Kinetics of oxidation of dimethyl sulphoxide by N-chloro-p-substituted benzenesulphonamides

Six sodium salts of N-chloro-p-substituted benzenesulphonamides (NCSBS) have been employed as oxidants for studying the kinetics of oxidation dimethyl sulphoxide (DMSO) in aqueous acid medium under varying conditions, to see how the oxidative strength of these reagents vary with substitution. The sodium salts of N-chloro-p-substituted benzenesulphonamides employed are of the general formulae: i-X-C6H4SO2NaNCI.xH(2)O (where i-X = H; 4-CH3; 4-C2H5; 4-F; 4-Cl or 4-Br). The reactions show first order kinetics each in [NCSBS] and [DMSO] and nearly zero order in [H+] and have been explained by a simple mechanism. The applicability of the Hammett equation has also been tested. The relation, 1g k(obs) = -3.31 + 0.69 sigma p, is found to be valid. The enthalpies and the free energies of activation for the reactions have also been correlated. The plot of Delta H* versus Delta S* was reasonably linear with isokinetic temperature of 307 K. Further, to see the effect of substituent on the energy of activation and lg A, both E-a and lg A values of NCSBS are optimised corresponding to the lg A and E a value of the parent oxidant, N-chlorobenzenesulphonamide, respectively. The energies of activation for the oxidants with electron-releasing groups in the benzene ring are slightly higher than that of the parent oxidant, while the E values are lower for the oxidants with the electron-withdrawing groups in the benzene ring. Enthalpies of activations have the same trend. Similarly lg A values are slightly higher for oxidants with the electron-withdrawing groups, while the effect of electron-releasing groups on lg A values is negligible. The free energies of activation remain almost the same in both the optimisations indicating the operation of similar mechanisms in all the cases.