Solvent effects and Brønsted coefficients in nucleophilic substitutions at tetraco-ordinate sulphur

Abstract

The rate of reaction of benzenesulphonyl chloride with pyridines or primary amines has been measured in a series of solvent mixtures. The kinetic data have been plotted in a Brønsted fashion against the pK_a of the conjugate acid of the nucleophile. A large effect of the solvent composition on the Brønsted slope was observed. For primary amines the slope changes from 0·53 in 45% dioxan–water at 25 °C. For pyridines, in the same solvent mixtures, the slopes are 0·43 and 0·79 respectively at 0 °C. Acetonitrile–water mixtures have also been investigated. Such changes are discussed, as usual, in terms of different degree of bond formation in the transition state. However, the difficulties of this unique interpretation are also pointed out.