thus the expressions
Dextro-tartaric Acid = r + + )
Laevo-tartaric Acid =T [Tartaric Acid
Mesotartaric Acid = r -| — )
can be understood without special explanation." In the case of relative asym- metry in unsaturated compounds and saturated rings, Baeyer proposes to use the
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 35
terms cis and trans. Maleic acid = I"*1"- ci* or briefly r5" ethylene-dicarboxylic acid, while fumaric acid=rci8«tranl1 ethylene-dicarboxylic acid.
Further considerations on the space-configuration of the ethylene and polymethylene derivatives lead to a broadening of the scope and to the correction of the law, that an asymmetric carbon atom must be present in every optically active compound (see above, p. 30). Optical activity can occur even in the absence of an asymmetric carbon atom in the ordinary sense, if the atoms are attached to a carbon skeleton in such a way in space, that there is no plane of symmetry' present — the object and its mirror-image do not correspond. This is found, for instance, in hexahydrohexahydroxybenzene, which exists in two enan- tiomorphic optically active forms, as d- and /- inositol :
d- and /- Inositol.
Another example is found in d- and /- methyl-cyclohexylidene-acetic acid,
CH, COOH HOOC CH8
| CH2-CH2 CH.-CH
, / . [*\ /CH.-CH.V |
*C< >C=C* *C=C< >c*
I N:H2— CH/ / \ XCH8— CH/ !
H H H H
in which the CH3 and H, COOH and H, attached to the *C atoms must lie in planes at right angles to each other as required by the condition of asymmetry (Aschan, B. 35, 3389 ; Marckwold and Meth} B. 39, 1171).
The particularly ready formation of carbocyclic and heterocyclic compounds when five or six carbon atoms take part in the ring forma- tion, is also a result of the position of the atoms in space. This aspect of stereochemistry will be considered in the introduction to the carbo- cyclic compounds, and there also to the heterocyclic bodies, as well as in the discussion of the cyclic carboxylic esters, or lactones, the cyclic acid amides or lactams, the anhydrides of dibasic acids, etc.
Hypotheses Relating to Multiple Unions of Carbon. — The multiple unions of carbon are so important in stereochemical considerations, that there has been a large amount of research into the nature of this union as well as attempts to represent it. All investigations in this direction demonstrate how difficult it is at present to understand so obscure a force as chemical attraction or affinity from a mechanical point of view. Despite the demand and necessity that may exist for the introduction of hypotheses dealing with the mechanics of multiple linkage the views so far presented are in many essentials contradictory, and not one has won general recognition for itself. See Baeyer (B. 18, 2277 ; 23, 1274) ; Wunderlich (Configuration organischer Molecule, Leipzig, 1886) ; Lossen (B. 20, 3306) ; Wislicenus (B. 21, 581) ; V. Meyer (B. 21, 265 Anm. ; 23, 581, 618) ; V. Meyer und Riecke (B. 21,
36 ORGANIC CHEMISTRY
946) ; Auwers (Entwicklung der Stereochemie, Heidelberg, 1890), pp. 22-25 ; Naumann (B. 23, 477) ; Bruhl (A. 211, 162, 371) ; Deslisle (A. 269, 97) ; Skraup (Wien. Monatsh. 12, 146) ; /. Thiele (A. 306, 87 ; 319, 129) ; Erlenmeyer, jun. (A. 316, 43 ; J. pr. Ch. [2] 62, 145) ; Vorlaender (A. 320, 66) ; Hinrichsen (A. 336, 168).
Stereochemistry of Nitrogen. — Isomeric phenomena of nitrogen-containing compounds of like chemical structure, which could not be ascribed to the same cause as prevailed in carbon compounds, led to the application of stereochemical views to the nitrogen atom. There appeared to be an absolute nitrogen asymmetry corresponding to the absolute carbon asymmetry, of which examples were cited by Le Bel in the unstable, optically active modification of methyl ethyl propyl isobutyl ammonium chloride (C. r. 112, 724 ; B. 32, 560, 722, 988, 1469, 3508 ; 33, 1003 ; C. 1900, II. 77 ; C. 1900, I. 26, 179 ; 1901, II. 206, 409, etc.).
The relative asymmetry, due to the doubly- bound carbon atom, is seen in the isomerism of the oximes (Hantesch and Werner', comp. also W attach, A. 332, 337), of the hydroxamic acids (Werner), and of the aromatic diazotates, diazo- sulphonic acids and diazocyanides (Hantzsch).
Stereochemistry of Tin: C. 1900, II. 34. Stereochemistry of Sulphur: C. 1900, 1.537; 11.623.
Intramolecular Atomic Rearrangements. — Many investigations have shown that certain modes of linking, apparently possible from a valence standpoint, cannot, in fact, occur, or when they do take place are possible only under certain definite conditions. In reactions, for example, in which two or three hydroxyl groups should unite with the same carbon atom, a loss of water almost invariably occurs and oxygen becomes doubly united with carbon, e.g. :
_H,o ^O
CHsc-Cl - > (CH3C-0— Hi --- > CH.cf
H
/°-H\ -HSO HC(-Q— H) ' —
xi xo— H/
On the other hand, the ethers derivable from these unstable " alcohols " are stable :
/O.C8H6 X).C8H.
CH,C^-O.C8H5 and HC^O.C,H8
>H \>.CaH5
In other cases there is a cleavage of a halogen hydride, water or ammonia, with the production of an unsaturated body, or an anhydride of a dibasic acid, or a cyclic ester (lactone), or a cyclic amide (lactam). In these reactions two molecules result from one molecule, in which atom-groups occur in unstable linkage-relations, an organic molecule and a simple inorganic body.
This type of decomposition of a labile molecule is similar to the phenomenon of intramolecular atomic rearrangement, where unstable atomic groupings pass at the moment of their formation into stable forms without the alteration of the size of the molecule. The hydrogen atom, especially, is inclined to wander, but groups, such as the alkyl, phenyl, and hydroxyl behave similarly. To-day, the number of examples of this phenomenon is remarkably large, of which a few
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 37
only need be cited. A free hydroxyl group becomes added in most cases to a carbon atom in double union with its neighbouring carbon atom. When intramolecular atomic rearrangements occur the hydro gen of the hydroxyl attaches itself to the adjacent carbon atom, and oxygen of hydroxyl unites doubly with carbon (Erlenmeyer's rule, B. 13, 309 ; 25, 1781).
|
CHBr ii |
(CH.OH\ ii i |
|
CH, |
II CH2 / |
|
Vinyl Alcohol. |
|
|
CHS |
(CH. \ |
|
doH\ C.O-M |
|
|
CBr i-y.Dr — II |
|
|
CHa |
CH2 / |
|
|8- Allyl Alcohol. |
Acetone.
However, the ethers obtained from vinyl alcohol (q.v.) are stable : CH2=CHO.C2H5 and CH2=C(O.C2Ha)— CHS are known.
It has also been observed that a transposition such as that described above can occur by two unstable and similar molecules rearranging with each other, so that two similar stable molecules result :
CHa=CH.OH CHj.CHO ?( ¥- ===
HO.CH=CH2 OCH.CH,
A rise of temperature is frequently necessary to induce many of these reactions to take place. Both compounds are capable of existence. Unsaturated acids pass into lactones. The intramolecular atomic rearrangement proceeds in a direction favouring the formation of a stable ring :
(CH,)aC _ (CH,)aC- — O
CH— CH2.COaH CHa— CHaCO
Isocaprolactone.
In other unsaturated compounds we observe that the unsymmetrical is transformed into a symmetrical body through the rearrangement of the double linking of carbon :
KCN
CHa : CH.CHaI •> CHa : CH.CHa.CN -> CH..CH : CH.CN -> Allyl Iodide. Nitrilc of Crotonic Acid.
CHS.CH : CH.COgH
Crotonic Acid.
CHa=C— CO CHS.C— CO
>o -> I
CO CH
CH8.CO CH— CO
Itaconic Anhydride. Citraconic Anhydride.
The esters of hydrothiocyanic acid, under the influence of heat, rearrange themselves into the isomeric mustard oils, sulphur unites doubly with carbon and the alcohol radical that had previously been in union with the sulphur wanders to nitrogen :
C3H5— S— C~N > S=C=N.C3H5
Allyl Thiocyanate. Allyl Mustard Oil.
ORGANIC CHEMISTRY
Isonitriles or carbylamines, when heated, pass into nithles ; the alcohol radical previously in union with nitrogen, wanders to carbon :
C6Hft— N=C-
Phenyl Carbylamine. (Vol. II.)
CaH5— C=S
Benzonitrile. (Vol. II.)
Other rearrangements among the atoms of compounds only take place in the presence of a strong acid or base. Indifferent bodies pass over into basic or acid compounds :
NH.C.Hj
HCl
Hydrazobenzene (indifferent). CO.C,H6
C6H4.NHa
C,H4.NH2
Benzidine (diacid base).
KOH
:o.c6H5
Benzil (indifferent).
Benzilic Acid (strong acid).
Further examples of intramolecular rearrangements of aromatic bodies are diazobenzoic acid, phenylhydroxylamine, diazoamido-com- pounds, etc. (see Vol. II.).
Pseudo-forms, Pseudomerism, Desmotropy, Merotropy, Tauto- merism, Phasotropism. — The study of these intramolecular atomic migrations led to the recognition of a large number of atomic groups as being labile and stable. In the case of many bodies it became known that apparently they could react in accordance with two different formula. In other words, as our constitutional formulae were deduced from chemical behaviour, it may be said that compounds existed to which two, and under certain circumstances more, constitutional formulae could be ascribed. Baeyer (B. 16, 2188) explained this pheno- menon in such a manner that the stable bodies, under the influence of heat or reagents, passed into unstable modifications. " These isomers are only known in compounds ; in the free state they revert to the original form. Their instability is referable to the mobility of the hydrogen atoms, since the replacement of the latter is followed by stability " (compare A. W. Hofmann, B. 19, 2084). Mention may be here made of :
^NH
sSH
or
Hydrothiocyan'c Acid.
Isothio- cyanic Acid.
'\S- Known.
.NR
Known (mustard oils).
f?
