Ruthenium(III)-catalyzed mechanistic studies of oxidation of benzhydrols by sodium N-chloro-p-toluenesulfonamide in HCl medium

The kinetics of oxidation of benzhydrol and its p-substituted derivatives (YBH, where Y=H, Cl, Br, NO₂, CH₃, and OCH₃) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT), catalyzed by ruthenium(III) chloride, in the presence of hydrochloric acid in 30% (v/v) MeOH medium has been studied at 35°C. The reaction rate shows a first-order dependence on [CAT]_O and a fractional-order each on [ YBH]_O, [Ru(III)], and [H⁺]. The reaction also has a negative fractional-order (−0.35) behavior in the reduction product of CAT, p-toluenesulfonamide (PTS). The increase in MeOH content of the solvent medium retards the rate. The variation of ionic strength of the medium has negligible effect on the rate. Rate studies in D₂O medium show that the solvent isotope effect, k′H₂O/k′D₂O, is equal to 0.60. Proton inventory studies have been made in H₂O(SINGLEBOND)D₂O mixtures. The rates correlate satisfactorily with Hammett σ relationship. The LFE relationship plot is biphasic and the reaction constant ρ=−2.3 for electron donating groups and ρ=−0.32 for electron withdrawing groups at 35°C. Activation parameters ΔH^≠, ΔS^≠, and ΔG^≠ have been calculated. The parameters, ΔH^≠ and ΔS^≠, are linearly related with an isokinetic temperature β=334 K indicating enthalpy as a controlling factor. A mechanism consistent with the observed kinetics has been proposed. © 1997 John Wiley & Sons, Inc.