Reactions between copper(II) and 2-mercaptosuccinic acid in aqueous perchlorate solution

Abstract

The anaerobic oxidation of 2-mercaptosuccinic acid (H₃L) by copper(II) in aqueous acidic perchlorate media (pH 2–4) proceeds by the formation of a sulphur-bonded transient [CuL]^– which absorbs maximally at 350 nm with an absorption coefficient of 1 600 ± 50 dm³ mol^–1 cm^–1 In conditions of a large excess of substrate the major redox path involves the acid-catalysed dimerization of [CuL]^–, equation (i), with a rate constant k₃= 3 dm³[Cu(HL)]+[CuL]^–→[Cu₂HL₂]^–(i) mol^–1 s^–l at /= 0.1 mol dm^–3 and 25 °C. Subsequent attack by substrate according to the overall equation (ii)[Cu₂HL₂]^–+ 2H₃L →[Cu₂(H₂L)₂]+[H₂L′–L′H]^–(ii) results in formation of the corresponding disulphide and a copper(I) dimer which is fully formed when the ratio [H₃L]₀ : [Cu²⁺]₀ exceeds 2 : 1. The mechanism is discussed in terms of the formation of a two-electron redox template which allows direct production of disulphide without formation of high-energy radical species. Various side reactions are noted.