Electrophilic aromatic substitution. Part XV. The kinetics, mechanism, and products of nitrodebromination in sulphuric acid

Abstract

The kinetics of nitration in sulphuric acid of o- and p-dibromobenzene, p-bromo- and p-chloro-toluene, 2-bromo-m-xylene, and p-bromochlorobenzene are reported. For these compounds and for bromobenzene and p-bromofluorobenzene the yields of products formed over a range of acidities have been determined. Nitrodebromination was not detected with o-dibromobenzene and 2-bromo-m-xylene, but was a major outcome of nitrating p-dibromobenzene, p-bromotoluene, and p-bromochlorobenzene. The degree of nitrodebromination increased with increasing dilution of the sulphuric acid and evidence is provided to show that Wheland intermediates formed at brominated carbon atoms (W_i^Br) are either debrominated or rearranged by nitro-group migration. There is no intramolecular migration of bromine and little or no nucleophilic capture of the Wheland intermediates.

When methyl groups are present ipso-nitration at C(Me) occurs and is followed by nucleophilic capture by water and by nitro-group migration in proportions which varied with the acidity. Nitrodechlorination was not observed, but even if attack at C(Cl) is assumed to occur a choice between subsequent decomposition of the Wheland intermediate to its components and nitro-group migration cannot be made. In p-dibromobenzene the C(Br) positions are at least as reactive as the C(H) positions.