attached an atom of hydrogen, but as carbon is usually considered quadrivalent, a fourth seemingly unused bond of affinity remains free to each of these carbon atoms. This affinity may be considered as the residual affinity. To this substance Kekulé has assigned the structure illustrated by the graphical formula:
The introduction of three double linkings between the alternate pairs of carbon atoms satisfies the demands for quadrivalence in these atoms. But Kekulé clearly called attention to the fact that a sort of equilibrium existed between all the carbon atoms, such that the presence of one of the three double linkings between any two adjacent carbon atoms, when both were involved in the formation of a derivative, would not necessarily change the properties of the derivative from that one formed when two adjacent carbon atoms were united by only a single bond. Now the Kekulé formula, and in fact all the older formula? assigned to this compound, represent only particular phases in the motions of the molecule. The space-formula proposed by Collie,[1] in which the atoms are represented as in a state of continual vibration, serves well for the basis of our modern conception. Upon examination of the absorption spectrum of benzol we note the presence of seven distinct bands all quite similar and closely situated with reference to each other, appearing between the oscillation frequencies 3,725 and 4,200. These bands are in that part of the ultra-violet spectrum where the absorption bands due to keto-enol tautomerism displayed themselves. At once the idea of a similar make-and-break of linkings between the carbon atoms suggested itself, and, in exact accordance with this hypothesis, the seven distinct bands may find here their cause of formation.
In keto-enol (aliphatic) tautomerism an even number of carbon atoms is always involved in the make-and-break of linkings. Accordingly with the benzol molecule we may assume that two, four or six carbon atoms may enter into this phase at one time. If the carbon atoms are numbered consecutively from 1 to 6, we should have in order the following conditions which represent the change of linkings between certain numbered carbon atoms: (1 and 2), (1 and 3), (1 and 4), (1 and 2, with 3 and 4), (1 and 2, with 3 and 5), (1 and 2, with 4 and 6), (1 and 2, with 3 and 4, with 5 and 6). At the outset we shall suppose the benzol ring to be elastic and capable of under-
- ↑ Chem. Soc. Trans., 71, 1013, 1897.
