NUCLEOPHILIC-SUBSTITUTION REACTIONS OF DIHALOMETHANES WITH HYDROGEN-SULFIDE SPECIES

Measurements of CH2Br2, CH2BrCl, and CH2Cl2 reaction kinetics as a function of pH and temperature indicate that HS--promoted reactions exceed hydrolysis rates at HS- concentrations greater than 2-17 muM, well within ranges common in sulfate-reducing environments. Experiments suggest that polysulfide-mediated reaction rates may exceed HS--mediated rates in waters in equilibrium with orthorhombic S8 at pH values greater than 7.0. The Swain-Scott model of nucleophilic reactivity underestimates rates at which these dihalomethanes react with HS-. For CH2Cl2, the observed reaction rate is 5 times the predicted rate; greater discrepancies are observed with the "softer" substrates CH2BrCl and CH2Br2, for which ratios of k(obs)/k(pred) are approximately 10-30. Examination of reaction products by FT-IR, solid-state C-13 NMR, GC-MS, and elemental analysis indicates the major product to be poly(thiomethylene), although minor amounts of dithiomethane were also observed. Published data suggest such thiols possess toxic properties. Reactions of dihalomethanes with HS- may thus yield products of greater toxicological significance than the parent compounds.