\NHa
or
|
Cyanamide. |
Carbodi-imide. |
|
— CH |
-CH2 |
|
li or |
c>t |
|
— C.OH |
-CO |
|
Hydroxyl |
Ketone |
|
orF.nol |
Form. |
|
Form (J. pr. |
|
|
Ch. [3] 50, us). |
Known.
Known.
CHCO,CaH6 CHj.COaC8H,
or | C.(OH).CH3 CO.CH, ^
Acetoacetic Ester.
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 39
_N _NH ^-N , NH
|| or e.g. CfH4< !| or C8H4\ |
— COH —CO XCOC.OH NCOCO
Lactime Lactam - — . — • ^-
Form. Form. Isatine.
Baeyer proposes to represent the unstable modifications by the designation " pseudo." Pseudomerism is the term that will be adopted in this work for the phenomenon in which one and the same carbon compound can react in accordance with different structural formulae. The unstable form of a derivative will, therefore, take the name " pseudo form " or " pseudo-modification." In some instances both forms are known.
Closely related to the conception of pseudomerism is that of Desmotropy, derived from Scares, a bond, and rpevciv, to change (P. Jacobson, B. 20, 1732, footnote ; 21, 2628, footnote ; L. Knorr, A. 303, 133 ; Hantzsch, B. 20, 2802 ; 21, 1754 ; Forster, B. 21, 1857). Michael suggested the name " Merotropy " (B. 27, 2128, footnote ; J. pr. Ch. [2] 45, 581, footnote ; 46, 208).
It is noteworthy that most pseudomeric compounds are acid in character; they can form salts. When these salts are treated with alkylogens or acy 1 halides the two classes of isomers appear. H. Goldschmidt (B. 23, 253) refers this phenomenon to the appearance of free ions. Hence in passing judgment upon
rstions of pseudomerism only those reactions can be considered, from which trolytic dissociation is excluded. Michael (J. pr. Ch. 37, 473) puts forward the noteworthy suggestion that in the transpositions of the salts by organic halides two independent processes, depending on the conditions present, take place : that there is a simple exchange whereby the organic radical takes the place of the metal ; or the radical halide first adds itself to the molecule and subsequently separates as a metallic halide. In the latter case the organic radical assumes a position different from that previously held by the metallic atom (compare acetoacetic ester and malonic ester). Nef has recently maintained the correctness of Michael's view.
Laar, on the contrary, following Butlerow (A. 189, 77), van 't Hoff (Ansichten liber die organische Chemie, 2, 263) and Zincke (B. 17, 3030), assumes that such compounds consist of a mixture of structural isomers, in that an easily mobile hydrogen atom oscillates between two positions in equilibria, and thereby the entire complex becomes mobile. He designates the phenomenon as tautomerism. Discarding the uncertainty introduced into the classification of the carbon com- pounds by the acceptance of this view, it has been noted that carbon compounds which Laar considers mixtures of structurally isomeric bodies do not differ in their physical properties from carbon compounds which offer no place in their structure for this equivocal assumption. By the assumption of tautomerism with the underlying meaning assigned it by Laar, the experimental solution of the problem as to the conditions under which pseudo-forms are capable of existence is without object. Although from the nature of the case the identification of easily alterable intermediate reaction-products must continue to be one of the most difficult problems, yet success has been met with in quite a number of cases. At a time when chemical investigations at very low temperatures can so easily be carried on by means of readily obtainable liquid air, experiments on the conditions of existence of labile modifications will be started afresh.
The preceding section was prepared in 1893. Since then, numerous confirma- tions of these views have been found. The ketones constitute the most important class of compounds, which are tautomeric. In them, as in acetoacetic ester, the oscillation is between the paraffin ketone and the olefine hydroxyl or enol formula (p. 40).
The investigations of Claisen (A. 291, 25 ; 297, i), Guthzeit (A. 285, 35), W. Wislicenus (A. 291, 147), Knorr (A. 293, 70 ; 303, 133 ; 306, 332), P. Raabe (B. 32, 84), Dimroth (A. 335, i), and others have demon- strated that there exist compounds of the form — C(OH)=C — CO — ,
40 ORGANIC CHEMISTRY
which readily pass into the form — CO — CH — CO — , and conversely are easily produced from the latter : " The character of the added residue, the temperature and the nature of the solvent, in the case of dissolved substances, determine which of the two forms will be the more stable." Claisen designates the acidic enol-form the a-compound and the neutral keto-iorm the £-bpdy, e.g.
COC6H6 I a-Tribenzoyl Methane C,H5C(OH)=C— COC,H6
COC6H6 j8-Tribenzoyl Methane CjH^CO— CH— COC6H6.
The system of nomenclature proposed by Hanlzsch for pseudomeric substances (B. 38, 1000) appears to be most suited for its purpose. If the accustomed name refers to a " pseudo-acid " (the weaker acid or neutral form), then the name of the real acid is characterized by the prefix " aci " ; for instance, CH3CO — CH2 — COOC2H5 is called aceto- acetic ester, and CH3C(OH) =CH — COOC2H5 is named aa-acetoacetic ester.
If the usual name denotes the strong acid, then that of the pseudo-acid is prefixed by the word " pseudo," as, for example,
dH2.C(OH) =CH-C0.6 is called tetronic acid, and CHa.CO-CH.-CO.O is pseudo-tetronic^dd.
Claisen was the first to show that, in the above example of the two tribenzoyl methanes, only compounds having the a- or aci- constitution form salts direct ; the ft- or pseudo-form yields no salts of the type CO — CMe --CO, but gradually changes when in contact with bases, into the salt of the aci-form CO — C=C(OMe) (see p. 41 ; slow or time isomerisation phenomena).
The change of phenyloxybiazole carboxylic acid ester from one pseudomeric form into the other has been quantitatively determined by Dimroth by titration with potassium iodo-iodate. He found that only the aci-form precipitated iodine while forming a salt, and that the pseudo-form remained unaltered.
Substances such as acetoacetic ester, malonic ester and others possessing the grouping — CO — CH2 — CO — are considered to exist in the pseudo-form, since only one form has been isolated, and this yielded no salts of its own ; those which have been obtained, are metallic hydro xyl-substitution compounds of the aci-form.
The phenomenon of pscudomerism in these compounds can be further complicated by the intervention of stereoisomerism (p. 32) in enol-forms (see Diacetosuccinic acid ester, Knorr, A. 306, 332 ; Formyl phenyl acetic ester, Z. phys. Ch. 34, 46, etc. ; on the other hand, see Michael, B. 39, 203).
Physical methods have proved exceedingly helpful in determining the constitution of the pseudomers, and in following the mutual interchange of forms. Molecular refractions in particular have been determined, as, for instance, in the case of acetoacetic ester and its salts (Briihl, J. pr. Ch. [2] 50, 119 ; B. 38, 1868 ; Holler and Midler,
CHEMICAL CONSTITUTION OF CARBON COMPOUNDS 4!
C. 1905, 1. 349, etc.) ; as well as dielectrical constants (Drude, Z. phys. Ch. 23, 308), and the magnetic rotation (Perkin, Sen.).
The investigations of Holleman (B. 33, 2912) and of Hanlzsch have enabled the presence of pseudomerism to be detected by electric con- ductivity measurements. This is only possible when one of the two possible forms is a weaker electrolyte than the other, as, for example, in the case of certain nitro-fatty bodies — R.CH2NO2, R.CH(NO2)2- Such compounds are gradually changed by alkalies into isonitro- bodies, RCH=NOOMe, etc. ; and from these salts the addition of the equivalent quantity of hydrochloric acid liberates the isonitro-body itself. In solution these iso-compounds revert to the true nitro- body with a greater or less velocity which can be followed by the diminution in electric conductivity, and the gradual disappearance of the red colour given with ferric chloride, which is a general cha racteristic for the aci-form of a compound (slow or time isomerizatiui. phenomena, B. 39, 2089, 3149, 2265).
Chromopseudomerism or Halochromism is the name given to the phenomenon of*a colourless or feebly coloured substance yielding a strongly coloured salt with colourless bases or acids. Such an occur- rence was referred by Hantzsch (B. 39, 3080) to pseudomerism, where a colourless pseudo-electrolytic radical yielded a coloured ion. Examples of this are found in the coloured salts of nitroform, violuric acid, etc.
Halochromism is specially developed in the ortho- and para-deriva- tives of the benzene series (see Vol. II.), which behave, on the one hand, like the mostly colourless true benzene compounds, and on the other like the mainly strongly coloured derivatives of quinone ; this class of bodies includes o- and p- nitroso- and nitrophenols, o- and p- amino- and oxyazo- bodies, derivatives of triphenyl carbinol, etc., classes of bodies to which the coal tar dyes belong, to which the study of pseudo- merism is of special importance. V. Baeyer and others (B. 38, 570 ; 39, 2977) consider halochromism can also occur in certain cases without any real alteration in structure occurring. This is brought about by one of the ordinary carbon valences charging into a so- called carbonium valence, which Baeyer represents by a wavy line ; as for example :
(C,H6)3C— OH (C.H6),C O.SO.H.
Triphenyl Carbinol, Triphenyl Carbinyl Sulphate,
colourless. coloured.
In all the cases which have been considered, the interchangeable isomers have belonged to two different classes of compounds with quite different chemical characteristics. There exist, however, substances which according to their mode of preparation should give rise to two forms belonging to the same class, but which have turned out to be identical with one another, as, for example, diazoamido-compounds, amidines, formazyl derivatives of the general type —
x,NX /NHX
R^ and R/
\NHY \>NY
where R represents N in diazoamido bodies, CH in the amidines, and N : CH.N in the formazyl derivatives. This explains the absence of certain isomerism phenomena in pyrrole, and such azoles as pyrazole
42 ORGANIC CHEMISTRY
and triazole (see Vol. II.), and also in the ortho-di-derivatives of ben- zene (Vol. II., the Constitution of Benzene), etc. Attempts have been made to explain these phenomena by assuming oscillations of Kekule's valences (Knorr, A. 279, 188) ; and this is further complicated, in the case of pyrrole and the azoles, by the wandering of a H atom. For the phenomenon itslef Briihl suggests the name Phasotropism (B. 27, 2396), whilst V. Pechmann puts forward the term virtual tautomerism (B. 28, 2362).
