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 N 2 O-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 × 10 9 dm 3 mol Ϫ1 s Ϫ1 . This dioxygen addition reaction was found to be practically irreversible, in contrast to other hydroxycyclohexadienyl radicals. The so-formed dihydroxycyclohexadienylperoxyl radicals subsequently eliminate HO 2 ؒ /O 2 Ϫ . By using tetranitromethane (TNM) as a probe for O 2 Ϫ (formation of the nitroform anion), the overall rate constant of HO 2 ؒ -elimination (mainly of the para-and ortho-isomers, formation of hydroquinone and catechol, respectively) was determined to be 1.3 × 10 5 s Ϫ1 . 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 × 10 3 ϩ 1.7 × 10 9 × [H ϩ ] s Ϫ1 , and that of the o-OH-adduct to be k 6 = 1.1 × 10 8 × [H ϩ ] s Ϫ1 (the uncatalysed reaction is too slow to be measurable by this technique). The HPO 4 2Ϫcatalysed dehydration rate constant of the p-OH-adduct was similarly determined to be 5.8 × 10 7 dm 3 mol Ϫ1 s Ϫ1 . 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 × 10 9 dm 3 mol Ϫ1 s Ϫ1 , and similarly that of the ortho-OH-adduct (on the yield of catechol) to be 2.1 × 10 8 dm 3 mol Ϫ1 s Ϫ1 .

Variable products in dielectric-barrier discharge assisted benzene oxidation

“…On the other hand, at higher temperatures, a similar direct reaction, While the radical-radical addition reaction (16) possibly falls in the minor category (due to prevailing low concentration of the either species, thus may arise only from a geminate recombination of reaction (9) products), oxygenation of the • OH adduct in reaction (12) would be an important channel for phenol generation. Similar reaction mechanism in condensed phase is reported in the literature [43]. The sequential steps in it are (a) formation of the 5-hydroxy-(1,3)-cyclohexadienyl radical, C 6 with H from C 5 ) generating phenol, C 6 H 5 OH.…”

Consecutive reactions of aromatic-OH adducts with NO, NO<sub>2</sub> and O<sub>2</sub>: benzene, naphthalene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol and aniline

“…This must immediately form HO 2 since a high yield of prompt HO 2 is observed in our FP/RF experiments employing cycling of radicals . Such a release of HO 2 radicals has been observed in aqueous-phase reactions of O 2 with a large number of OH-adducts (Fang et al, 1995) including toluene-OH, benzene-OH (Pan et al, 1993) and phenol-OH (Mvula et al, 2001) and awaits to be studied in the gas phase in the absence of NO x . The almost temperature-independent yield of phenol from benzene, determined between 286 and 306 K by Volkamer et al (2002a) and the enthalpy data of Lay et al (1996), scaled to the experimental value for k 7 /k −7 by Bohn and Zetsch (1999), imply that the direct channel contributes a fraction of 0.8 to the effective Reaction (6) and that the isomerisation of the bicyclic peroxy radical, releasing HO 2 amounts to a fraction of 0.2, see Volkamer et al (2002a).…”

Degradation of catechol by ionizing radiation, ozone and the combined process ozone-electron-beam

“…A similar reactivity as for hydroquinone can be expected for catechol. For phenol degradation by ozone it was estimated that the indirect pathway via OH radicals is contributing to about 30% (Mvula et al, 2001).…”