Oxidation of sulphur-containing compounds

Mercaptans are oxidised to disulphides by peroxodisulphate. Eager and Winkler studied the kinetics of the oxidations of n-butyl, n-octyl, and n-dodecyl mercaptans in acetic acid/water solvent (80 ml acid-I-20 ml water) and found first-order kinetics with respect to peroxodisulphate. The first-order rate coefficient increases with increase of mercaptan concentration, and reaches a limit at about 5 x 10 Af mercaptan. A decrease in the first-order rate coefficient with increase of the initial peroxodisulphate concentration was observed and attributed to a salt effect. Eager and Winkler suggested a mechanism involving sulphate radical ions. Levitt proposed a mechanism involving sulphur te-troxide, but there is no evidence for its formation in peroxodisulphate oxidations. 

Diphenyl sulphoxide, thiodiglycol sulphoxide, and diethyl sulphoxide are oxidised to the corresponding sulphones. Howard and Levitt studied the kinetics of oxidation of these compounds at pH 8 in a phosphate buffer (aqueous solution) and found the rates to be first-order with respect to peroxodisulphate and independent of the substrate concentration over the limited range employed (0.01-0.02 M). The removal of oxygen had no effect on the rate. Diethyl sulphide is oxidised very rapidly initially, then at the same rate as diethyl sulphoxide. Howard and Levitt concluded that the sulphide is first oxidised to the sulphoxide which in turn is oxidised to the sulphone, but Wilmarth and Haim point out that this interpretation cannot be correct, and conclude that the reaction must be more complex. 

The reaction is approximately first-order with respect to each reactant (the second-order rate coefficient increases with increase of substrate concentration), and catalysis by hydroxide ions is observed. Henderson and Winkler studied the ferrous ion-catalysed oxidation of thioglycolicacidto dithioglycolic acid. The rate is sensitive to traces of metal ions, and reproducible results could not be obtained in the absence of the catalyst. The oxidation is first-order with respect to both peroxodisulphate and ferrous ions, and zero-order with respect to the substrate. The second-order rate coefficient is approximately equal to that determined in the absence of the substrate, so Henderson and Winkler suggested that the ratedetermining step is the oxidation of ferrous to ferric ions, as in reaction (96), and that this is followed by reaction (97) and then rapid oxidation of thioglycolic acid by ferric ions. 

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The preparation and use of supported polyoxometal complexes with H2O2 for the oxidation of sulphur containing compounds to sulphoxides, sulphonic acids, and epoxidation of alkenes to epoxides. 

In conclusion, we have studied the supported polyoxometalate/H202/solvent system for the oxidation of sulphur containing compounds, and found that significant advantages in terms of product selectivity can be obtained when homogeneous species are immobilized. 

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