THE NOMENCLATURE OF THE CARBON COMPOUNDS
The steadily increasing number of carbon derivatives has shown the absolute necessity that definite principles should determine their designation. The absence of such general and international rules (where they were possible) has led to great confusion in the nomenclature.
Compounds originating from plants and animals received names that indicated their origin, and often at the same time their characteristic chemical properties : urea, uric acid, tartar, tartaric acid, formic, oxalic, malic, citric, salicylic acids, etc. With a large class of bodies, e.g. the bases, glucosides, bitter principles, fats, etc., it was customary to employ the ending " ine " : coniine, nicotine, guanidine, creatine, betaine, salicine, amygdaline, glycerine, stearine, etc., and in the terminations al, ol, an, en, yl, ylene, ylidene, the effort was made to show the similarity of certain compounds, without, however, proceeding in a connected way.
The more thoroughly the constitution of bodies became known, the greater was the desire to indicate by names the manner in which the atoms were united. This was especially true in the case of isomeric compounds. The manner in which this was done, however, was left to the choice of the individual, and thus it happened that often one and the same derivative received different names, which possessed fundamentally equivalent meanings.
Of the early suggestions on nomenclature, that of Kolbe (A. 113, 307) on carbinol deserves special consideration. As is known, Kolbe referred the names of the monohydroxy saturated alcohols back to the name carbinol. In order to make this principle more general, it becomes necessary to ascertain the carbinol or carbinols for each class of compounds — that is, to find those bodies from which the homologues might be derived, just as the monohydroxy saturated alcohols might be deduced from methyl alcohol or carbinol. Without attempting at this time to determine the limits of the " carbinol nomenclature," it will suffice to remark that in the case of the paraffin dicarboxylic acids all the normal homo- logues are the carbinols ; e.g. malonic acid, succinic acid, normal glutaric acid, adipic acid, etc. Indeed, names such as monomethyl malonic acid, ethyl methyl malonic acid, symmetric and unsymmetric dimethyl succinic acid, etc., are so readily understood that they are preferred by many chemists.
In order to minimize as far as possible the arbitrary nomenclature of organic compounds, a meeting was convened in Geneva, in 1892, of the chemists of nearly all the civilized countries, for the purpose of agreeing on a method of indicating the constitution of carbon compounds in a consistent and clear manner. The new " official " names adopted by the Geneva Convention will, in the case of certain important series of compounds, be observed in the present text ; they will be enclosed in brackets — e.g. [ethene] for ethylene, [ethine] for acetylene, etc. The designations of the simpler bodies — the names justified from an historical stand- point and deduced from important reactions — will not be wholly eliminated. Thus, the names ethyl hydride, dimethyl or methyl methane will be used for ethane, depending upon what relations are especially to be emphasized.
The new nomenclature proceeds from, or begins with, the hydrocarbons. The name of the hydrocarbon serves as the root for the names of those substances which contain their carbon atoms arranged in a similar manner. The different classes of bodies are distinguished by the addition of suffixes to the names of the hydrocarbons : alcohols end in ol, aldehydes in al, ketones in one, and the acids in acid— e.g. [ethanol]= ethyl alcohol, [ethanal]=acetaldehyde, [propanone] a»acetone, [propanal] =propionic aldehyde, [ethane-acid] = acetic acid. These examples will suffice. The more detailed consideration will be given to the various
PHYSICAL PROPERTIES OF THE CARBON COMPOUNDS 43
classes of bodies, which are discussed. The principles of this nomenclature have already been found difficult of application, especially in attempting to indicate in name a compound having a mixed character — e.g. the body COH — CH2 — CHOH — CO — CO2H, which would be pentanolalone-acid. The accumulation of suffixes, each of which possesses a meaning peculiar to itself, lias " conduit rapidement a des termes bizarres, d'une complication facheuse et d'une prononciation difficile " (Ame Pictet).
For the decisions of the International Congress of Geneva, convened igth to 22nd April, 1892, for the purpose of co-ordinating chemical nomenclature, see Tiemann (B. 26, 1595) : Istrate's proposals (C. 1898, I. 17). On the nomen- clature of ring-compounds, see Vol. II.; also M. M. Richter (B. 29, 586).
In order to distinguish the more frequently occurring radicals of the same kind, such as the univalent hydrocarbon residues, both aliphatic and aromatic, the name alky I has been accepted. In differentiating between the two classes alphyl refers to the aliphatic residues and aryl to the aromatic ; whilst aromatic residues possessing aliphatic characteristics are referred to as alpharyle. Carboxylic acid residues, too, are referred to as acyl and differentiated into alphacyl and aracyl (C. 1899, I- 825).
PHYSICAL PROPERTIES OF THE CARBON COMPOUNDS
It can, in general, be foreseen that the physical as well as the chemical properties of carbon compounds must be dependent on their composition and constitution. Such a regular connection has, how- ever, only been determined for a few properties, of which the following serve chiefly for external characterization : —
1. Crystalline form.
2. Specific gravity, density.
3. Melting point.
4. Boiling point.
5. Solubility.
For the investigation of constitution the following properties are of importance : —
6. Optical properties.
(a) Refraction.
(b) Dielectric constants.*
(c) Optical rotation.
(d) Magnetic rotation.
7. Electrical conductivity.
I. CRYSTALLINE FORM OF CARBON COMPOUNDS
The crystalline form of a carbon derivative is one of its most im- portant distinctions, whereby a body may be recognized most definitely and differentiated from other substances ; so that the preparation of organic substances in the form of crystals and their examination has been of the greatest value in organic chemistry. The more com- plex the constitution of a substance, the less the symmetry of its crystals (B. 27, R. 843). The crystalline forms of isomeric bodies are always different. Many substances may assume two or more forms ; they are dimorphous, polymorphous, but each is characterized very definitely by particular conditions of formation and existence.
* This is, strictly, an electrical constant, but owing to its close connection with optical refraction, it is convenient to include it here, as in the German edition. (Translator's note 1
44
ORGANIC CHEMISTRY
When it is possible for a compound to crystallize from the same solvent in different forms, only one can separate within definite ranges of temperature. The limit between these zones, the temperature of transformation, is theoretically expressed by the point of intersection of the solubility curves of the two crystalline forms. It is only the one or the other form that can appear under normal conditions above or below this temperature. From a super- saturated solution, and indeed a supersaturated solution of the two forms, it is possible by the introduction of one or the other form, to obtain each of the two kinds of crystals, and, indeed, both together, but only so long as the supersaturation continues. After that, one of the two forms will gradually dissolve and that one will remain which is the more stable at the temperature of experiment. The temperature of transformation varies for each solvent, and when impurities are present in the substances a greater or less variation in the temperature will occur, according to the degree of impurity.
The existence and stability of a definite modification of a polymorphic sub- stance depends to a great extent on the temperature, of which the influence, how- ever, is not always the same. In the case of perchlorethane C2C16, rhombic, tri- clinic, and regular crystal forms are successively assumed during a gradual rise in temperature, whilst on cooling, the same series is passed through in reversed order. The change is said, therefore, to be reversible, and polymorphic substances of this kind are called enantiotropic (Lehmann). With other bodies, however, one modification may be labile and the other stable, so that the first form changes into the second, and not vice versa. As an example, paranitrophenol C6H4.OH.NO2 (1,4) may be taken. On solidification from the molten state, or from a hot solution, it crystallizes in the colourless labile form. This, on standing, turns into the stable yellowish-red modification, which is quite different in its cleav- age and optical properties from the first. It can also be obtained by crystallizing from a cold solution. Such substances, which undergo a change in one direction only, are called monotropic. In many cases, however, a rigid grouping of the numerous polymorphic organic bodies in one or other of the two groups is not always easy. For the assumptions necessary for the explanation of the pheno- menon, see Zincke (A. 182, 244) and Lehmann (Molecular physik, Leipzig, 1888/89) ; Graham-Otto (Lehrbuch der Chemie, Vol. I., Part 3, p. 22, 1898).
At the present time little is known about the inner connection between the crystalline form and chemical constitution of carbon compounds, but it has been found, for example, that -the slightest variation in chemical constitution does affect the amount of rotation exhibited by optically active compounds. Many such substances possess a hemihedral form, and the two optically active modi- fications of a carbon compound, although they exhibit the same geometrical constants, are distinguished by peculiar left and right types (enantiomorphous forms) ; they are not superposable. The difference between two such com- pounds, in which the atoms are similarly united, is only due, according to the hypothesis of an asymmetric carbon atom (p. 30), to the difference in arrangement of the atoms within the molecule. From this it follows that this variation in arrangement finds expression in the crystalline form (comp. B. 29, 1692).
Laurent, Nichlts, de la Provostave, Pasteur, Hjortdahl (see F. N. Hdw. 3, 855) investigated the influence that chemical relations of organic bodies exerted upon the geometrical properties of their crystals. This problem, however, first appeared in the forefront of crystallographic study after P. Groth introduced the idea of morphotropy (Pogg. A. 141, 31). By this term was understood the phenomenon of regular alteration of crystalline form produced by the entrance of a new atom or atomic group for hydrogen. Groth, Hintze, Bodewig, Arzruni, and others frequently called attention to such morphotropic relations particularly with the aromatic bodies (comp. Physikal. Chemie der Krystalle von Andreas Arzruni, 1893).
The recognition of the connection between crystalline form and chemical constitution is rendered more difficult by the fact that as yet an accurate determination of the magnitude of the crystal-molecule or crystal-element cannot be made. The possibility of doing this in the future may perhaps be found in van 't Hoff's theory of solid solutions. As to the role of water of crystallization in the salts of organic acids, consult Z. phys. Ch. 19, 441.
SPECIFIC GRAVITY OR DENSITY 45
2. SPECIFIC GRAVITY OR DENSITY
By this term is understood the relation of the absolute weight of a substance to the weight of an equal volume of a standard body. Conventional units of comparison are water for solids and liquids, and air or hydrogen for gaseous bodies (see p. n). The number repre- senting the specific gravity of a compound is as great as that repre- senting its density. It frequently occurs, therefore, that the terms specific gravity and density are used interchangeably.
