Crystal Structures of 1-Chloro-2,4-Dinitrobenzene and 1-Bromo-2,4-Dinitrobenzene

Abstract

THE rates of reaction for nucleophilic aromatic substitution are affected if the replaced group has one, or two, groups ortho- to it. In particular, the effects of nitro groups in these positions (and in meta- and para-positions) have been studied in many reactions. Generally, these groups contribute two competing effects which control the reaction-rate. First, a maximum activation through electron transfer is attained if the nitro group is planar with the benzene ring. However, spatial considerations often forbid this coplanarity in the case of ortho-substitution, and there exists a competing de-activation due to the twist of the nitro group from the plane of the benzene ring. In the extreme case of two ortho-substituents, the activating effect of the nitro groups is greatly diminished by this steric effect and may result in a net de-activation.