Mechanistic significance of the inhibitory role of oxygen in Cu(II)-catalyzed oxidation of 2-mercaptoethylamine hydrochloride by methylene blue in methanol-water medium

The title reaction was investigated in methanol - water medium (5 % v / v) and in the presence of sulphuric acid under anaerobic conditions. The stoichiometry of the reaction is 2 : 1 (2-ME : MB) and the corresponding disulphide and leucobase are the end products. The order in methylene blue (MB) is unity at lower concentrations but the order changes to 3/2, at higher [MB]. Similarly, the order in MB is 3/2 at higher concentrations of 2-mercaptoethylamine hydrochloride (2-ME or ca. > 9.04 x 10(-3) mol/l) which changes to 2 at lower [2-ME]. The order in the substrate is 1/2. The rate constant shows an optimum with respect to [H+] while a fractional dependence of rate on [Cu2+] has been noticed. The rate decreases on increasing the ionic strength and decreasing the dielectric constant of the medium. The rate appreciably depends on the time of equilibration of the catalyst with 2-ME and shows a maximum at 60 min. The addition of the disulphide increases the rate while that of the leucobase retards it. The experiments were carried out under aerobic conditions to verify the proposed mechanism which postulates the symmetry restricted interaction of methylene blue dimer with Cu(II)-2-ME complex as the rate limiting step.