Density of Gaseous Bodies. — For these, as we have already seen, the relation of the specific gravity (gas density) to the chemical composi- tion is very simple. Since, according to Avogadro's law, an equal number of molecules are present in equal volumes, the gas densities stand in the same ratio as the molecular weights. Being referred to hydrogen as unit, the gas densities are one-half the molecular weights. Therefore, the molecular volume, i.e. the quotient of the molecular weight and specific gravity, is a constant quantity for all gases (at like pressure and temperature).
Density of Liquid and Solid Carbon Derivatives. — In the liquid and solid states the molecules are considerably nearer each other than when in the gaseous condition. The size of the molecules and their distance from each other, which increases in different degrees with rise of temperature, are unknown, so that the theoretical bases for deducing the specific gravity are lacking. However, some regularities have been established empirically, which, by comparison with the specific or molecular volumes, give the ratio of molecular weight to specific gravity.
The relations between the specific volumes of carbon compounds were first systematically studied by H. Kopp, in 1842 (A. 64, 212 ; 92, i ; 94, 257 ; 96, 153, etc., to 250, i). He felt justified from his observations in proposing : " That the specific volume of a liquid compound (molecular volume) at its boiling point is equal to the sum of the specific volumes of its constituents (of the atomic volumes), and that every element has a definite atomic volume in its compounds."
From this it would follow that : (i) Isomeric compounds possess approxi- mately like specific volumes ; (2) like differences in specific volumes correspond to like differences in composition.
The more recent researches (Lossen and others (A. 214, 81, 138 ; 221, 61; 224, 56 ; 225, 109 ; 233, 249, 316 ; 243, i) ; R. Schiff (A. 220, 71, 278) ; Horst- mann (B. 19, 1579; 20, 766 and 21, 2211, etc.), based upon an abundance of material, and at the same time giving due consideration to the structural relations of the carbon compounds, prove conclusively that the supposed regularities, mentioned above, are unfounded. In fact, isomeric compounds do not possess equal molecular volumes, and their atomic volumes are not constant. The volume for the difference CH2 is not constant in the different homologous series, nor is that of hydrogen (A. 233, 318 ; B. 20, 767), nor that of oxygen (A. 233, 322 ; B. 19, 1594). M. W. Richards has shown that the atomic volume is a function of temperature and pressure, and probably, also, of electric potential (Z. phys. L hem. 40. 169). For the molecular solution- volume, see Traube (A. 290, 43 : B. 28, 2722).
Hence the molecular volumes do not represent the sums of the atomic volumes (the latter are scarcely determinable), and the specific gravities and molecular volumes depend less upon the volume of the atoms than upon their manner of linkage and upon the structure of the molecules. Therefore, to deduce regularities in the specific volumes it is first necessary to consider carefully the chemical structure of the compounds. In this connection the influence of the double union of the C- atoms in the unsaturated compounds and the ring-linkage
46
ORGANIC CHEMISTRY
in the benzene derivatives, is significant. Assuming that the molecular volume of hydrogen is known and is equal to 5-6, it becomes possible to calculate the molecular volume of an unsaturated olefine compound if the molecular volume of the corresponding saturated paraffin body is known. Thus, pentane =117-17; therefore amylene = ii7'i7 — 2 X5'6 = io5'97. In fact, the molecular volume of amylene equals 109-95. Consequently 109-95 — 105-97 = 3-98 — the increase in molecular volumes caused by the double linkage in amylene (A. 220, 298 ; 221, 104 ; B. 19, 1591 ; 20, 779). The divalent union is therefore less intimate (pp. 21, 35), and the unsaturated compounds consequently show a greater heat of combustion (A. 220, 321).
In the conversion of benzene hydrocarbons into their hexahydrides there is an increase in volume which is three times as great as in the conversion of the olefines into their corresponding paraffins. This would empjhasize the theory that in the benzene nucleus there are three doubly-linked carbon atoms. The specific gravities of the benzene hexahydrides are notably greater (consequently the molecular volumes are smaller) than those of their corresponding olefines, and that accounts for the fact that in the ring-linking of the C- atoms in the benzene nucleus there is an appreciable contraction in volume (A. 225, 114 and B. 20, 773) ; Horstmann (B. 21, 2211) ; Neubeck (Z. phys. Chem. 1, 649).
Schroeder determined the specific volumes of a number of solids (B. 10, 848, 1871 ; 12, 567, 1613 ; 14, 21, 1607, etc.).
In determining the specific gravity of liquid com- pounds, a small bottle — a pyknometer — is used, of which the narrow neck carries an engraved mark. More complicated apparatus, such as that designed by Bruhl, based on Sprengel's form, is employed where greater accuracy is sought (A. 203, 4) (Fig. 7). De- scriptions of modified pyknometers will be found in Ladenburg's Handworterbuch, 3, 238. A convenient form by Ostwald is described in J. pr. Ch. 16, 396. To obtain comparable results, it is recommended to make all determinations at a temperature of 20° C., and refer these to water at 4° and a vacuum. If m represents the weight of substance, v that of an equal volume of water at 20°, then the specific gravity at 20° referred to water at 4° and a vacuum (with an accuracy of four decimals), may be ascertained by the following equation (A. 203, 8) : —
^5 4?
r
FIG. 7.
To find the specific volumes at the boiling temperature, the specific gravity at some definite temperature, the coefficient of expansion and the boiling point must be ascertained ; with these data the specific gravity at the boiling point is calculated, and by dividing the molecular weight by this, there results the specific or molecular volume. Kopp's dilatometer (A. 94, 257), Thorpe (J. Ch. S., 37, 141), Weger (A. 221, 64), is employed in obtaining the expansion of liquids. For a method of obtaining the direct specific gravity at the boiling point, see Ramsay (B. 12, 1024), Schiff (A. 220, 78; B. 14, 2761), Schall (B. 17, 2201). Neubeck (Z. phys. Ch., 1, 652).
Kanonnikow, as well as Kopp and his followers, employed the " true density " in his calculations, not the figure as found directly. This he took as being the reciprocal of Lorenz's refraction constant, since, according to the Clausius and Mosotti theory, it constitutes the fraction of the total volume of a body which is actually occupied by the molecules themselves (C. 1899, II. 858 ; 1901, I. 1190).
3. MELTING POINT (FUSION POINT BP.)
Every pure compound, if at all fusible or volatile, exhibits a definite melting temperature. It is customary to determine this for
MELTING POINT (FUSION POINT BP.) 47
the characterization of the substance, and as a test of its purity. The melting point of a pure compound is not changed by recrystallization. The slightest impurities frequently lower the melting point very con- siderably, whereas when foreign substances are present in larger amounts the melting point is irregular and not well defined— i.e. there is not a definite melting point. If two different substances have the same melting point, a mixture of them will show a considerably lowered melting point. The converse of this is of importance when establishing the identity of two bodies — the mixture must have the same melting point as each of the separate substances. Pressure influences the melting point to a very slight degree.
In many crystalline carbon compounds a double melting point is observed. When heated, the substance first melts to a doubly refracting, turbid " crystalline liquid " (Lj), which becomes clear and isotropic at a higher temperature (La, the " clearing point "). On cooling the reverse order of changes may be observed :
LI , L2
Solid crystals -^ " Crystalline liquid " ^ Amorphous liquid.
The phenomenon apparently depends on chemical constitution, and is observed mainly in aromatic compounds, chiefly acids, acid esters, ketones, and phenolic ethers, which belong to the azoxy- or azo- series, or which contain the group ArC-NAr
\/ or ArC=NAr ; and also in the cholesterol compounds, etc. (see
O B. 39, 803, bibliography; Z. phys. Ch. 57, 357).
Determination of the Melting Point. — The most accurate method would be to immerse the thermometer in the molten substance ; this, however, would require large quantities of material (Landolt, B. 22, R. 638).
Ordinarily, a small quantity of the finely pulverized material is introduced into a capillary tube, closed at one end, which is attached to a thermometer, for instance^ by a thin platinum wire, in such a way that the thermometer and capillary tube are'on the same level. Alternatively, the substances may be pressed between two cover glasses (C. 1900, I. 241). A beaker containing sulphuric acid or liquid paraffin is used to furnish the heat, which is kept uniform throughout the liquid by agitation with a glass stirrer. A long-necked flask, containing sulphuric acid, is sometimes employed, in which a test tube is inserted or fused : in the latter case it is necessary that the flask should be provided with a side-tubulure (Fig. 8) (B. 10, 1800; 19, 1971 ; 5, 337; C. 1900, II. 409).
When the mercury thread of the thermometer extends far above the surface of the bath, it is necessary, in accurate determinations, to introduce a correction, by adding the value n(T— t) 0*000154 to the observed point of fusion, where n is the length of the mercury column projecting beyond the bath expressed in degrees of the thermometer, T is the observed temperature, and t the tempera- ture registered in the middle of the projecting portion of the mercury column ; 0^000154 is the apparent coefficient of expansion of mercury in glass (B. 22, 3072 : Literature and Tables). After the melting point has been approximately determined with an ordinary thermometer a more accurate determination may be made by introducing a shorter thermometer, divided into fifths, with a scale carry- ing a limited number of degrees (about 50°). (See Fig. 8.)
The lack of agreement between the melting points of the same compound as determined by different workers, is often sufficient to prevent identification. This is not so much due to the thermometers as to the manner in which the deter- mination is made. By rapid heating the mercury of the thermometer will not have time to assume the fusion temperature. In the region of the melting point the heat must be moderated so that during the course of the fusion the thermometer rises very slowly. Far more concordant figures might be obtained if a general use of short-scale thermometers were adopted and the time agreed upon for the mercury of the thermometer to rise through one degree of the scale during the
48
ORGANIC CHEMISTRY
observation. For the determination of low melting points by means ot the
,-iir thermometer, see B. 26, 1052; B. 33, 637. For the determination of the
melting points of organic bodies fusing at high temperatures, see B. 28, 1629 ; at red- heat, B. 27, 3129; of coloured compounds, B. 8, 687 ; 20, 3290.
