In the preceding post, a nice discussion broke out about Kekulé’s 1872 model for benzene.[1] This model has become known as the oscillation hypothesis between two extreme forms of benzene (below). The discussion centered around the semantics of the term oscillation compared to vibration (a synonym or not?) and the timescale implied by each word. The original article is in german, but more significantly, obtainable only with difficulty. Thus I cannot access[1] the article directly since my university does not have the appropriate “back-number” subscription.‡ So it was with delight that I tracked down an English translation in a journal that I could easily access.[2] Here I discuss what I found (on pages 614-615, the translation does not have its own DOI).
The bent bond formula
The translation is by no other than Henry Armstrong, whose own contributions I have documented elsewhere. The pertinent points (it’s a long explanation) seem to be:
- Kekulé does not use the word oscillation anywhere. This seems to have been added by subsequent commentators.
- He does describe the atoms as being in continuous movement, actually using the very modern term intramolecular motion (as translated of course).
- He also describes this motion as returning to a mean position of equilibrium, and the separate atoms as possessing rectilinear motion, striking and recoiling against adjacent partners.
- He finally concludes by describing at some length what happens during two units of time involving what we would regard as one complete vibration to return the atoms to their starting point. This description is couched in words, and refers to what we would now call a normal (vibrational) mode evolving in time. You can see that written description below for yourself (in translation). It IS quite verbose; if ever a case could be made for replacing 1000 words with one picture, this is it!
Armstrong’s translation
Perhaps I can attempt to replace the (1000?) words above with that one picture (below). Here, I think Kekulé does manage to complicate things by including a hydrogen (h) as part of his scheme. Carbon C1 is described as contacting C2, and then immediately a hydrogen (although since he does not number the hydrogens it is not absolutely clear he means the hydrogen on carbon 2 at this stage). The modern equivalent below shows relatively little motion from the (light) hydrogen atoms, and certainly no obvious contact between e.g. C1 and any hydrogen other than the one it is bonded to.
We now replace the description above by using far more concise vectors to describe the movement of the atoms with respect to time. And of course Kekulé had no real idea of how long his cycle took (only that it must be short as inferred from the laboratory observation of not being able to isolate geometric isomers, perhaps shorter than 100 seconds?); we now know that it is about 10-14 s. Commentators to this day describe this as Kekulé’s oscillation hypothesis, but since Kekulé did not use the term at all but did use (thrice) the word vibration we really should call it his vibration hypothesis, as indeed Paul Schleyer noted in his comment on the original post.
‡There is little doubt that historical researches have become severely endangered by the increasing lack of access to older issues of many journals. In some cases, older can mean as little as ten years!
References
- A. Kekulé, "Ueber einige Condensationsproducte des Aldehyds", Justus Liebigs Annalen der Chemie, vol. 162, pp. 77-124, 1872. https://doi.org/10.1002/jlac.18721620110
- "Organic chemistry", Journal of the Chemical Society, vol. 25, pp. 605, 1872. https://doi.org/10.1039/js8722500605