Nucleophilic substitution at bivalent sulphur. Part I. Kinetics and mechanism of the base-catalysed hydrolysis of 2-nitrobenzenesulphenate esters

Abstract

The kinetics of the base-catalysed hydrolysis of ethyl and of t-butyl 2-nitrobenzenesulphenates have been studied in 50%(v/v) aqueous dioxan at 25 °C. The reactions are of the second order, first order in ester and first order in hydroxide ion, the uncatalysed reaction being slow. An absorption at 588 nm., attributed to the 2-nitrobenzene-sulphenate anion, ArSO^–, was observed during the reaction and the extinction coefficient determined for this intermediate. As the reaction proceeds, the concentration of this intermediate passes through two maxima before decreasing to zero. Evidence is adduced that the reaction of the sulphenate anion with the ester is not the major reaction path leading to products. The rate of reaction of the sulphenate anion was determined during the early portion of the reaction and found to be proportional to [ArSO^–]². From the nature of the proportionality it is suggested that the major reaction of the sulphenate anion is best represented by the equation ArSO^–+ ArS·OH = ArSO·SAr + OH^–. Consequently the velocity constants for the hydrolyses give a measure of the rate of nucleophilic substitution at sulphur and are not complicated by later reactions. The mechanism of the hydrolysis is discussed.