Kinetics and mechanism of the reaction of benzenesulphinic acid with nitrous acid

Abstract

Rate measurements on the reaction between benzenesulphinic acid (BSA) and nitrous acid over a range of acidity. Show that reaction occurs rapidly via both the neutral acid molecule of BSA and its anion. The latter process is the faster, as expected: reaction takes place with the positive nitrosating species (H₂NO₂⁺ or NO⁺) at the diffusion rate. The first-order dependence on [BSA] shows that the first stage, in which the intermediate nitrosobenzenesulphinate is formed, is rate-limiting; the intermediate then reacts with another BSA molecule (or its anion) to yield the hydroxylamine derivative as the final product Catalysis by Cl^–, Br^–, SCN^–, and SC(NH₂)₂ was found, and the bimolecular rate constants for attack by NOCI etc. on both BSA and its anion form, were extracted by a kinetic analysis of the variation of the rate constant with acidity in the presence of Cl^–, Br^–, or SCN^–. Again the sulphinate anion is the more reactive, and the sequence NOCI > NOBr > NOSCN was established; the rate of reaction of NOCI is close to that calculated for the diffusion limit.