Regularities in Melting Points. — (i) In the case of isomers it has been observed that the member possessing the most symmetrical structure generally shows the highest melting point ; for instance, among the aromatic series, para-compounds melt at a higher temperature than ortho- or meta-compounds. (2) Of the alkyl esters of the carboxylic acids those with the methyl residue have a higher melting point than that of the next homologues (see oxalic esters). (3) In homologous series with like link- ages the melting point alternately rises and falls (see saturated normal aliphatic mono- and dicar- boxylic acids, B. 29, R. 411 ; C. 1900, 1. 749). The members, having an uneven number of carbon atoms, have the lower melting points (Baeyer, B. 10, 1286). This is also true of acid amides having from 6 to 14 carbon atoms (B. 27, R. 551), and for the normal primary diamines (C. 1900, II. 1063 ; 1901, I. 610, etc. ; Z. phys. Ch. 50, 43). (4) In the case of the benzene nitro- compounds and their derivatives — the azoxy-, azo-, hydrazo-, and amido- bodies — as well as the corresponding diphenyl compounds, it has been observed that as oxygen is withdrawn the melting point rises until the azo-derivatives are. reached, when it descends to the amido- bodies (G. Schultz, A. 207, 362). To all these regularities among melting points there exist numerous exceptions (Graham-Otto, Lehrbuch der Chemie, Vol. I. part 3 (1898), p. 505 ;
Franchimont, C. 1897, II. 256). For the melting points of mixtures, see B.
29, R, 75-
FIG. 8.
4. BOILING POINT ; DISTILLATION
The boiling points of carbon derivatives, which are volatile without decomposition, are as important for the purpose of characterization as the melting points. In case ®f the latter the influence of pressure is so slight that it can be neglected, but the former vary very markedly when comparatively inappreciable changes in pressure occur. Hence in stating a boiling point accurately it is necessary to add the pressure at which it was observed. When the quantity of material is ample the boiling point is determined by distillation. For the determination of the boiling points of very small amounts of liquids, see B. 24, 2251, 944 ; 19, 795 ; 14, 88.
Distillation under Ordinary Pressure. — For this purpose a special flask is employed, the long neck of which is provided with a side tube pointing downwards at an angle. The neck of the flask is closed with a stopper, bearing a thermometer. It must not be forgotten that very frequently the vapours of organic substances attack ordinary corks or those of rubber, therefore the exit tube should be placed a considerable distance from the end of the neck ; or the neck may be narrowed at the upper end and the thermometer held in position by means of a piece of india-rubber tubing passed outside it. The mercury bulb of the thermometer
BOILING POINT; DISTILLATION
49
should be slightly below the level of the exit tube in the neck of the flask. The latter should be at least one-half filled with the liquid to be distilled.
If the thermometer is not wholly immersed in me vapour, the external mercury column Avill not be heated to the same degree as that on the interior, hence the recorded temperature will be less than the true one. The necessary correction is the same as that which has already been given for the melting point. By using a shorter thermometer with a scale not exceeding 50°, which can be wholly surrounded by the vapour, the correction becomes unnecessary.
In general, when the boiling point " under ordinary pressure " is recorded, it is understood to mean at 760 mm. of mercury. If the barometric column docs not indicate this amount during the distillation, a second correction is necessi- tated (B. 20, 709 ; Landolt-Boernstein, Tabellen, 3rd edition, 1905, p. 177). To avoid this it is advisable to adjust the pressure in the apparatus to the normal, for which purpose the regulators of Bunte (A. 168, 139) and Lothar Meyer (A. 165, 303) are suitable.
Distillation under Reduced Pressure* — Attention has already been directed to the great variation in boiling points witn variation in temperature. Many carbon derivatives whose decomposition temperature, at the ordinary pressure, is lower than that of their boiling points, can be boiled under reduced pressure at temperatures below the point at which they break down. Distillation under reduced pressure is often the only means of purifying liquids which decompose when boiled at the ordinary pressure, and which cannot be crystallized. This
method is of primary import- ance in scientific research in the laboratory, and is rapidly being introduced into technical opera- tions with much
success.
Distillation under reduced
pressure of easily solidifying bodies has been facilitated by the introduction of flasks to which receivers are fused or ground in (Fig. 9). The thermometer is introduced into a thin- walled tube drawn out into a capillary, the other end of which is closed FIG. 9. with rubber tubing and a clip. A slow current of gas is drawn
through the liquid during distillation, and in this way bumping is avoided. The distillation flask is best heated in a bath. Usually the pressure is lowered by means of a water pump, but when it is desired to distil at pressures lying near the absolute vacuum, it will be found advantageous to use a Sprengel mercury pump, which is set into motion, according to Babo's method, by means of a water suction pump ; compare Kahlbaum (B. 27, 1386) ; F. Krafft and H. Weilandt (B. 29, 1316) ; Precht (B. 29, 1143).
A still simpler method of attaining very low pressures consists in the employ- ment of liquid air. A vessel, containing very finely divided pure blood-charcoal, or cocoanut charcoal, is interposed between the apparatus illustrated in Fig. 9 and the air pump. On cooling it with liquid air the small amount of gas left in the apparatus condenses in the charcoal, and the pressure falls to a fraction
* Compare Anschiitz and Reitter, Die Destination unter vermindertem Druck im Laboratorium, and ed., 1895, Bonn. The tables in this book record the boiling points of over 400 inorganic and organic substances under reduced pressure. George W. Kahlbaum, Siedetemperatur und Druck, Leipzig, 1885. Dampfspannkraftsmessungen, Basel, 1893. Meyer Wildermann, Die Siedetemper- aturen der Korper sind eine Funktion ihrer chemischen Natur (B. 23, 1254, 1468). W. Nernst and A. Hesse, Siede- und Schmelzpunkte, Braunschweig, 1893.
VOL. I. £
5o ORGANIC CHEMISTRY
of a millimetre. If the apparatus is filled beforehand with CO,, the charcoal can be omitted (B. 38, 4149)-
For distillation under any pressure, the apparatus of Staedel (A. 195, 218 ; B. 13, 839), »nd Schumann (B. 18, 2085), may be used. For mercury thermometers registering temperatures to 550°, see B. 26, 1815 ; to 700°, B. 27, 470.
Fractional Distillation. — Liquids having different boiling points can be separated from mixtures by fractional distillation — an operation that is per- formed in almost every distillation. Portions boiling between definite tempera- ture intervals (from 1-10°, etc.) are collected separately and subjected to repeated distillation, those portions boiling alike being united. To attain a more rapid separation of the rising vapours, these should be passed through a vertical tube, in which the vapours of the higher boiling compound condense and flow back, as in the apparatus employed in the rectification of spirit or benzene. To this end there is placed on the boiling flask a so-called fractionating column of Wiiriz. Excellent modifications of this have been described by Linncmann, Le Bel, Hempel, Young, and others. For the action of these "heads," see A. 224, 259 ; B. 18, R. loi, and A. 247, 3 ; B. 28, R. 352, 938 ; 29, R. 187. The action of these fractionating columns is increased if enclosed by a highly evacuated jacket (B. 39, 893, footnote).
Relation of Boiling Point to Constitution.* — (i) Generally the boiling point of members of a homologous series rises with the increasing number of carbon atoms. (2) Among isomeric compounds of equal carbon content, that possessing the more normal structure boils at a higher temperature. The addition of the methyl groups depresses the boiling point. It is noteworthy that the lowest boiling isomers possess the greatest specific volume (B. 15, 2571) . (3) Unsaturated compounds boil at a higher temperature than those which are saturated. (4) The substitution of a hydrogen atom by a hydroxyl group raises the boiling point about 100°.
The connection existing between the boiling points and chemical constitution of the compounds will be discussed later in the several homologous groups.
5. SOLUBILITY
The hydrocarbons and their halogen substitution products are either insoluble, or only very slightly soluble, in water. They dissolve, however, very readily in alcohol and in ether, in which most other carbon derivatives are also soluble.
Ether, but slightly miscible with water, is employed to extract many substances from their aqueous solutions, separating funnels being used for this purpose.
The more oxygen a compound contains, the more readily soluble is it in water ; especially is this true when several of the oxygen atoms are combined with hydrogen, i.e. when hydroxyl groups are present in the organic compound.
The first members of homologous series of alcohols, aldehydes, ketones, and acids are soluble in water, but as the carbon content increases, the hydrocarbon character, in relation to solubility, becomes more and more evident, and the compounds become more and more insoluble in water.
In addition to water, alcohol, and ether, other solvents are employed as solvents, such as carbon disulphide, chloroform, carbon tetrachloride, methylal, acetone, glacial acetic acid, ethyl acetate, benzene, toluene, xylene, aniline, nitrobenzene, phenol, etc. Light petroleum spirit, derived from American petroleum, is especially valuable ; it is composed of lower paraffins, and is often used to separate compounds from solvents with which it is miscible, because very many organic substances are insoluble or dissolve with difficulty in it.
The solubility of a compound is dependent upon the temperature, and is constant for a definite temperature. This means is frequently employed for purposes of identification.
* On the connection between the boiling point and the chemical constitution of a substance, as known at present, see Graham-Otto, Lehrbuch der Chemie, Vol. I. part 3, p. 535 (1898) ; also Menschuthin, C. 1897, II. 1067.
OPTICAL PROPERTIES 51
For the regularities among the solubilities of isomeric carbon derivatives, consult Carnelley, Phil. Mag. [6] 13, 180 ; Carnelley and Thomson, J. Ch. S. 53, 8or.
For apparatus suitable for determining solubility, see V. Meyer, B. 8, 998, and Kohler, Z. anal. Ch. 18, 239 ; B. 30, 1752.
6. OPTICAL PROPERTIES
Colour. — Most organic compounds are colourless, many are coloured ; e.g. iodoform is yellow, whilst carbon tetraiodide is dark red. The presence of certain atomic groups is connected with definite colours, particularly in the case of the aromatic derivatives. The nitro- bodies, for example, are more or less yellow, whilst the azo- derivatives vary from orange to red, etc. The colour of the solution of coloured substances depends to a large extent on the nature of the solvent (B. 27, R. 20 ; 39, 4153).
Dye-stuffs. — Many coloured compounds, belonging almost ex- clusively to the aromatic series, possess the property of dyeing vegetable or animal fibres, either directly or through the agency of mordants.
