1. Chemistry 125: Lecture 23 Valence Theory and Constitutional Structure (1858) Youthful chemists Couper and Kekulé replaced radical and type theories with a new approach involving atomic valence and molecular structure, and based on the tetravalence and self-linking of carbon. Valence structures offered the first explanation for isomerism, and led to the invention of nomenclature, notation, and molecular models closely related to those in use today. Synchronize when the speaker finishes saying “We started with Lavoisier.” Synchrony can be adjusted by using the pause(||) and run(>) controls. For copyright notice see final page of this file

2. Genealogy Top

3. Archibald Scott Couper (1831-1892) 1851-1856 Dilettante Edinburgh/Berlin Classics, Metaphysics Logic, Moral Philosophy, Concerts 1855 Chemist Berlin 1856 Chemist Paris (Wurtz) - Salicylic Acid Fired 1859-1892 Mental Invalid 1858

4. Kirkintilloch (near Glasgow)

5. Endrick Water where Couper (age 28) suffered sunstroke while fishing during recuperation

6. “Laurel Bank” Kirkintilloch where his mother cared for him during his final 33 years

7. Theory and Types

8. p. 106 - on the French Type Theory Should the principle which is therein adopted be applied to the common events of life, it will be found that it is simply absurd. Suppose that some one were to systematize the formation of letters into words that formed the contents of a book.

9. p. 106 - on the French Type Theory Were he to begin by saying that he had discovered a certain word which would serve as a type, and from which by substitution and double decomposition all the others are to be derived, - that he by this means not only could form new words, but new books, and books almost an infinitum, - he would state certainly an empirical truth.

10. p. 106 - on the French Type Theory At the same time, however, his method would, judged by the light of common sense, be an absurdity. But a principle which common sense brands with absurdity, is philosophically false and a scientific blunder.

11. p. 107 - on the German Radical Theory I can only remark that it is not merely an unprofitable figure of language, but is injurious to science, inasmuch as it tends to arrest scientific inquiry by adopting the notion that these quasi elements contain some unknown and ultimate power which it is impossible to explain.

12. p. 107 - on the German Radical Theory It stifles inquiry at the very point where an explanation is demanded, by putting the seal of elements, of ultimate powers, on bodies which are known to be anything but this. but stated undiplomatically, especially for a newcomer. Absolutely true

13. Science demands the strict adherence to a principle in direct contradiction to this view. That first principle, without which research cannot advance a step, dare not be ignored; namely, that a whole is simply a derivative of its parts. p. 108 - Look to the Elements [?]

14. As a consequence of this, it follows that it is absolutely necessary to scientific unity and research to consider these bodies as entirely derivative, and as containing no secret ultimate power whatever, and that the properties which these so-called quasi elements possess are a direct consequence of the properties of the individual elements of which they are made up. p. 108 - Look to the Elements

15. p. 109 - Focus on Carbon In applying this method, I propose at present to consider the single element carbon. This body is found to have two highly distinguishing characteristics: - 1. It combines with equal numbers of hydrogen, chlorine, oxygen, sulfur, &c. 2. It enters into chemical union with itself. These two properties, in my opinion, explain all that is characteristic of organic chemistry.

16. 1858 Tetravalence and Self-Linking of Carbon

17. C2 as nucleus H = 1 C = 6 O = 8 Cl = 35.5

18. (Cf. W ö hler/S.C.H.Windler hoax)

19. "Structural" Formulae

20. CH 3 CH 2 -OH CH 3 -OH

21. Ethyl Ether... CH 3 CH 2 -O-CH 2 CH 3 There is a Printer’s Error. (This was unfamiliar territory.)

22. Couper C  12

23. Glycerine CH(OH) 2 CH 2 OH CH 2 CH(OH) 2 CO 2 H CH 2 2 OH OH 2 OH OH guessed a bit wrong

24. English Glucose CH 2 OH CHOH CH(OH) 2 O + H 2 O Try 2- or 3-step HOMO/LUMO scheme for H 2 O + R 2 C=O guessed right for hydrate!

25. 2 OH English Glucose CH 2 OH CHOH CH (Typo: Addition of H 2, not H 2 O) French version of same paper; lines for bonds

26. Old Aisle Cemetery Kirkintilloch Oct. 28, 2008 Special thanks to our agent Susan Frew

27. from K. Hafner Angew. Chem. Int. Ed. 18, 641 (1979) Drawing by Kekulé age 13

28. By Kekulé age 18 Giessen for architeture from K. Hafner Angew. Chem. Int. Ed. 18, 641 (1979)

29. Liebig's advice to Paris-bound graduate August Kekulé (1851) There you will broaden your horizons, there you will learn a new language, there you will learn to know the life of a great city, but there you will not learn chemistry!

