STUDIES ON THE INTERACTION OF SELENITE AND SELENIUM WITH SULFUR DONORS .2. A KINETIC-STUDY OF THE REACTION WITH 2-MERCAPTOETHANOL

Selenous acid is reduced by 2-mercaptoethanol in water to elemental se lenium in two stages. H2SeO3 + 4HOC(2)H(4)SH --> HOC2H4SSeSC2H4OH + HO C2H4SSC2H4OH + 3H(2)O HOC2H4SSeSC2H4OH --> Se + HOC2H4SSC2H4OH The rea ctions have been followed by spectrophotometry and by proton and Se-77 NMR spectroscopy. The mechanism of the first stage parallels that pro posed for butyl thiols in 60% dioxane but nucleophilic attack by RS(-) rather than RSH on the Se(IV) intermediate, RSSeO(2)(-), is of greate r significance for 2-mercaptoethanol than for n-butylthiol. The signal s of three intermediates are observed in the Se-77 NMR spectra of a mi xture undergoing the first reaction stage. These arise from HOC2H4SSeO 2-, (HOC2H4S)(2)SeO, and an asymmetric intermediate that chemical shif t evidence suggests is the sulfinic acid ester, HOC2H4S(O)SeSC2H4OH. T he second stage involves coordination of the bis(thio)selenide by thio late followed by decomposition to disulfide and selenium. Evidence is presented to support a reaction mechanism involving nucleophilic attac k 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 ab ove, both the first and second stages can take place simultaneously.