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 (WiBr) 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.