Radical cations from one-electron oxidation of aliphatic sulphoxides in aqueous solution. A radiation chemical study

Abstract

Sulphoxide radical cations, (R2SO)+˙, have been observed upon one-electron oxidation of simple aliphatic sulphoxides by strongly oxidizing radicals with redox potentials ≥+ 2 V, e.g. SO4–˙, (CH3I)+˙, (CH3I∴ICH3)+, or Ti2+ in pulse-irradiated aqueous solutions. They exhibit optical absorptions in the u.v. with X λmax depending on the substituent (e.g. 300, 320, and 330 nm for R = Me, Et, and Pr, respectively). Extinction coefficients are of the order of 103 mol–1 dm–3 cm–1. The sulphoxide radical cations exist only at low pH and are probably best formulated in terms of an adduct with one water molecule, (R2SO∴OH2)+. The pK values for the equilibrium (R2SO∴OH2)+⇌ R2SO(OH)˙+ H+ have been estimated to be 5.6, 6.1, and 6.5, for R = Me, Et, and Pr, respectively, from yield measurements. The neutral R2SO(OH)˙ is identical with the hydroxyl radical adduct to sulphoxides formed at any pH in the reaction of R2SO +˙OH, and decays irreversibly into R˙+ RSO2–+ H+. The sulphoxide radical cations are very good oxidants. Absolute rate constants have been measured for their reactions with a variety of electron donors, namely, organic sulphides, dithia compounds, disulphides, Br–, I–, and SCN–. The optical and kinetic results are discussed in the light of the electronic properties of the radical species.