Kinetics and mechanism of oxidation of aromatic aldehydes by peroxomonosulphate

Abstract

The kinetics of oxidation of benzaldehydes by peroxomonosulphate (PMS) have been measured in aqueous acetic acid medium (30–70%v/v) in the presence of HClO₄(0.10–1.00 mol dm^–3) at constant ionic strength (1.20 mol dm^–3) over the temperature range 10–60 °C. The reactions have been found to obey the rate law (i) for the disappearance of PMS, indicating the concurrent occurrence of both the Rate ={k_a[H⁺]+k₀}[PMS][aldehyde](i) acid-dependent and acid-independent reaction paths. Oxidation of substituted benzaldehydes afforded phenols by aryl migration and benzoic acids by hydride migration. The characteristic ρ values, +1.81 for hydride and –1.70 for aryl migration, are discussed in terms of the mechanism of oxidation involving the fast nucleophilic addition of peroxide to the carbonyl group followed by the rate-determining acidcatalysed and uncatalysed reaction paths, the other experimental observations also being taken into account. The product distribution and the difference in the reactivity of peroxomonosulphate observed in the present investigation from those of the corresponding reactions of peroxomonophosphate are attributed to the better oxidising power of, and the better leaving group (SO₄^2– or HSO₄^–) from, peroxomonosulphate. Thermodynamic parameters have been evaluated to substantiate the mechanism proposed.