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

Department of Post-Graduate Studies and Research in Chemistry, Mangalore University, Mangalagangothri-574 199, India E-mail : gowdabt@yahoo.com Fax : 91-824-7423671742424 Manuscript received 18 October 2000, accepted 22 March 2001 The kinetics of oxidation of thiosemicarbazide by N-chlorobenzamide, N-chloro-p-methyl-, N-chloro-p-chloro- and N-chloro-p-nitrobenzamides have been studied in aqueous methanol in the presence of perchloric acid. The reactions show first order kinetics in [oxidant], fractional order in [TSC] and inverse first order in [H+]. The rates slightly decrease with increase in ionic strength of the medium (0.05-0.50. mol dm-3), but increase with decrease in dielectric constant of the medium, effected by increasing the methanol composition of the solvent. Addition of the reduced product of the oxidant, benzamide has no significant effect on the rate. The observed results have been explained by a two-pathway mechanism and the related rate law deduced. Activation parameters for both the pathways have been evaluated by computing the rate coefficients for both the pathways at different temperatures. The activation parameters have also been optimised corresponding to the parent 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