According to O. N. Witt, an aromatic substance behaves as a dye when it includes a chromophoric group, e.g. NO2, N3, etc., as well as an auxochrome group, such as an OH or amino -group, in its composition. The latter occupy the ortho- or para- position to the chromophor. A substance containing a chromophoric group alone is called a chromogen (B. 9, 522 ; 35, 4225 ; 36, 3008).
Fluorescence. — This property, like that of colour, results from the presence in the molecule of certain fluorophoric groups (R. Meyer, B. 31, 510 ; C. 1900, II. 308; Chem. Ztg. 29, 1027).
Refraction. — The carbon compounds (like all transparent sub- stances) possess the power of refracting light to a varying degree.
The coefficient of refraction or refractive index (n) for homogeneous light passing from medium i into medium 2, represents the ratio of the velocities of propa- gation vl and vz in both media ; n=-L. For single refracting media, in which
similar optical behaviour is observed in all directions (a condition which is seldom found in crystals) n is independent of the direction of the incident light, so that
if * and r are the incident and refractive angles w=^=s!— , a constant number
v % sin r for light of a definite wave-length.
Specific Refractive Power. — The refractive index (n) varies with the tempera- ture, consequently also with the specific gravity of the liquid.
Their relation to each other is approximately expressed by the equation ;
~ = const, or n ~l . i = const.* d n*+z d
(Gladstone's formula). (Lorenz and Lorentz's formula), n-formula. nMormula.
where d is the sp. gr. of the liquid, determined at the same temperature as the refractive index. The constant remains practically unchanged for any tempera- ture.
Molecular Refractive Power or Molecular Refraction is the specific refractive
* See Graham-Otto, Lehrbuch der Chemie, Vol. I. part 3, p. 567, 1898.
52 ORGANIC CHEMISTRY
power of a substance multiplied by its molecular weight. It is represented by M or fiH, according to whether Gladstone's or the w2 formula is adopted :
M =
n — x
P-
It is immaterial which of the two formulae is employed in the examination of stoichiometrical questions, so long as fluid substances are referred to. In a comparison of liquids with their vapours the n9 formula only can be used, and it is also to be preferred when dealing with aromatic substances.
The molecular refraction of a liquid carbon compound is equal to the sum of the atomic refractions r, r' , /' :
M = ar + W + cr",
in which a, b, c, represent the number of elementary atoms in the compound. The atomic refractions of the elements are deduced from the molecular refractions of the compounds obtained empirically, in the same manner as the atomic volumes are obtained from the mole- cular volumes. Whilst it was formerly assumed that but one atomic refraction existed for each element in its compounds, later researches have proved that only the univalent elements have a constant atomic refraction, and that of the polyvalent elements, e.g. oxygen, sulphur, carbon, is influenced by their manner of union.
This is seen in the rise in the molecular refraction by a constant quantity, amounting to 2-4 for the ^-formula, and 1-84 in the case of the w2-formula, for each double bond of a carbon atom. A treble bond possesses the nz value of approximately 2-2.
The refraction is determined either for the yellow sodium line (the D line in the solar spectrum), or for the red hydrogen line Ha (C in the solar spectrum). These values are affected by the disturbing influence of " dispersion," and a refractive index free from this factor has not yet been developed (see Dielectric Constant, p. 53). The molecular refraction ascertained by means of the above formula from the observed values of the refraction and density, can be compared with that calculated by the addition of the particular atomic refractions, as given in the accompanying table.
|
— |
Gladstone's formula. |
Lorenz's formula. |
|||
|
fa |
fD |
'« |
'D |
||
|
Carbon (single bond) .... Hydrogen |
C' H O' o< Cl Br I |
5-00 I-30 I 2-80 3HO 9-79 15-34 24-87 2-4 |
4-71 1-47 2-65 { 3'33 10-05 15-34 25-01 2-64 |
2-365 1-103 I-506 1-655 2-328 6-014 8-863 13-808 1-836 2-22 |
2-501 1-051 1-521 1-683 2-287 5-998 8-927 14-12 1-71 |
|
Oxygen (in hydroxyl) .... Oxygen (in ethers) Oxygen (carbonyl) Chlorine . ..'... |
|||||
|
Ethylene linkage . . . |
|||||
|
Acetylene linkage |
The atomic refraction of nitrogen in its various combinations has been minutely investigated by Brtihl, but final results have not, as yet, been attained.
It is, therefore, obvious that important data relating to the manner of union of the atoms in the molecule of a carbon compound can be
OPTICAL PROPERTIES 53
obtained from the molecular refractions. When the observed mole- cular refraction is in excess of the calculated value, the presence of a double or treble bond is indicated. Thus the greater molecular refraction (by 3 X 178 = 5-34 units) of the benzene bodies, confirms the view, previously deduced from chemical facts, that there are present in the benzene nucleus three doubly-linked carbon atoms. Among the terpenes the change from a ring formation to an open chain with a double bond can be followed (B. 20, 2288 ; 22, 2736 ; 23, 855 ; 24, 656, 2450 ; 25, 2638). In many cases among the sub- stances referred to by Laar as being tautomeric, it has been possible to ascertain whether they exist in the enol- or keto- form (B. 25, 366, 3078 ; 38, 1868). However, the regularities noted above only hold good for bodies with slight dispersive power, such as the fatty bodies. In the case of substances possessing a greater dispersive power than cinnamyl alcohol, the molecular refraction is valueless for the deter- mination of chemical structure (B. 19, 2746 ; 24, 1823).
On the employment, for the elucidation of stoichiometrical problems, of the molecular dispersion of bodies, i.e. the difference between the refractions measured with blue and red hydrogen lines, see Bruhl (Z. phys. Ch. 7, 140).
The refraction stere of /. Traube is the quotient obtained by the division of the molecular refraction by the number of atomic valencies. Within certain limits it approximates to a constant (0-787) which is of special significance in the theory of valency (B. 40, 130, 723).
The Abbi total refractometer, and Pulfrich's total reflectometer are much more convenient than the spectrometer for rapid and sufficiently accurate working (Z. phys. Ch. 18, 294 '. B. 24, 286)."
Dielectric Constant. — The electrostatic force by which two electrified bodies affect one another varies with the nature of the insulating " dielectric medium " which separates them. Taking air as unity, the measurement made with another substance under similar circumstances gives the dielectric constant of that medium. This value, usually indicated by K, has been taken for a large number of carbon compounds ; * for example : —
K K
Gases and Vapours, about . i-o Fatty Acids, about . 2-6-7-0
Liquid Hydrocarbons . 2-0-2-5 Fatty Acid Esters . 5-9
Carbon Bisulphide . . .2-6 Fatty Alcohols . . 16-35
Ethyl Ether 4-5 Water 80
The electromagnetic theory of light is based on the fundamental principle that light and electromagnetic waves are of the same nature, differing from one another only in length. The refractive index, for an infinitely long wave can be closely connected to the dielectric constant, by the relation v//T= no.. The determination of the dielectric constant thus supplies directly a value for the refractive index free from dispersion, analogous to the Lorenz formula (p. 51).
P . * ~ I . 4- = const.
K + 2 d
The values obtained in investigations so far carried out f have not led in general to a good correspondence with those derived by optical methods, whilst the optical molecular refraction measurements show an additive character (at least for compounds of similar constitution), the values obtained by electrical methods are influenced by insignificant differences in constitution of each sub- stance. In this case there is no possibility of calculating " atomic refractions,"
* On the method of measurement for chemical purposes, see Nernst (Z. phys. Ch. 14, 622 ; 24, 21) ; Wied (A. 57, 215 ; 60, 600) ; Drude (Z. phys. Ch. 23, 267).
f Landolt and Jahn (Z. phys. Ch. 10, 289). See also Graham-Otto, Lehrb. der Chemie, I. part 3, p. 650, 1888.
54 ORGANIC CHEMISTRY
but rather to trace and disclose differences in constitution by electrical means, for which purpose it is of great assistance. Under certain circumstances the attendant phenomenon of anomalous electrical absorption is to be observed, i.e. the partial change of electrical into heat energy. Almost all the non-conducting carbon compounds which give rise to this absorption contain the hydroxyl group. On this observation is based a method of detecting and demonstrating the mutual change of keto- and enol- forms (Drude, B. 30, 94° ; z- Phys. Ch. 23, 308, 318). Further progress in this investigation will doubtless yield important results.
The vapours of many groups of aliphatic and specially aromatic bodies absorb Tesla currents at ordinary pressure and change them into light waves. Such substances, for example, are the primary aromatic amines, and the simple aliphatic aldehydes and ketones. In the latter case the keto- group seems to be the vehicle of the luminescence, at any rate neither the vapours of paraldehyde nor of acetaldehyde become illuminated (H. Kauffmann, B. 35, 473).
Optical Rotatory Power,* Rotation of the Plane of Polarization by Liquid or Dissolved Carbon Compounds. — Biot, in 1815, observed that many naturally occurring bodies such as the sugars, the terpenes, and camphors, were capable of rotating the plane of polarized light. He also showed, in 1817, how the vapours of turpentine also deviated the plane of polarization, and concluded that this power was a property of chemical molecules. Such bodies are termed optically active carbon compounds.
Specific Rotatory Power [a]. — The angle of rotation o is proportional to the length / of the rotating column (usually expressed in decimetres) ; hence the ex- pression - is a constant quantity. To compare substances of different density, in
which very unequal masses may be contained in this column, they must be referred to a like density, and hence the rotation must be divided by the sp. gr. of the substance at a definite temperature. The expression
C«]D or [a],= j-d
is called the specific rotatory power and is designated by [o]D or [a]jt according as the rotation is referred to the yellow sodium line D or the " transitional colour " j. For solid, active substances, in an indifferent solvent, the equation employed is
1000
W-?EP
where p represents the quantity of substance in 100 parts by weight of the, solution, and d represents the specific gravity of the latter.
This specific rotatory power is constant for every substance at a definite temperature ; it varies, however, with the latter, and, in the case of solutions, with the nature and quantity of the solvent. So much is this the case, that under various conditions the angle of rotation for one and the same substance can become zero or even change in sign. Therefore, in the statement of the specific rotatory power of dissolved substances the temperature and percentage strength of the solution are always given.
