Kinetics and mechanism of the oxidation of dimethyl sulfide by hydroperoxides in aqueous medium Study on the potential contribution of liquid-phase oxidation of dimethyl sulfide in the atmosphere

Abstract

Conventional and multi-wavelength stopped-flow spectrophotometry has been used to study the kinetics of the oxidation of dimethyl sulfide (DMS) by hydroperoxides, ROOH=hydrogen peroxide (H ₂ O ₂ ), peroxo formic acid (HCO ₃ H), peroxo acetic acid (CH ₃ CO ₃ H), peroxo nitrous acid (ONOOH), peroxo monosulfuric acid anion (PMS=HSO ₅ ^- ), and the H ₂ O ₂ analogue hypochlorous acid (HOCl), in aqueous solution in the pH range 0–14 at 293 K and I=1.0 M. The reaction between DMS and ROOH and between dimethyl sulfoxide (DMSO) and ROOH is a second-order process, leading to DMSO and dimethyl sulfone (DMSO ₂ ), respectively. It was shown by gas chromatography that, except for the oxidant ONOOH, DMSO and DMSO ₂ are the only oxidation products. It follows from the pH dependence of the second-order rate constant k ₂ that both the hydroperoxide ROOH, rate constant k _ROOH , and its anion ROO ^- , rate constant k _ROO , oxidize DMS and DMSO, respectively. The data for k _ROOH (dm3 mol ^-1 s ^-1 ) and for k _ROO (dm3 mol ^-1 s ^-1 ) at 293 K for the formation of DMSO and DMSO ₂ are presented. For the oxidation of DMS k _ROOH >k _ROO ; for the step DMS→DMSO, k _ROOH ranges from 4780 (PMS) to 0.018 (H ₂ O ₂ ), whereas k _ROO lies in the range 88 (PMS) to 0.0018 (H ₂ O ₂ ); for the step DMSO→DMSO ₂ , k _ROOH ranges from 349 (HOCl) to 2.7×10 ^-6 (H ₂ O ₂ ), whereas k _ROO lies in the range 18 (PMS) to 8.4×10 ^-5 (H ₂ O ₂ ). A mechanistic interpretation of the oxidation reactions, based on the ambifunctional character of both ROOH and DMSO, is presented. The relevance of in-cloud oxidation of DMS by atmospheric hydroperoxides such as CH ₃ CO ₃ H and HCO ₃ H is discussed and substantiated.