Reactions of Charged Substrates. 8. The Nucleophilic Substitution Reactions of (4-Methoxybenzyl)dimethylsulfonium Chloride.

Displacement reactions on the title compound (1) occur only for nucleophiles with intermediate hardness. Nucleophiles that react display a range of mechanisms. 1 reacts with the neutral nucleophile pyridine-d(5) through a mixed S(N)1/S(N)2 mechanism; salt added to control ionic strength affects the rate for the unimolecular process, but has no effect on the bimolecular rate constant. The mechanism of displacement by N(3)(-) and SO(3)(2)(-) depends on the presence or absence of exogenous salt. At constant ionic strength, the mechanism is mixed S(N)1/S(N)2 over most of the range of [Nu]. With nucleophile only present, plots of k(obsd) vs [Nu] exhibit severe breaks that are not the result of salt effects. Analysis of rate constants and product ratios suggests that at low [Nu] reaction occurs simultaneously through concerted Hughes-Ingold S(N)2 and preassociation-concerted mechanisms. At high [Nu], displacement occurs only through the preassociation-concerted mechanism. Comparison of these results with data for gas-phase dissociation of benzyl dimethylsulfoniums and with solution results for benzyl pyridiniums suggests that the intrinsic stability of the intermediate does not necessarily determine the mechanism.