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

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 x 10(9) dm(3) 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./O-2(.-). By using tetranitromethane (TNM) as a probe for O-2(.-) (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 x 10(5) 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 k(7) = 1.8 x 10(3) + 1.7 x 10(9) x [H+] s(-1), and that of the o-OH-adduct to be k(6) = 1.1 x 10(8) x [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 x 10(7) dm(3) 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 x 10(9) dm(3) mol(-1) s(-1), and similarly that of the ortho-OH-adduct (on the yield of catechol) to be 2.1 x 10(8) dm(3) mol(-1) s(-1).