KINETIC MODELING OF THE PHOTOOXIDATION OF DIMETHYLDISULFIDE IN THE LIQUID PHASE

A reaction mechanism for the photooxidation of dimethyldisulfide (DMDS) in aqueous acetonitrile has been established by kinetic modeling the UV absorbance vs. time curves under continuous irradiation. The model, built according to the known solution reactivity of oxysulfur radicals [1], consists of 22 steps involving 6 radical and 10 nonradical species. The first steps of the mechanism are the homolytic cleavage of the DMDS S—S bond with formation of methanethiyl radicals (CH3S·) followed by addition of these radicals to molecular oxygen. There are photoequilibria between thiyl (CH3S·), sulfinyl (CH3S·), and sulfonyl (CH3SO2·) radicals and the corresponding molecular species (methyl methanethiosulfinate CH3S(O)SCH3 or MMTSI, methyl methanethiosulfonate CH3S(O)2SCH3 or MMTS and meth-anesulfinic acid CH3S(O)OH or MSIA) which appear as long lived intermediates. Reactions of sulfonyl radicals with oxygen lead to methanesulfonic acid (CH3S(O)2OH) or MSA. Cleavage of sulfonyl radicals gives SO2 and CH3·, the parent compounds of sulfuric (H2SO4) and methanoic (HCOOH) acids. The predictive power of the model was tested at higher initial concentration of DMDS in anhydrous and aqueous acetonitrile. In these conditions, the proposed mechanism gives a semiquantitative description of the course of the reaction and reproduces the kinetic behavior of the long lived intermediates. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 825–834, 1997