Mechanisms of elimination reactions. The effect of added dimethyl sulphoxide on rates, isotope effects, and substituent effects in the reaction of 2-arylethyldimethylsulphonium bromides with sodium hydroxide in water

Abstract

Rates of elimination have been determined for the reactions of 2-arylethyldimethylsulphonium bromides with sodium hydroxide in water containing from 0 to 85 mole % of dimethyl sulphoxide. Kinetic deuterium isotope effects (k_H/k_D) were determined using dimethyl-2-phenylethylsulphonium bromide and its 2,2-dideuterio-isomer. Hammett reaction constants (ρ) were determined with the p-substituents methoxy, methyl, hydrogen, and chlorine. The values of k_H/k_D and ρ have been used, in conjunction with the sulphur isotope effects previously measured, to assess the influence of solvent composition on the E2 transition state. As the solvent is gradually enriched in dimethyl sulphoxide, the ρ value increases sharply and then remains almost constant, while k_H/k_D rises from the value reported in water, passes through a maximum, and then decreases. In going from water to 85% dimethyl sulphoxide, the rate increases by 10⁷, and rates measured in the range 30–60° indicate a large reduction in both the enthalpy and the entropy of activation as the medium is made less aqueous. It is suggested that dimethyl sulphoxide induces a more “reactant-like” transition state than is encountered in the elimination reaction in water.