Mechanistic information on the copper-catalysed autoxidation of mercaptosuccinic acid in aqueous solution

Copper(II) ions react rapidly with sulfur from thiol groups, forming two distinct, intensely absorbing, short-lived intermediates, which decompose in a subsequent redox reaction to produce reduced copper and disulfides. In this study we report the results of a mechanistic study on the reaction between mercaptosuccinic acid, HO(2)CCH(2)CH(SH)CO(2)H, and Cu(2+)(aq) and [Cu(tren)H(2)O](2+), tren = tris(2-aminoethyl)amine. Spectroscopic and kinetic data indicate that in the presence of an excess of thiol, at least two distinct complexes are formed, with very different decomposition rate constants and an absorption maximum at 346 nm. Upon addition of thiol to [Cu(tren)H(2)O](2+)(1:1), a transient with a maximum at 380 nm appears, whereas in an excess of thiol this complex decomposes and again the 346 nm band is observed. The use of [Cu(tren)H(2)O](2+) enables to study the reaction of thiol with copper also in alkaline solution, where the rate of the overall process is slowed down greatly. The reactions were studied in detail, including the effect of dioxygen, and a possible reaction mechanism for the catalysed autoxidation process is proposed and discussed in reference to available literature data.