Deviations from the additivity principle in rates of aromatic nitration and aromatic bromination (by ‘positive bromine’)

Abstract

Kinetic studies have been carried out on the bromination by hypobromous acid of a series of disubstituted benzenes with substituents Me, OMe, and [CH₂]_nX where n= 0–3 and X = NMe₃, NH₃, or SMe₂. Deviations from the additivity principle are small but appear to be systematic. With two strongly deactivating substituents, the substrate is less reactive than expected (negative deviation). With one activating and one deactivating substituent the deviation is usually negligible or positive. Unexpectedly, two weakly deactivating substituents of type [CH₂]₂X also give a positive deviation.

For deactivating substituents, the results in nitration parallel those in bromination. However, for the combination of a methyl substituent and a deactivating substituent the deviations in nitration are much greater than in bromination. This difference is attributed in part to the effect of diffusion control in distorting the relative reactivity of toluene in nitration. From the comparison with bromination, the relative reactivity of toluene to benzene in reaction with the nitronium ion in the absence of diffusion control is estimated to be about 79. This value is unexpectedly high in relation to direct measurements on the nitration of toluene.