Kinetics and mechanism of oxidation of thiosemicarbazide in the pure state and in its metal complex by chloramine T, bromamine T, and dichloramine T in acid medium

Abstract

The kinetics and mechanism of oxidation of thiosemicarbazide (TSC) in the pure state and in its zinc metal complex by chloramine T(CAT) and bromamine T(BAT) in aqueous perchloric acid medium and by dichloramine T(DCT) in 1 : 1 (v/v) water–methanol medium have been investigated. The rate followed first-order kinetics in [oxidant] and fractional order in [TSC] in both the pure state and in the metal complex with all the oxidants. With chloramine T, the rate had an inverse first-order and inverse fractional order dependence in [H+] for two ranges of [HClO4], 0.01–0.05 and 0.05–0.03 mol dm–3, respectively. The rate had inverse fractional and inverse first-order dependence in [H+] with BAT and DCT respectively over the entire [HClO4] range (0.01–0.2 mol dm–3). Addition of the reduced product of the oxidants (toluene-p-sulphonamide) had no significant effect on the rate with all the oxidants. But the increase of ionic strength of the medium decreased the rate. Decrease of dielectric constant of the reaction medium by adding methanol increased the rate indicating positive ion–dipole interactions in the rate-limiting steps. The coefficients of the rate-limiting steps have been computed at different temperatures with all the oxidants and these constants were used to calculate the activation parameters from the Arrhenius plots. The oxidation process under the conditions employed has been shown to proceed by two paths, one involving the direct interaction of monochloramine T(RNHCl) or dichloramine T(RNCl2) with TSC to give an N-chloro intermediate of the substrate which subsequently undergoes disproportionation and further reactions with the oxidant molecules in fast steps to give the final products, and the other involving the interaction of H2OCl+ or H2OBr+, produced from the disproportionation of oxidants, with the substrate to yield the products. The mechanisms proposed and the rate laws deduced are in conformity with the observed kinetics. The rate constants predicted by the derived rate laws as [TSC], [Complex], or [H+] vary are in good agreement with the experimental rate constants thus justifying the rate laws and hence the suggested mechanistic schemes.