MECHANISM OF OXIDATION OF ALIPHATIC THIOETHERS TO SULFOXIDES BY HYDROXYL RADICALS - THE IMPORTANCE OF MOLECULAR-OXYGEN

The sulfoxide yield in the .OH radical-induced oxidation of dimethylsulfide in aqueous solution is dramatically enhanced by admission of molecular oxygen. Various possible mechanistic pathways leading to sulfoxide can now be eliminated from speculation as a result of a detailed study on the influence of sulfide concentration, pH, isotope effects, radical cation stability and deprotonation kinetics, and electron-transfer reactions from transient radicals to special additives. The major route to sulfoxide is demonstrated to involve a 2sigma/1sigma* three-electron-bonded radical cation, (>S:S<)+, which, after reaction with OH- (H2O), generates a transient neutral sulfuranyl-type radical, >S(OH)S.<. The latter is prone to O2-addition, and the thioperoxyl radical, >S(OH)S(OO.)<, generated thereby decays intramolecularly into one molecule each of sulfoxide, sulfide, and superoxide. The mechanism may be generalized to the free radical-induced oxidation of any sulfide (at least aliphatic ones) and would thus potentially be of interest, for example, for the stability of sulfide-based drugs or biological material such as methionine-containing peptides and proteins.