Structure of 1.2-diphenylbenzene (C 18 H 14 )

Abstract

Benzene derivatives, as a particular example of conjugated compounds, tend to form plane molecules unless some kind of hindrance exists. Until recently it was thought that the carbon-carbon linkage between the individual nuclei in, say, diphenyl was a single bond, capable of free rotation, and that the co-planar configuration was the most likely because it was the most symmetrical (Adams and Yuan 1933). X-ray analysis has shown in fact that the molecules of diphenyl (Dhar 1932), p -diphenylbenzene (Pickett 1933) and p -diphenylbiphenyl (Pickett 1936) are all plane, but the observation that the carbon-carbon internuclear distance is less than the normal single-bond distance indicates a measure of conjugation in the internuclear bond as a more fundamental reason for the molecular planarity. X-ray analysis has further shown that in aromatic compounds of all varieties of structure, carbon atoms in separate molecules do not approach nearer to each other than about 3·5 A (between centres) (Robertson 1934), owing to van der Waals repulsion. There are, therefore, in such compounds as 1.2-diphenylbenzene, two forces opposing one another: (1) The tendency for the whole of the atomic nuclei to lie in one plane, because of conjugation in the interatomic bonds. (2) The tendency for atoms in the substituted phenyl groups to repel each other. The latter force is exhibited even in such a compound as durene (1. 2. 4. 6-tetramethylbenzene) for which X-ray analysis has shown (Robertson 1933) that the adjacent methyl groups repel each other in the plane of the molecule, so that there is a straining of the normal valency angles.