Kinetic and mechanism of oxidation of thiocarbohydrazide and its metal complex by acid bromate in aqueous and partial aqueous media

Abstract

AbstractThe kinetics of oxidation of thiocarbohydrazide in the free and zinc(II)‐bound states by acid bromate have been studied in aqueous and water‐acetic acid (1:1, V/V) media under varying conditions, both in the absence and presence of added bromide ion. The rates of oxidations show first order kinetics in [bromate] in all the cases, but exhibit different kinetic behaviour in [substrate] and [H+]. Oxidation of TCH in aqueous medium shows zero order in [TCH] and nearly second order in [H+], while oxidation in aqueous acetic acid shows two ranges in [H+]. The rate shows first and fractional order kinetics in [TCH] in the first and second acid ranges. Kinetics observed in the presence of Br− are similar to those observed for oxidation of TCH in second acid range. In addition, the reaction shows fractional order in [Br−]. Oxidation of TCH in Zn(II)‐bound state exhibits first order kinetics in [substrate] and second order in [H+]. Increase in ionic strength of the medium decreases the rate in all the cases. Increase in acetic acid composition of the solvent increases the rate. Mechanisms consistent with the observed results have been considered and the rate laws deduced. The rate limiting steps have been identified and the coefficients of these steps have been calculated at different temperatures. The related activation parameters have also been computed. The validity of the deduced rate laws has also been tested by recalculating the rate constants from them as [TCH] and [H+] are varied.