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 λ_max 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.