Primary processes in the reaction of OH·-radicals with sulphoxides

Abstract

The primary processes in the OH· radical-induced oxidation mechanism of sulphoxides have been investigated by pulse radiolysis and, in particular, by an improved conductivity detection method with a time resolution of ca. 50 ns in aqueous solution. Electrophilic addition of the OH· radical to the sulphoxide group leads to a transient adduct, R₂SO(OH)· which decays unimolecularly with t_1/2 up to 100 ns into a sulphinic acid, RSO₂H, and a radical R·. The various RSO₂H have been identified by their pK_a, and R·(including t-butyl and phenyl radicals) by direct optical measurement or chemical scavenging experiments. The probability of radical split-off from R₂SO(OH)· for mixed sulphoxides depends on the stability of the radical leaving. Depending on the nature of the sulphoxide substituents two other OH· radical reactions compete with, and may even predominate over, the addition at the sulphoxide group. Thus hydrogen-atom abstraction readily occurs from longer chain and branched aliphatic groups and in the presence of aromatic substituents OH· radicals add to the π-system to form a hydroxycyclohexadienyl radical. The respective yields, kinetics and some physico-chemical properties of the primary species are presented and discussed.