Studies on the interaction of selenite and selenium with sulphur donors. Part 2. A kinetic study of the reaction with 2-mercaptoethanol

Abstract

Selenous acid is reduced by 2-mercaptoethanol in water to elemental selenium in two stages.[Formula: see text]The reactions have been followed by spectrophotometry and by proton and 77Se NMR spectroscopy. The mechanism of the first stage parallels that proposed for butyl thiols in 60% dioxane but nucleophilic attack by RS− rather than RSH on the Se(IV) intermediate, RSSeO2−, is of greater significance for 2-mercaptoethanol than for n-butylthiol. The signals of three intermediates are observed in the Se-77 NMR spectra of a mixture undergoing the first reaction stage. These arise from, HOC2H4SSeO2−, (HOC2H4S)2SeO, and an asymmetric intermediate that chemical shift evidence suggests is the sulfinic acid ester, HOC2H4S(O)SeSC2H4OH. The second stage involves coordination of the bis(thio)selenide by thiolate followed by decomposition to disulfide and selenium. Evidence is presented to support a reaction mechanism involving nucleophilic attack exclusively at Se(II) and not at S(II) as suggested in previous work. This stage is strongly accelerated at basic pH and, at pH 8.3 and above, both the first and second stages can take place simultaneously.