Mechanistic investigations on N-halosulphonamides. Kinetics of oxidation of thiocarbonohydrazide in the free and metal-bound states and diethylidene thiocarbonohydrazide, in aqueous and partially aqueous media

Abstract

The kinetics of oxidation of thiocarbonohydrazide (TCH) in the free and Zn(II)-bound states and its hydrazone, by N-chlorobenzenesulphonamide (chloroamine-B) in aqueous perchioric acid and by N,N-dichlorobenzenesulphonamide (dichloramine-B) in water–methanol (1:1, v/v) in the presence of perchloric acid, have been investigated. Oxidation by both chloramine-B and dichloramine-B shows first-order kinetics in [oxidant] and fractional order in [substrate]. The rates of reactions are independent of [H⁺]. Addition of the reduced product of the oxidant (benzenesulphonamide) or a variation in ionic strength of the reaction medium has no significant effect on the rate of oxidation, in all cases. However, a decrease in the relative permittivity of the medium, effected by changing the solvent composition with methanol, decreases the rate. Both the Michaelis–Menten type and two-pathway mechanisms are discussed in order to explain the observed results. The rate-limiting steps have been identified and the rate coefficients of these steps calculated at different temperatures by varying the substrate concentrations at each temperature. The latter constants have been used to compute the activation parameters from the Arrhenius plots. The metal complexation of thiocarbonohydrazide has little effect either on the rate of oxidation or kinetic order, while conversion to its hydrazone enhances the rate of oxidations with both the oxidants.