Oxidation of mephenesin and guaifenesin with chloramine-B in hydrochloric acid medium: Design of kinetic model

Abstract

Kinetics of oxidation of mephenesin and guaifenesin by sodium N-chlorobenzenesulfonamide (CAB) have been investigated in HCl medium at 303 K. The oxidation behaviour is similar for both the substrates. The rate shows a first order dependence on both [CAB]₀ and [HCl], and is fractional in [substrate]₀. The orders individually in [H⁺] and [Cl⁻] are fractional. The variation of ionic strength and addition of the reduction product, benzenesulfonamide, has no significant effect on the reaction rate. The solvent isotope effect has been studied using D₂O, and the oxidation products have been identified. Composite activation parameters for the reaction have been determined from Arrhenius plots. Michaelis-Menten type of kinetics is observed and activation parameters for the rate limiting steps have also been computed. The proposed mechanism assumes the simultaneous catalysis by H⁺ and Cl⁻ ions. The reaction is found to be moderately faster in guaifenesin in comparison with mephenesin, which may be attributed to the involvement of methoxy and methyl groups of the substrate. The observed results have been explained by a plausible mechanism and the related rate law has been proposed.