Nucleophilic substitution at trigonal carbon. Part 4. Substituent effects in the reactions of aliphatic acyl chlorides with methanol and phenol in acetonitrile

Abstract

A previous study of the methanolysis and phenolysis of acetyl chloride in acetonitrile has been extended to the influence of the substitution of up to two chlorine atoms or up to three methyl groups into the acetyl chloride. For each of the substrates so obtained, the methanolysis continues to exhibit pathways with both second-and third-order kinetics. The introduction of up to two methyl groups has only a minor influence but an appreciable fall in the magnitude of both rate coefficients is observed upon introduction of the third methyl group. The introduction of chlorine atoms leads to fairly large accelerations, and a tightening of the loose S_N2 transition state or, possibly, a change to an addition–elimination mechanism. The phenolysis reactions, in the presence of methyl substituents, can be described at fairly low phenol concentrations in terms of second-order rate coefficients, which fall off slightly in value as methyl groups are introduced. In contrast to methanolysis, the phenolysis shows a large decrease in rate on introduction of an electron-withdrawing chlorine substituent; this reduction is consistent with a retardation of the electrophilically assisted ionization mechanism proposed for phenolysis of the parent substrate.