MECHANISM OF THE CENTER-DOT-OH RADICAL-INDUCED DECARBOXYLATION OF 2-(ALKYLTHIO)ETHANOIC ACID-DERIVATIVES

The reactions of the hydroxyl radical with 2-(methylthio)ethanoic acid and 2,2'-thiodiethanoic acid have been investigated in H2O and D2O. The initial step is a formation of an OH adduct at the sulfur moiety (absorption maximum at lambda = 340 nm) with absolute rate constants of k(OH+2-MTEA) = 8.7 X 10(9) M(-1) s(-1) and k(OH+2.2'-TDEA) = 9.1 X 10(9) M(-1) s(-1). The subsequent decay pathways of these adducts strongly depend on pH but do not lead to the respective intermolecularly S therefore S-bonded dimeric radical cations even at high concentrations of solute (similar to 10(-2) M) and protons (similar to 10(-1) M). The S therefore S-bonded radical cations are typically formed upon oxidation of unsubstituted thioethers. Instead, very high radiation chemical yields of CO2 (G = 3.5-6.0) and of alpha-(alkylthio)-alkyl radicals are observed over the entire investigated pH region (1.0-7.5). Mechanistically, the formation of CO2 and the associated reaction kinetics including solvent kinetic isotope effects suggest the occurrence of an intramolecular electron transfer from the carboxyl group to the oxidized sulfur function followed by homolytic carbon-carboxyl bond breakage into carbon dioxide and the alpha-(alkylthio)alkyl radical. The (OH)-O-. radical-induced decarboxylation can receive part of its driving force from the resonance stabilization of the R-S-CH2. radical resulting from CO2 cleavage.