Kinetics and mechanism of oxidation of thiocarbohydrazide in the free state and in its metal complex and thiocarbohydrazone by chloramine-T and dichloramine-T

Abstract

Kinetics of oxidation of thiocarbohydrazide (TCH) in the free state and as its metal complex, and as a hydrazone by chloramine-T (CAT) in aqueous HC104 medium, and by dichloramine-T (DCT) in 1 : 1 (v/v) water-methanol medium in the presence of HC10 4 have been studied. Rates of oxidation of TCH in the free state and in metal complex by CAT were determined. The rate law for the oxidation of TCH at high [H + ] and for complex oxidations were identical to that for CAT oxidations. The conversion of TCH into its hydrazone changed the order in [H + ] from a positive to a negative value, probably signalling the change of reaction site. The rate law for oxidation under these conditions was determined. Addition of the reduced product of the oxidants had no effect on the rate of oxidations. Variation in ionic strength of the medium had little positive effect, while decrease in dielectric constant of the medium decreased the rate in both the oxidations. Oxidation processes generally follow a Michaelis-Menten type of mechanism. Constants of the rate limiting steps have been calculated at different temperatures and these constants have been used to calculate the activation parameters from the Arrhenius plots. The proposed mechanisms are supported by investigations with HOCI under identical reaction conditions. Metal complexation of the substrate decreased the reactivity, while conversion of TCH into its hydrazone changed the rate dependence on [H+].