Nucleophilic substitution at two-co-ordinate sulphur. Effect of the leaving group

Abstract

The effect of the basicity of the leaving group has been measured in the reaction of pyridine and butylamine with para-substituted phenyl triphenylmethanesulphenates. The kinetic data plotted in a Brønsted fashion against the pK_a of the conjugate acid of the leaving group indicate a large sensitivity to the basicity of the leaving group; β–1·06 and –1·09 for pyridine and butylamine, respectively. The element effect has been measured on a number of triphenylmethanesulphenyl derivatives with n-butylamine and hydroxide. The specific rate constants of displacement with I, SCN, Br, and Cl, respectively, as leaving groups, are the following: (a) with n-butylamine, 0·44, 0·41, 85, 147; (b) with OH^–, 3·6, 1·03, 21, 28. These data indicate small sensitivity to the nature of the element displaced. The results are discussed in terms of an intermediate complex mechanism with bond formation as the rate-limiting step.