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 (Ge ≈ 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.