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

Conventional and multi-wavelength stopped-flow spectrophotometry has b een used to study the kinetics of the oxidation of dimethyl sulfide (D MS) by hydroperoxides, ROOH = hydrogen peroxide (H2O2), peroxo formic acid (HCO3H), peroxo acetic acid (CH3CO3H), peroxo nitrous acid (ONOOH ), peroxo monosulfuric acid anion (PMS = HSO5-), and the H2O2 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 di methyl sulfoxide (DMSO) and ROOH is a second-order process, leading to DMSO and dimethyl sulfone (DMSO2), respectively. It was shown by gas chromatography that, except for the oxidant ONOOH, DMSO and DMSO2 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 D MS and DMSO, respectively. The data for k(ROOH)(dm(3) mol(-1) s(-1)) a nd for k(ROO) (dm(3) mol(-1) s(-1)) at 293 K for the formation of DMSO and DMSO2 are presented. For the oxidation of DMS k(ROOH) > k(ROO); f or the step DMS --> DMSO, k(ROOH) ranges from 4780 (PMS) to 0.018 (H2O 2), whereas k(ROO) lies in the range 88 (PMS) to 0.0018 (H2O2); for th e step DMSO --> DMSO2, k(ROOH) ranges from 349 (HOCl) to 2.7 x 10(-6)( H2O2), whereas k(ROO) lies in the range 18 (PMS) to 8.4 x 10(-5) (H2O2 ). A mechanistic interpretation of the oxidation reactions, based on t he ambifunctional character of both ROOH and DMSO, is presented. The r elevance of in-cloud oxidation of Dh?IS by atmospheric hydroperoxides such as CH3CO3H and HCO3H is discussed and substantiated.