ALEXANDER GERO Hahnemann Medical College, Philadelphia, Pennsylvania
INTEXTBOOKS of
organic chemistry F. A. Kekul6 is given the attention due to a giant of chemistry to whom science owes, among other things, the concepts of the tetrahedral carbon atom and of the benzene ring. Most texts also report in some detail that Kekul6 conceived of the double bonds in benzene as oscillating around the ring and of benzene, therefore, as a tautomeric mixture of two structures:
0-0
It is interesting, and probably not widely known, that Kekul6 did nothing of the sort. What he did was much more subtle and much closer to present-day ideas. It may therefore be useful to remove the crust of misunderstanding and t o present to a modem audience what Kekul6 really wrote in his famous paper "Uber einige Condensationsprodukte des Aldehyds" (Ann. chem., Justus Liebigs, 162,77 (1877)). I n this paper he reviews the several structures proposed by various investigators for benzene and puts forward the following argument in favor of his structure: (1) It is as good as all other structures proposed in accounting for the formation of benzene from acetylene. (2) It accounts better than other structures for the relationship of benzene to naphthalene and anthracene. (3) It explains the addition of halogens to benzene. 'The one phenomenon which seemingly contradicts the Kekul6 structure is the nonexistence of ortho-disuhstitution isomerism, and Kekul6 devotes a considerable part of his paper to showing that his structure does not call for isomers of ortho-disubstituted benzenes. He bases his argument on a hypothesis of valence in which the oscillations of atoms around equilibrium positions within the molecule and their rhythmic collisions with each other are looked upon as the forces holding the molecule together. I n particular, Kekul6 equates valence with the relative number, in unit time, of the collisions which an atom undergoes with other atoms. I n the same time in which the monovalent atoms of a diatomic molecule collide once, the divalent atoms of a diatomic molecule collide twice, and in a triatomic molecule (such as H-0-H or O=C=O) the middle atom suffers twice as many collisions in each time unit as the other two atoms. Applied to carbon compounds this hypothesis amounts to the statement that, during the time in which a hydrogen atom performs one full oscillation, two car-
ban atoms held together by a single bond collide once with each other and once with each of six other atoms, two double-bonded carbon atoms collide twice with. each other and once with each of four other atoms; (assuming that there are otherwise no multiple bonds i n the molecules), and so on. I n benzene, therefore, each carbon atom collides three times with its neighbor carbons during the time it collides once with the hydrogen atom it holds. Thua C , in the molecule
collides in the first period with the atoms Ce, Cs, H, C2, in this order; in the next period it collides with Cs, C2, H, Cs in this order. Hence during the second period the structure is
Therefore, whether any particular C-C bond in benzene is single or double depends on when we start counting time. This may perhaps be made clearer by the following chart: Time measure I: 1 I I I I CLoollidea aith . Cz O d C n CI C B C e Ca CsHCs Cs C,HC. C? CsH Time measure 11: I I I I I
.
Clearly, when the molecule appears as (I) according to time measure I, it appears mostly as (11) according to time measure 11, and vice versa. Obviously it is purely arbitrary, and of little physical simificance, when we beein to count time. But then the distinction between (fi and (11)is also arbitrary and meaningless. What counts is only the time-average distribution of the collisions of C, with its neighboring carbon atoms: and it is easy to see that on this assumption the difference between (I) and (11) disappears. To put i t differently,we eliminate the arbitrariness of the time measure by doubling the periods:
202
then we get the same result by time measure I and by time measure 11,namely, that in each double period CI collides three times with C2and three times with Cs. But if this is so then the "KekulB structure" does not describe the complete state of the benzene molecule, only one phase of it. In the real benzene ring all bonds are identical, neither single nor double but partaking of the character of both single and double bonds; neither (I) nor (11) describes the benzene molecule adequately, only both (I) and (11) combined; and the difference between 1 , 2 and 1,6-disubstituted benzene is only apparent, not real. I t is impossible not to be struck by the close correspondence between KekulB's ideas and modern concepts of resonance in the benzene ring. Of course, Kekul6
JOURNAL OF CHEMICAL EDUCATION
went the wrong way in trying to identify valence with the number of atomic collisions; but if we consider that his paper wrts published more than twenty years before the electron was discovered, and almost half a century before the first groping attempts were made to find an electronic interpretation of valence, we cannot help admiring KekulB's great insight and scientific instinct. From one sentence in particular it is clear that KekuM knew that however fanciful his hypothesis of valence, it contained an essential element of truth as far as the structure of benzene is concerned. In discussing the difference between (I) and (11) he writes: "If my concept or some similar one" (my emphasis) "should prove correct, it follows that this difference is only apparent but not real."
