Reactions of phosphate radicals with substituted benzenes. A structure–reactivity correlation study

Abstract

The bimolecular rate constants (k) for the reactions of HPO₄⁻ radicals with phenol, fluorobenzene, chlorobenzene, bromobenzene, iodobenzene, benzonitrile, tert-butylbenzene and anisole (k = 5.3 × 10⁸, 7 × 10⁶, 6.9 × 10⁶, 4 × 10⁶, 1.2 × 10⁷, 6.5 × 10⁶, 1.0 × 10⁸ and 4.6 × 10⁷ M⁻¹ s⁻¹, respectively) have been determined by flash-photolysis of P₂O₈⁴⁻ solutions of pH 7.1 containing variable amounts of the organic substrates. The trend observed for the rate constants with the electron withdrawing ability of the substituents in the aromatic ring is discussed in terms of a Hammett type correlation. Phenoxyl type radical formation was observed in the reactions of HPO₄˙⁻ with chlorobenzene and phenol. Based on the experimental results, a possible reaction mechanism is proposed. Additional information on the decay kinetics of hydroxycyclohexadienyl radicals of chlorobenzene in the presence of molecular oxygen is also obtained from independent experiments with hydrogen peroxide solutions containing chlorobenzene under nitrogen, air and oxygen saturation.