A pulse radiolysis study of the OH radical induced autoxidation of methanesulfinic acid

Methanesulfinic acid, CH3SO2H, reacts with OH radicals at pH 7 forming CH3SO2 radicals with a rate constant k = (6.0 ± 1.0) x 109 M-1 s-1. The CH3SO2 radical absorbs at 325 nm with an extinction coefficient of 900 ± 100 M-1 cm-1 and disappears in a second order self-reaction with k (1.0 ± 0.2) x 109 M-1s-1. This radical reacts with oxygen, k = (1.2 ± 0.3) x 109 M-1 s-1, forming a peroxy radical which absorbs in the UV below 300 nm. The peroxy radical reactions in turn with methanesulfinic acid reforming the CH3SO2 radical whereby a chain oxidation of sulfinic acid takes place. During the course of the chain oxidation a peroxyacid, presumably methaneperoxymonosulfonic acid, is formed and accumulated. This acid absorbs in the UV and eventually decays by reaction with excess methanesulfinic acid k = 5 x 103 M-1 s-1. The final product of the chain autooxidation is methanesulfonic acid. The chain is very efficient and proceeds until either oxygen or methanesulfinic acid is exhausted. The mechanism is compared to the chain autooxidation of sulphite. (author)