30. Kekulé in Heidelberg (1856-58) Performed Research on (CH 3 ) 2 As (Prof. Bunsen's cacodyl radical) in his apartment's kitchen! 1857 - Proposed a new type, "Marsh Gas" HHHHHHHH C 1858 - Proposed self-linking of Carbon (tetravalence)

31. Kekulé Claims Priority (1858) Observations on Mr. Couper's New Chemical Theory "In fact, in two memoirs which have appeared in Liebig's Annalen...I have put forward different views, which, in my opinion, should furnish a clearer insight into the constitution of chemical compounds. I may be allowed to indicate that [my first paper] lays down the principle…which I have called the basicity of atoms [we would say 'valence']. If Mr. Couper thinks he has discovered the cause of this difference of basicity in the existence of a special kind of affinity, I am the first to admit that I have no right to contest his priority in this."

32. Ghent (1865) Structure of Benzene Kekulé

33. Bonn (1872) Kekulé

34. Nomenclature Facts, Ideas, and Words  Notation  Models

35. Hofmann's Systematic Hydrocarbon Nomenclature for Radicals (1866) Quartane, (C 4 H 10 )° Quartyl (C 4 H 9 )' Quartene, (C 4 H 8 )'' Quartenyl, (C 4 H 7 )''' Quartine, (C 4 H 6 )iv Quartinyl, (C 4 H 5 )v Quartone, (C 4 H 4 )vi Quartonyl, (C 4 H 3 )vii Quartune, (C 4 H 2 )viii Quartunyl, (C 4 H) ix

36. Kekulé on Notation ("Benzene" Paper, 1865) "For greater clarity I am presenting at the end of this note a table giving graphical formulae for most of the substances mentioned. The idea that these formulae are designed to express is rather well known now; so it will not be necessary to dwell upon it. I am keeping the form that I had adopted in 1859 when expressing for the first time my views on the atomic constitution of molecules. This form is nearly identical with that which M. Wurtz used in his beautiful lectures on chemical philosophy. It seems to me preferable to the modifications proposed by MM. Loschmidt and Crum-Brown."

37. What SHOULD a Formula Show? Constitution: Nature & Sequence of Bonds Composition: Elements & number of Atoms Isomers Berzelius is fine for this, but now we also need to show

38. Lohschmidt Acetic Acid (1861)

39. Crum-Brown (1861-1866) isomers benzene reaction

40. What do you think Kekul é thought? How Many Isomers? Kekulé's "Preferable" Notation (1865) WRONG CH 3 CH 2 CH 2 OH CH 3 CCH 3 O = CH 3 CHOH CH 3 - CH 3 CHCH 3 OH - Constitution same as 28! Kekul é thought that his notation said these are different.

41. Molecular Models

42. Gaines Models Now, if a class thus instructed be at the recitation bench, and one of them is requested to form carbonic acid, he will walk quickly to the table in front, where the box is open, and place on one of the shelves a cube marked 6, and two marked 8 – carbon, one, and oxygen, two. If requested to form lime, he will place on the shelf a cube marked 20, (calcium,) and another marked 8, (oxygen,) these being the elements of lime. These cubes being all placed in contact, and the pupil being asked, “What have we now?” answers, “Carbonate of lime.” Another pupil may then be asked to give the proximate analysis for carbonate of lime, and the cubes before him will indicate the answer and fix it in his memory: “It is composed of one equivalent of carbonic acid, (22) and one of lime, (28).” Another may be requested to give the ultimate analysis, and by the same means he is furnished with the answer: “Carbonic acid is composed of one atom of carbon and two of oxygen, and the lime is composed of one atom of calcium and one of oxygen.” The intelligent educator will see at a glance, that in this chemical alphabet he has the means of at once arresting the attention of his class, and converting what has hitherto been an irksome task into a pleasant recreation. The atomic theory, the characteristics of affinity, the law of multiple proportions, the nomenclature, isomerism, &c., may all be learned in half the time required heretofore, and learned so as not likely ever to be forgotten. ? ?

43. James Dewar's Brass Strip Models (1866) "to make the combination look like an atom, a thin round disc of blackened brass can be placed under the central nut" Prepared ~100 years later, and called "Dewar Benzene" 4-center bond?

44. “CPK” Model of Alanine an amino acid Hofmann Croquet-Ball Models (1865) Established the “standard” colors we still use!

45. Hofmann Croquet-Ball Models (1865)

46. O _ Cl O O O O Oxidizing agent for Gay-Lussac “Chlorox” bleach Explosive _ NH 4 ClO 4 (AP) is a component of military explosives.

47. Hofmann Croquet-Ball Models (1865) Isomers? Constitutional Models Try to Show Nature & Sequence of Bonds NOT Position in Space. Successive Substitution Products  Science Museum, London by permission

48. Hofmann Croquet-Ball Models (1865) Reactivity! “Unfinished” or “Non-Saturated” (now called “unsaturated”) Cl 2 addition not substitution  ClH 2 C-CH 2 Cl Cl 2 Actually, as we’ve seen: H 2 C=CH 2

49. End of Lecture 23 Oct. 29, 2008 Copyright © J. M. McBride 2009. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0