Kinetics and mechanism of oxidation of thiosemicarbazide by substituted-N-chlorobenzamides in aqueous methanol medium

The kinetics of oxidation of thiosemicarbazide by N-chlorobenzamide, N-chlo ro-p-methyl-, N-chloro-p-chloro- and N-chloro-p-nitrobenzamides have been s tudied in aqueous methanol in the presence of perchloric acid. The reaction s show first order kinetics in [oxidant], fractional order in [H+] and inve rse first order in [W]. The rates slightly decrease with increase in ionic strength of the medium (0.05-0.50 mol dm(-3)), but increase with decrease i n dielectric constant of the medium, effected by increasing the methanol co mposition of the solvent. Addition of the reduced product of the oxidant, b enzamide has no significant effect on the rate. The observed results have b een explained by a two-pathway mechanism and the related rate law deduced. Activation parameters for both the pathways have been evaluated by computin g the rate coefficients for both the pathways at different temperatures. Th e activation parameters have also been optimised corresponding to the paren t oxidant. The constancy of the free energies of activation may signal the operation of similar mechanism. Validity of Hammett equation and isokinetic relationship has also been tested.