Formation and reactivity of phenylperoxyl radicals in aqueous solutions

The reaction of phenyl radicals with oxygen, to produce phenylperoxyl radicals, and the reactions of several phenylperoxyl radicals with a number of organic compounds in aqueous solutions have been studied by pulse radiolysis. Phenyl radicals were produced by reduction of aryl halides with hydrated electrons. The rate constant for the reaction of 4-carboxyphenyl with O₂ was determined from the rate of buildup of the peroxyl radical absorption at 520 nm as a function of [O₂] and found to be 1.6 × 10⁹ L mol^-1 s^-1. Phenyl radicals react with 2-PrOH by H abstraction; a rate constants of 4 × 10⁶ L mol^-1 s^-1 was determined for 4-carboxyphenyl by competition with the reaction of this radical with O₂. Phenylperoxyl radicals react with 4-methoxyphenolate ions, trolox C(6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), ascorbate ions, chlorpromazine, and ABTS [2,2′-azinobis(3-ethylbenzothiazoline-6-sulffonate ion)] by one-electron oxidation. The rate constants for such reactions, determined from the rate of formation of the one-electron oxidation product as a function of substrate concentration, were found to be near 10⁸-10⁹ L mol^-1 s^-1. The reaction with neutral phenols, however, was much slower and could not be observed under the pulse radiolysis conditions. Phenylperoxyl radicals are found to be much more reactive than methylperoxyl and more reactive than most substituted methylperoxyls, except for the halogen-substituted radicals. Electron-withdrawing substituents at the 4-position of phenylperoxyl increase the rate constant and electron-donating groups decrease the rate constant for oxidation by this radical, in accordance with the Hammett substituent constants.