Kinetics and mechanism of oxidation of thiosemicarbazones by acid bromate

Abstract

The kinetics of oxidation of benzaldehyde, propanaldehyde, acetone and acetophenone thiosemicarbazones by acid bromate have been studied in aqueous acetic acid medium (1:1, v/v). The rates show first order dependence on [oxidant) in all the cases, but show varying orders in [H⁺] depending upon [H⁺], In the [H⁺] range 0.001-0.02 mol dm^-3 the reaction order is fractional in [H⁺]and it is > 1 in the [H⁺] range > 0.02 mol dm^-3. The reactions show fractional order in [hydrazone], in both the ranges of [HCIO₄]. Addition of KBr has little effect on the rates of oxidations. Suitable mechanisms consistent with the observed results have been suggested and the related rate laws deduced. Coefficients of the rate limiting steps have been calculated at different temperatures and the activation parameters are computed. The validity of rate laws is also tested by recalculating the rate constants from the deduced rate laws as [hydrazone] and [H⁺] are varied. Reasonably good agreement between the recalculated values and the experimental constants provides support to the proposed mechanisms.