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 pKa 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.