OH-Radical-induced oxidation of methanesulfinic acid. The reactions of the methanesulfonyl radical in the absence and presence of dioxygen

Abstract

Hydroxyl radicals were generated radiolytically in N₂O-saturated solutions. As shown by pulse radiolysis, methanesulfinic acid reacts with ˙OH (k = 5.3 × 10⁹ dm³ mol⁻¹ s⁻¹) and N₃˙ giving rise to an intermediate which has an absorption maximum at 330 nm (Gε ≈ 5.2 × 10⁻⁵ m² mol⁻¹). This is attributed to the methanesulfonyl radical, CH₃S(O)O˙. Pulse radiolysis with conductometric detection indicates that CH₃S(O)O˙ is formed in only ∼80% yield. This is confirmed by scavenging experiments with Fe(CN)₆⁴⁻, ascorbate and sulfite, which are oxidised by CH₃S(O)O˙ with rate constants of ∼2 × 10⁹ dm³ mol⁻¹ s⁻¹. Steady-state radiolysis of methanesulfinic acid shows that methanesulfonic acid is the major product (G = 2.1 × 10⁻⁷ mol J⁻¹). Further products are sulfate (0.7 × 10⁻⁷ mol J⁻¹), methane (0.3 × 10⁻⁷ mol J⁻¹), ethane (0.08 × 10⁻⁷ mol J⁻¹) and dimethyl sulfone (not quantified). It is suggested that an OH-adduct is formed initially which mainly eliminates OH⁻, but also decomposes yielding ˙CH₃ and bisulfite. The formation of methanesulfonic acid can be explained by a disproportionation of the methanesulfonyl radicals via recombination and subsequent hydrolysis. In the presence of dioxygen, a chain reaction occurs whereby 2 mol methanesulfonic acid are formed per 1 mol dioxygen consumed. G(methanesulfonic acid) ≈ 250 × 10⁻⁷ mol J⁻¹ was found to be independent of dose rate (0.011–0.165 Gy s⁻¹), methanesulfinic acid concentration [(0.1–4) × 10⁻³ mol dm⁻³] and dioxygen concentration. An efficient chain process was also observed upon electron beam irradiation [G(methanesulfonic acid) ≈ 200 × 10⁻⁷ mol J⁻¹ at 0.8 Gy per pulse and 75 × 10⁻⁷ mol J⁻¹ at 9 Gy per pulse (pulse duration 2 μs)]. It is proposed that the oxidation of the substrate by the methylsulfonylperoxyl radical, CH₃S(O₂)OO˙, to give the strongly oxidising CH₃S(O₂)O˙ radical, initiates the chain reaction, with the latter propagating the chain by reacting with a substrate molecule to give methanesulfonic acid and the methanesulfonyl radical. Branching and partial removal of the chain-carrying CH₃S(O₂)O˙ radicals by H-abstraction from the substrate is suggested as the likely path leading to chain termination. Oxidation of methanesulfinate by ozone (k = 2 × 10⁶ dm³ mol⁻¹ s⁻¹) occurs only by O-atom transfer, and an electron transfer that would start a chain reaction was not observed.