Nucleophilic substitution at a trigonal carbon. Part 6. Substituent and bromide/chloride leaving group effects in the reactions of aromatic acyl chlorides with methanol in acetonitrile

For the methanolyses in acetonitrile at 25.0 degrees C of benzoyl chloride and eleven para- or meta-substituted derivatives, the kinetics of methyl es ter formation have contributions both first-order and second-order in metha nol. The Hammett plots for the overall second- and third-order processes ar e curved and both plots can be analyzed in terms of the simultaneous operat ion of two reaction channels. An addition-elimination process is favored by electron-withdrawing substituents and a loose S(N)2 process by electron-su pplying substituents. Comparisons of widely varying halogen leaving-group e ffects suggest that the addition-elimination processes for solvolyses of ha loformate esters and acyl halides have an identical addition step but they then differ in the sequence of the two elimination steps.