The oxidation of arenethiols by peroxides
The oxidation of 4-substituted-benzenethiolate ions by hydrogen peroxide or t-butyl hydroperoxide gave the corresponding disulphide, sulphinic acid and sulphonic acid. With hydrogen peroxide the oxidation was of the second-order with p = -0.71 and showed a small positive salt effect. Benzenethiolate ion gave a small increase in rate with increasing solvent polarity and in 50% v/v dioxan at 25.0°C had ΔHdag 54.7 kJ mol-1, ΔS^dag -52.2 J mol^-1K^-1 and ΔGdag 70.2 kJ mol-1. 2-Nitrobenzenethiolate similarlygave ΔH^dag 39.7 kJ mol^-1, ΔSdag-113.2 J mol-1K-1 and ΔG^dag 73.4 kJ mol^-1 and the rate was not affected by added radical traps. The oxidation of these arene-thiolate ions was entropy controlled above 255.4 K. These oxidations are proposed to be polar and to proceed initially by a nucleophilic attack of the thiolate ion on the peroxide producing a sulphenic acid intermediate. Disulphide and sulphinic acid are formed by further reaction of the sulphenic acid. Disulphides give the same products on oxidation but are not necessary intermediates. Undissociated 4-substituted-thiols give only the corresponding disulphides under these conditions. Oxidation of benzenethiolate ion with t-butyl hydroperoxide gave unusual rate curves, a negligible solvent effect and apparently a large positive ΔSdag value. The mechanism of this oxidation differs from that previously described. 2,4-Dinitrobenzenethiolate ion gave anomalous rates of oxidation. With hydrogen peroxide (2 equivalents) the main product was the sulphinic acid, a positive salt effect was observed and excess hydroxide ion further increased the rate. The reaction may involve general base catalysis and a similar mechanism to that previously given. With t-butyl hydroperoxide (2 equivalents), less sulphinic acid was obtained together with large yields of di-(2,4-dinitrophenyl) sulphide and 2,4-dinitrophenol, presumably formed from the sulphinic acid. Added hydroxide ion markedly decreased the rate but a negative salt effect was observed. A greater difference in the mechanism is apparent in this case. The initial rate of aerial oxidation of benzenethiolate ion is of the same order of magnitude as that attributed to the medium in the peroxide oxidations and may be a radical process.