In many cases the addition of substances such as salts, etc., causes a change in the rotation. Such active bodies, including tartaric acid, malic acid, mandelic acid, and others, which contain an alcoholic hydroxyl group, are powerfully influenced by the addition of alkali borates, molybdates, tungstates, and uranates. The phenomenon depends apparently on the formation of complex combinations (B. 38, 3874, etc.), and can sometimes be used to increase the rotation of active substances, of which the rotatory power would otherwise be too small to be measured alone, either on account of specific value being insignificant or because the solution employed is too weak. (See Landolt, previous reference, footnote, p. 220 ; Walden (B. 30, 2889).)
* Landolt, Das optische Drehungsverm gen organischer Substanzen und die practische Andwendung derselben, 2nd edition, Braunschweig, 1898. Waldtn, Ueher das Drehungsvermogen optisch aktiver Korpcr, B. 38, 345.
OPTICAL PROPERTIES 55
Molecular Rotatory Power is the product of the specific rotatory power [a] and the molecular weight P. As these values are usually high, the molecular weight is divided by 100.
M-Lifel.
100
The most suitable apparatus for measuring rotation are described in the above- mentioned work of Landolt (p. 54, footnote).
In 1848 Pasteur demonstrated that in optically active substances, such as tartaric acid and its salts, the rotatory power is intimately connected with the crystalline form, and is usually connected with the presence of hemihedral faces. In the discussion of the stereochemical or spacial theories, reference was made to the fact that Pasteur considered the asymmetric structure of the molecules of optically active carbon compounds to be the cause of their remarkable action upon polarized light.
According to the theory of van 't Hoff and Le Bel, the activity of the carbon compounds is dependent upon the presence of asymmetric carbon atoms or on the asymmetric arrangement of atoms attached to a carbon skeleton in space (p. 30).
So far as they have been investigated, all optically active carbon compounds contain one or more asymmetric carbon atoms. However, there are many compounds containing such atoms, which, when they exist as liquids, or when in solution, have no effect upon polarized light. This is true when two molecules of opposite but equal rotatory power unite to form a molecule of a physical, polymeric compound, e.g. inactive lactic acid, inactive malic acid, inactive asparagihe, inactive aspartic acid, racemic acid, etc. ; also, when the half of a molecule neutralizes the rotation produced by the other half, as in mesotartaric acid.
It has also been shown that in the conversion of optically active bodies into their derivatives the activity continues so long as the latter contain asymmetric carbon atoms ; when the asymmetry disappears, the derivatives become inactive. The two active tartaric acids yield two active malic acids ; active asparagine yields active aspartic acid, active malic acid, etc., whilst the symmetrical succinic acid that is obtained by further reduction is inactive.
If various groups, each containing an asymmetric carbon 'atom, be introduced into a molecule, the final rotation will be the algebraic sum of the rotations of the single groups: see especially, Guye (C.r. 119, 953 ; 120, 632 ; 121, 827 ; 122, 932) ; and W olden (Z. phys. Ch. 17, 721).
By changing or substituting a single group or element, connected with an asymmetric C atom, the rotatory power is often very considerably influenced ; as, for instance, by the production of an ethylenic linkage or by ring- formation (C. 1903, II. 116 ; 1905, II. 31 ; A. 327, 157) ; or when alkyl groups are intrp,- duced into NH or OH groups (B. 34, 2420 ; C. 1905, II. 455). In the case of malic acid the optical antipodes can be transformed into onfc another by a con- tinuous series of changes ; 1-malic ester, with PC15, gives d-chlorosuccinic ester, the acid of which with silver oxide yields d-malic acid. Conversely, d-malic ester, with PC18, gives 1-chlorosuccinic ester, of which the acid can be converted into 1-malic acid. Similarly, 1-bromo- or 1-chlorosuccinic acid, acted on by ammonia in methyl alcohol solution, yields d-aminosuccinic acid, which is changed into d-malic acid by barium hydroxide. Finally, the halogen substitution products of the active succinic acids, when acted on by potassium hydroxide instead of silver oxide, have their halogens replaced by hydroxyl to form the hydroxy-acids, possessing not the same but the opposite direction of rotation (Walden, B. 30, 3146). Similar " reversed rotations " can be observed among the simple amino- acids, such as alanine and leucine (q.v.) (B. 39, 2895 ; 40, 1051).
Asymmetric compounds prepared in the laboratory from inactive substances are inactive. This results from the simultaneous formation in equal quantities of the two optical antipodes which manifest a tendency to combine to form the inactive, physically polymeric molecules. Asymmetric syntheses, i.e. the pre- paration of one active body from an inactive one without the intermediate formation of a racemic body, can, however, sometimes be effected, by combining the inactive compound with an active one and then carrying out the change which will produce the active substance sought : methyl ethyl malonic acid combines with the active alkaloid brucine forming an acid salt. On heating, COf escapes, and when the resulting brucine methyl acetyl acetate is decomposed with
56 ORGANIC CHEMISTRY
hydrochloric acid, optically active methyl ethyl acetic acid is obtained (B. 37, 1368; C. 1906, I. 1613; II. 53).
Racemic Bodies. — The typical substance, racemic acid, has given its name to all similar inactive mixtures of the two optical antipodes. The racemic sub- stance differs from its components also in that it forms crystals which do not give rise to enantiomorphic modifications. The density of the racemic body is, as a rule, greater and its solubility lets than the corresponding active substances, but not always ; similarly there is no general rule for the relative position of the melting point.
When the crystalline form of an inactive substance cannot be observed with accuracy, as often happens, and when at the same time, the melting point lies lower than that of either of the optically active components, then doubt may arise whether it is a true racemate or a mixture of equal quantities of the optical antipodes. A variety of tests can be applied. The melting point may be taken after a small quantity of one of the active components has been added to the inactive substance. The composition may be determined, as well as the optical behaviour, of a concentrated solution of the inactive body as compared with that of a mixture of the inactive and one active substance. If the addition of the active body causes a lowering of the melting point of the inactive substance, a change in the concentration and in the optical activity of the saturated solution, then the substance is a racemic one ; if, on the other hand, the melting point rises, and the concentration and inactivity of the solution are unaltered, then the inactive body is a mixture.
The formation of a racemic substance is dependent on the temperature. Above or below its transformation temperature the body may be a racemic body or an enantiomorphic mixture. The results of the above experiments hold good, then, only for the particular temperature at which they are carried out, and a series of experiments over a wide range of temperature is necessary to obtain a complete insight into the matter.
These practical tests are, in part, the direct result of the considerations on heterogeneous equilibrium as put forward in Gibb's phase rule (van 't Hoff, B. 31, 528 ; Ladenburg, B. 32, 1822 ; Roozeboom, Z. phys. Ch. 28, 494, etc. Also B. 33, 1082).
Pseudo-racemic mixed crystals, although inactive, possess the form of the active modifications, without, however, the hemihedric faces (J. Ch. S. 71, 889 ; 75, 42).
Resolution of Inactive Carbon Compounds into their Optically Active Com- ponents.— The synthesis of optically active^ carbon compounds is easily realized by direct methods, because it is possible to separate the dextro- and laevo- rotatory components in an inactive molecule. The following methods, i, 2, and 5, were employed by Pasteur (1848) in his study of the racemates and racemic acid. This classic investigation supplies the firm experimental basis for the theory of stereochemistry or the space chemistry of carbon (p. 29).
Method i, based upon resolution by crystallization. — The substance itself, or its derivatives with optically inactive compounds, is crystallized at varying temperatures and from various solvents. In the case under consideration it is possible to separate two substances showing enantiomorphous hemihedrism by actually picking out those crystals exhibiting the particular forms. Thus, from a solution of sodium ammonium racemate below 28° hemihedral crystals of sodium ammonium dextro- and laevo-tartrates can be obtained (B. 19, 2148).
Method 2, dependent upon the formation of compounds with optically active- substances. — Pasteur succeeded in separating d- and 1-tartaric acids through their quinicine and cinchonine salts. This was because these, being no longer enantiomorphous, were distinguished by their varying solubility, and so could be very easily separated from each other.
Ladenburg first used the latter method to resolve inactive bases by forming salts of the latter with an active acid. It was thus that he decomposed synthetic inactive coniine (a-n-propyl piperidine) by means of dextro tartaric acid into its active components, and completed the synthesis of the first optically active vegetable alkaloid — con:ine — wmch occurs in hemlock (q.v.).
The resolution of racemic substances does not always immediately result from the combination with active bodies and the subsequent precipitation of the more insoluble of the new compounds. Under certain conditions the racemic
OPTICAL PROPERTIES
57
body unites, as such, with the added active body, forming a se mi-racemio compound (such as strychnine racemate), which can only be decomposed into compounds of its active components at a particular temperature (Ladenburg, B. 31, 1969 ; 32, 50) .
Method 3, based on the formation of esters or amides between racemic arid optically active substances. — Racemic mandelic acid can be partially turned into the 1-menthol ester, whereby the residue consists of an excess of 1-mandelic acid. If 1-quinic acid be heated with rac. a-phenyl ethylamine, the dextro-rotatory acid, which does not take part in the amide formations, remains behind (B. 38, 801).
Method 4. — Enzymes, such as maltase or emulsin, decompose racemic glucosides (E. Fischer, B. 28, 1429).
Method 5. — On introducing some suitable fungus such as Penicillium glaucum into an aqueous solution of an inactive mixture, capable of resolution, one modi- fication of the mixture will be destroyed during the life-process of the fungus ; thus racemic acid yields l-tartaric acid, inactive amyl alcohol yields d-amyl alcohol, methyl propyl carbinol yields \-methyl propyl carbinol, propylene glycol yields \~propylene glycol, etc.
One fungus may leave an optical modification untouched which another may destroy.
Penicillium glaucum or Bacterium termo will leave d-mandelic acid from the synthetic inactive racemic acid, whilst Saccharomycesellipsoideus or Schizomycetes leave the 1-acid untouched. For the literature of the resolution of racemic compounds, see Landolt, Optisches Drehungsvermogen, etc., 2nd edition, p. 86, 1888.
Carbon compounds, in which an asymmetric carbon atom is not pr«sent, could not be decomposed by these methods (A. 239, 164 ; B. 18, 1394).
