Oxidation of indole by N‐sodio‐N‐chlorobenzenesulphonamide (chloramine‐B) in alkaline medium catalysed by os(viii): A kinetic and mechanistic study

The kinetics of oxidation of indole (In) and 5-substituted indoles (OCH3, Br and Cl) by chloramine-B (CAB) were studied in alkaline medium with Os(VIII) as catalyst at 303 K. At low In]0 the mte law rate = kCAB]Os(VIII)]In]0 is obeyed, which changes to mte = kCAB]Os(VIII)]OH-]-1 at higher substrate concentrations. Variation of ionic strength has no effect on the rate and the dielectric effect is negative. The reaction was studied at different temperatures and activation parameters were evaluated. Hammett correlation of substituent effects indicated a linear free energy relationship with rho+ = -1.0, showing the formation of an electron-deficient transition state. From enthalpy-entropy relationships and Exner correlations, the isokinetic temperature beta was found to be 330 K, indicating enthalpy as a controlling factor. The mechanism assumes the formation of a complex between oxidant and Os(VIII) at high and low In]0. Proton inventory studies in H2O-D2O mixtures showed the involvement of a single exchangeable proton of OH- ion in the transition state