The effects of electronic buttressing on the conformations of some ortho-disubstituted benzenes

Abstract

When one of a pair of substituents ortho to each other on a benzene ring is conjugated with an appropriate para substituent, it is pulled towards coplanarity with the aromatic ring and the other substituent has to take a greater share of the twisting necessary to avert steric conflict. This effect, termed electronic buttressing, is demonstrated in the proton magnetic resonance spectra of 5-dimethylamino-2- nitroanilines, 5-substituted methyl 2-nitrobenzoates, and 4-substituted 1,2-dinitrobenzenes. In this latter group the size of the effect is shown to depend on the conjugative ability of the 4-substituent.