Mechanistic Studies of Oxidation of Thiols to Disulfides by Sodium N-Chloro-p-toluenesulfonamide in an Alkaline Medium: A Kinetic Approach

The kinetics of oxidation of five alkanethiols, viz., 1-ethanethiol, 1-propanethiol, 1-butanethiol, 1-pentanethiol, and 1-hexanethiol to symmetrical disulfides by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in the presence of NaOH has been investigated at 15 °C. Reactions in all the five cases followed the identical kinetics with a first-order dependence each on [CAT]o and [Thiol]o and fractional-order dependence on [OH-]. Addition of p-toluenesulfonamide, a reduction product, retards the rate, and the dielectric effect is negative. Variation of ionic strength and halide ions showed no effect on the rate. The solvent isotope effect was studied using heavy water. Reaction mixture failed to induce polymerization of acrylonitrile. Activation parameters have been computed from the reactions studied at different temperatures. A two-pathway mechanism is proposed to account for the kinetic data obtained in which the hypochlorous acid (HOCl) and the anion of CAT (CH3C6H4SO2NCl-) interact with the substrate in rate-limiting steps. An isokinetic relationship is observed with (beta) = 322 K, indicating that enthalpy factors are controlling the rate. The rate of oxidation of thiols follows the order: 1-hexanethiol > 1-pentanethiol > 1-butanethiol > 1-propanethiol > 1-ethanethiol. A Taft linear free energy relationship is obeyed for the reaction with (rho)* = -5.28 and -7.50 and (delta) = -2.00, indicating that the electron donating groups enhance the rate. The observed results have been explained by a two-pathway mechanism, and the related rate law has been deduced. The method adopted for the oxidation of thiols to disulfides in the present work offers several advantages and can be scaled up to industrial operation.