Conversion of Optically Active Substances into their Optically Inactive Modifi- cations.— Whilst soluble salts of optically inactive, resolvable carbon compounds may be resolved by crystallization under proper conditions of temperature, many others reunite to form a salt of the inactive body, especially if the latter dissolves with difficulty. Solutions of laevo- and dextro-tartrate of calcium when mixed yield a precipitate of calcium tartrate, which dissolves with difficulty. The free, optically active modifications unite, as a rule, very easily when mixed in solu- tion, to form the inactive decomposable modification, e.g. laevo- and dextro- tartaric acid yield racemic acid. The esters of these acids behave in a similar manner : laevo- and dextro-tartaric methyl esters unite directly and in solution to form racemic methyl ester (B. 18, 1397). Also, in energetic reactions, or when heated, the active varieties rapidly pass into the inactive forms, e.g. dextro- tartaric at 175° yields racemic acid, and at 165° mesotartaric acid. At 180° dextro- and laevo-mandelic acids pass into inactive mandelic acid. Some optically active halogen substitution products of carboxylic acids undergo auto-racemation, even at ordinary temperatures (B. 31, 1416).
A corresponding behaviour is observed in the decomposition of albumins, when heated with barium hydroxide, into inactive leucine, tyrosine, and glutamine, whilst at a lower temperature hydrochloric acid produces the active modifications (B. 18, 388). For an experimental explanation of the transformation of optically active substances into their inactive modifications, compare A. Werner in R. Meyer's Jahrbuch der Chemie 1, 130.
Magnetic Rotatory Power.* — Faraday, in 1846, discovered that trans- parent, isotropic, optically inactive bodies were capable of rotating the plane of polarized light when a column of the substance was brought into the magnetic field, as, for example, when it was surrounded by an electric current. The power of rotation only continued as long as these influences were active, and was reversed when the position of the magnetic poles were reversed ; this distinguished magnetic rotatory power from the rotatory power of optically active carbon compounds.
Specific magnetic rotatory power is the degree of rotation that the plane of polarization of a ray of light undergoes when it passes through a layer of liquid of definite thickness, exposed to the influence of a magnet. The unit of com- parison is the rotation produced by a layer of water of the same temperature and thickness when exposed to the same magnetic field.
f Graham-Otto, Lehrbuch der Chemie, Vol. I. part 3, p. 793, 1898.
58 ORGANIC CHEMISTRY
Molecular Magnetic Rotatory Power. — This is the degree of rotation produced by columns of liquids chosen of such a length that similar cross-sections will each contain a molecular weight of the substance. The unit in this case can also be the molecular rotatory power of water.
W. H. Perkin, Sr., has investigated minutely the connection between the magnetic rotatory power and the constitution of carbon derivatives. Numerical relations between the increase of rotation and change of composition have been established for many groups of aliphatic and aromatic compounds (C. 1900, I. 797 '• I9°2, I. 621). Deviations from the theoretical values are encountered particularly in the reactive benzene substitution compounds (see Table, T. pr. Ch. [2] 67, 334).
7. ELECTRIC CONDUCTIVITY
Substances which are capable of conducting electricity arrange themselves into two groups : conductors of the first class, or those which conduct electricity without undergoing any change, and conductors of the second class, known as electrolytes, in which con- duction is only possible through the agency of the ions in which the solutes separate when dissolved. The greater the conductivity of a substance the less is the resistance to the passage of the current ; in other words, the resistance is inversely proportional to the conduc- tivity. The unit of measurement of resistance is the ohm — the resis- tance of a column of mercury ro6 metres long, and I mm. in cross section, at o° C.
Ostwald's investigations have demonstrated that the conductivity of electrolytes is intimately related to chemical affinity, and forms a direct measure of the chemical affinity of acids and bases. Therefore, the determination of the conductivity of electrolytes (in aqueous solution), to which all organic acids and their salts belong, is of great interest and importance for all carbon derivatives.
Kohlrausch (Wied, A. 6, i) has suggested a very simple and accurate means of determining the conductivity of electrolytes, which has been extensively applied by Ostwald (J. pr. Ch. 32, 300, and 33, 352 ; Z. phys. Ch. 2, 561). (See also C. 1900, I. 577.) It is dependent on the application of alternating currents, produced by an induction coil, so that the disturbing influence of galvanic polarization is avoided.
The conductivity of electrolytes is not referred to the percentage content of their aqueous solutions, but (as the conductivity is deter- mined by the equivalent ions) to solutions containing a gram-mole- cule, or a gram-equivalent of substance in one litre. This value is the molecular (or equivalent) conductivity of the substance (Z. phys. Ch. 2, 567).
F. Kohlrausch and Holborn, in their book, "Das Leitungsvermogen der Elektrolyte," refer the conductivity of a solution to a unit consisting of a column i cm. long, and i cm.1 in section which has a resistance of i ohm. In this case the conductivity becomes 10,600 times as great as the above. Also, they employ the gram-equivalent in place of the gram-molecule, and the cubic centimetre in place of the litre.
The strong acids have the greatest molecular conductivity, and are followed by the fixed alkalies and alkali salts. Most organic acids, on the contrary (e.g. acetic acid), are poor conductors in a free condition, whilst their alkali salts approach those of the strong acids in conductivity. The molecular conductivity increases by about 2 per cent, per degree rise of temperature. It also increases with increasing dilution, and in the case of the poor conductors it is far more rapid
ELECTRIC CONDUCTIVITY 59
than with the good conductors ; in both instances it ultimately approaches a maximum (limiting) value. With good conductors this is attained at a dilution of about 1000 litres to the gram- molecule ; whilst with those poor in conducting power it is only reached when the dilution is indefinitely large. In fact, in such cases the conductivity is practically indeterminable.
An interesting observation in connection with the alkali salts of all acids is the variable increase of the molecular conductivity with increasing dilution. This is true both in the case of the strong and the weak acids (most organic acids belong to the latter class), and it varies according to their basicity. With sodium salts of monobasic acids, this increase equals from 10-13 units, by dilution of 32-1024 litres for the equivalent of substance ; for the salts of dibasic acids from 20-25 units, for those of the tribasic 28-31, for those of the tetrabasic about 40, and those of the pentabasic about 50 units.
Thus it may be seen that the increase in conductivity of acids, in the form of their sodium salts, offers a means of determining the basicity and, consequently, the molecular magnitude of acids (Ostwald, Z. phys. Ch. 1, 74, 97 ; 2, 901 ; Walden, ibid., 1, 530 ; 2, 49).
If a certain quantity of an acid be neutralized with N/32 sodium hydroxide solution, and the conductivity of the neutral salt be measured before and after dilution to 32 tinjes its volume, the difference of the conductivities divided by 10 gives the basicity of the acid.
Molecular conductivity has acquired still greater importance by its application to the measurement of the dissociation of the electrolytes ; it is at the same time the measure of the reactivity or chemical affinity, first, of acids, then bases, and, finally, of salts.
Arrhenius's electrolytic* dissociation theory maintains that in aqueous solution the electrolytes are more or less separated into their ions ; this would give a simple explanation for the variations of solu- tions from the general laws of osmotic pressure, the depression of the freezing point, etc. (see p. 16). The dissociation is also manifest in the molecular conductivity, for the latter is directly proportional to the degree of dissociation, the number of free ions and the speed of migration of the free ions.
Molecular conductivity increases with dilution and dissociation. When the latter is complete, it attains its maximum (/HQO). The degree of dissociation (m) (or the fraction of the electrolyte split up into ions) for any dilution is found from the ratio of the molecular conductivity at this dilution (/i) to the maximum conductivity (for an indefinite dilution) :
The latter (fi^ ) cannot be directly measured in the case of free organic acids, because most of them are poor conductors. But it can be obtained from the molecular conductivity of their sodium salts, by deducting from their maximum values the speed of migration of the sodium-ions (49*2), and adding those of the hydrogen-ions (352).
Since the molecular conductivity depends upon the dissociation of the electrolytes into their ions, the effect of dilution must follow the same laws as those prevailing in the dissociation of gases. This influence of dilution or volume (v) upon the molecular conductivity, or the degree of dissociation (m) is, there- fore, expressed in the equation :
which represents the law of dilution advanced by Ostwald (Z. phys. Ch. 2, 36, 270).
6o ORGANIC CHEMISTRY
This law has beeri fully confirmed by the perfect agreement of the calculated and observed values (van 't Hoff, Z. phys. Ch. 2, 777). In the case of strong electro- lytes, such as strong acids and bases, and most salts, the equation of Rudolphi
is preferable to that of Ostwald, even though it is empirical : — m' _ K
y(i — w)2
The value, K, is the same at all dilutions for every monobasic acid ; hence it is a characteristic value for each acid, and is the measure of its chemical affinity. The determination of these chemical affinity-constants by Ostwald for more than 240 acids, has proved that they are closely related to the structure and constitution of the bodies (Z. phys. Ch. 3, 170, 241, 369). Literature : see W olden (Z. phys. Ch. 8, 833). Affinity values of stereoisomeric compounds : Hantzsch and Miolatti (B. 25, R. 844).
Addendum : Determination of affinity-coefficients : Conrad, Hecht, and Bruckner (Z. phys. Ch. 3, 450 ; 4, 273, 631 ; 5, 289). Lellmann (B. 22, 2101 ; A. 260, 269 ; 263, 286 ; 270, 204, 208 ; 274, 121, 141, 156). Nernst (R. Meyer's Jahrbuch 2, 31).
HEAT OF COMBUSTION OF CARBON COMPOUNDS*
" The quantity of heat evolved in any chemical change is a measure of the total work, both physical and chemical, expended." The determination of the quantity of heat developed in complete combus- tion is alone adapted for the determination of the energy content of carbon compounds.
The heat of combustion of a carbon compound by the method of Berthelot is determined by combustion with oxygen at a pressure of 25 atmospheres in a calorimetric bomb, lined internally with platinum or enamel. Ignition is effected by means of an electric spark, or by the incandescent products of combustion formed by a thin iron wire heated electrically.
The method is so accurate that it can be employed for the detection of quite small quantities of impurity in an organic compound, the heat of combustion of which is known