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

摘要

The reactions of OH radicals generated radiolytically in N2O-saturated aqueous solutions with phenol have beennexamined, focusing special attention on the addition of dioxygen and the competing dehydration reactions of thenOH-adduct radicals. Using the pulse radiolysis technique, the overall rate constant of dioxygen addition to thendihydroxycyclohexadienyl radicals was determined to be k = 1.2 × 109 dm3 molu00021 su00021. This dioxygen addition reactionnwas found to be practically irreversible, in contrast to other hydroxycyclohexadienyl radicals. The so-formed dihydroxycyclohexadienylperoxylnradicals subsequently eliminate HO2nu0001/O2u0001u0002. By using tetranitromethane (TNM) as a probenfor O2u0001u0002 (formation of the nitroform anion), the overall rate constant of HO2nu0001-elimination (mainly of the para- andnortho-isomers, formation of hydroquinone and catechol, respectively) was determined to be 1.3 × 105 su00021. The ratenconstant of the dehydration of the p-OH-adduct in neutral to acidic solution was determined by monitoring thenabsorbance build-up of the phenoxyl radical to be k7 = 1.8 × 103 u0001 1.7 × 109 × [Hu0001] su00021, and that of the o-OH-adductnto be k6 = 1.1 × 108 × [Hu0001] su00021 (the uncatalysed reaction is too slow to be measurable by this technique). The HPO4n2u0002-ncatalysed dehydration rate constant of the p-OH-adduct was similarly determined to be 5.8 × 107 dm3 molu00021 su00021.nBased on the above rate constant of dioxygen addition, the rate constant of the proton-catalysed dehydration of thenp-OH-adduct was determined by the competition of these two reactions on the yield of hydroquinone to be 1.0 ×n109 dm3 molu00021 su00021, and similarly that of the ortho-OH-adduct (on the yield of catechol) to be 2.1 × 108 dm3 molu00021 su00021.