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 radica ls [1], consists of 22 steps involving 6 radical and 10 nonradical spe cies. The first steps of the mechanism are the homolytic cleavage of t he DMDS S-S bond with formation of methanethiyl radicals (CH3S.) follo wed by addition of these radicals to molecular oxygen. There are photo equilibria between thiyl (CH3S.), sulfinyl (CH3S.), and sulfonyl (CH3S O2.) radicals and the corresponding molecular species (methyl methanet hiosulfinate CH3S(O)SCH3 or MMTSI, methyl methanethiosulfonate CH3S(O) (2)SCH3 or MMTS and methanesulfinic acid CH3S(O)OH or MSIA) which appe ar as long lived intermediates. Reactions of sulfonyl radicals with ox ygen lead to methanesulfonic acid (CH3S(O)(2)OH) or MSA. Cleavage of s ulfonyl 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 a queous acetonitrile. In these conditions, the proposed mechanism gives a semiquantitative description of the course of the reaction and repr oduces the kinetic behavior of the long lived intermediates. (C) 1997 John Wiley & Sons, Inc.