Nucleophilic substitution at a trigonal carbon. Part 5. Substituent effects in the reactions of aromatic acyl bromides with methanol in acetonitrile

Abstract

The kinetics of the methanolysis of benzoyl bromide and eight para- or meta-substituted derivatives in acetonitrile at 25.0 °C can be analysed in terms of the simultaneous operation of overall second- and third-order processes. In turn, each of these processes can be analysed in terms of the simultaneous operation of two reaction channels, which are proposed to involve a carbonyl addition–elimination mechanism (favoured for electron-withdrawing substituents) and a process (SN2–SN1) proceeding through a loose SN2-type transition state. For the p-methoxybenzoyl bromide substrate, a third reaction channel is observed, but only for the overall second-order process (first-order in methanol), and this is tentatively described as involving electrophilic assistance by a methanol molecule to an ionization (SN1) pathway.