Reactions of phenol-OH-adduct radicals. Phenoxyl radical formation by water elimination vs. oxidation by dioxygen

Abstract

The reactions of OH radicals generated radiolytically in N₂O-saturated aqueous solutions with phenol have been examined, focusing special attention on the addition of dioxygen and the competing dehydration reactions of the OH-adduct radicals. Using the pulse radiolysis technique, the overall rate constant of dioxygen addition to the dihydroxycyclohexadienyl radicals was determined to be k = 1.2 × 10⁹ dm³ mol⁻¹ s⁻¹. This dioxygen addition reaction was found to be practically irreversible, in contrast to other hydroxycyclohexadienyl radicals. The so-formed dihydroxycyclohexadienylperoxyl radicals subsequently eliminate HO₂˙/O₂˙⁻. By using tetranitromethane (TNM) as a probe for O₂˙⁻ (formation of the nitroform anion), the overall rate constant of HO₂˙-elimination (mainly of the para- and ortho-isomers, formation of hydroquinone and catechol, respectively) was determined to be 1.3 × 10⁵ s⁻¹. The rate constant of the dehydration of the p-OH-adduct in neutral to acidic solution was determined by monitoring the absorbance build-up of the phenoxyl radical to be k₇ = 1.8 × 10³ + 1.7 × 10⁹ × [H⁺] s⁻¹, and that of the o-OH-adduct to be k₆ = 1.1 × 10⁸ × [H⁺] s⁻¹ (the uncatalysed reaction is too slow to be measurable by this technique). The HPO₄²⁻-catalysed dehydration rate constant of the p-OH-adduct was similarly determined to be 5.8 × 10⁷ dm³ mol⁻¹ s⁻¹. Based on the above rate constant of dioxygen addition, the rate constant of the proton-catalysed dehydration of the p-OH-adduct was determined by the competition of these two reactions on the yield of hydroquinone to be 1.0 × 10⁹ dm³ mol⁻¹ s⁻¹, and similarly that of the ortho-OH-adduct (on the yield of catechol) to be 2.1 × 10⁸ dm³ mol⁻¹ s⁻¹.