Mechanisms of peroxide decomposition: an electron paramagnetic resonance study of the reaction of the peroxomonosulphate anion (HOOSO –3) with CuI. A marked contrast in behaviour with that of TiIII and FeII

Abstract

EPR spectroscopy has been employed in conjunction with a continuous-flow system to show that Cu^I(produced from the rapid reaction of Cu^II with Ti^III) reacts rapidly with the peroxomonosulphate anion (k≈ 2 × 10⁴ dm³ mol^–1 s^–1 at pH 2 and 20 °C) to give the hydroxyl radical (HO˙) which is trapped via its addition to alkenes. In contrast, Fe^II and Ti^III react with HOOSO^–₃ predominantly to give the sulphate radical-anion (SO^˙–₄). It is argued that with Cu^I the reaction shows characteristics of an outer-sphere electron-transfer process, whereas in the inner-sphere reaction of Fe^II and Ti^III the reaction is facilitated by attack of the hydroxyl oxygen atom on the smaller, more highly charged metal ions.

Evidence is also presented that radicals formed by the addition of both HO˙ and SO^˙–₄ to alkenes (˙CR¹R²CR³R⁴Y, Y = OH, OSO^–₃) are readily oxidized by HOOSO^–₃(k≈ 10⁵ dm³ mol^–1 s^–1), a reaction which leads to the formation of SO^˙–₄.