Solvent Effects in the Reactions of Peroxyl Radicals with Organic Reductants. Evidence for Proton-Transfer-Mediated Electron Transfer

Article abstract of DOI:10.1021/j100359a025

Absolute rate constants for the reaction of substituted methylperoxyl radicals with ascorbate, urate, trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) and TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) have been determined by pulse radiolysis in different solvents.In water-alcohol or water-dioxane solutions, the rate constants for trihalomethylperoxyl radicals generally increase with increasing water content.The rate constant for the reaction of CCl3O2* radicals with trolox was measured in water, MeOH, i-PrOH, t-BuOH, ethylene glycol, diethyl ether, dioxane, acetone, acetonitrile, formamide, dimethylformamide, pyridie, and CCl4.The rate constants were found to correlate well with a two-parameter equation that includes the dielectric constant of the solvent and the coordinate covalency parameter, a measure of the proton-transfer basicity of the solvent.Kinetic isotope effects in H2O/D2O of about 2 and the activation entropies of about -10 eu for reduction of RO2* by the organic reductants indicate that electron transfer to the peroxyl radical is concerted with the transfer of proton from the solvent to the incipient hydroperoxide anion.