Mercaptans autoxidation

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

Autoxidation of mercaptans gives rise to thiyl radicals, but these, like phenoxy radicals, are inert toward oxygen and normally dimerize to disulfides. Their participation in a chain reaction can be achieved in the co-oxidation of olefins and mercaptans, first demonstrated by Khar-asch (12), which takes advantage of the rapid addition of thiyl radicals to double bonds. 

Alternatively, the fast autoxidation of mercaptans is achieved by working in basic solution and in the presence of transition metal catalysts, via reactions involving RS" anions and redox steps, much as for phenols (22). 

Oxidation of Mercaptans by Ov. Mercaptans are autoxidized in the presence of 02 in alkaline medium. In general, the oxidation is slow in the absence of catalyst because of unfavorable spin state symmetries that result from differences in the electronic configuration of the reactants (54). However, the reaction proceeds rapidly in the presence of traces of metal ions or transition metal phthalocyanines (55—58). The catalyst tends to alter the electronic structure of either the reductant and/or 02 so as to surmount the activation energy barrier imposed on the reaction by spin-state symmetry restriction. The coupled oxidation system in the presence of catalyst can be represented by ... 

Hydrogen peroxide and mercaptan radicals, RS, have been identified as reaction intermediates in the autoxidation of both substrates. These intermediates react further to produce the corresponding disulfide, RSSR, as the final product of the autoxidation. 

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