Primary radicals in the photo-oxidation of aromatics — reactions of xylenols with •OH, N 3• and H • radicals and formation and characterization of dimethylphenoxyl, dihydroxydimethylcyclohexadienyl and hydroxydimethylcyclohexadienyl radicals by pulse radiolysis

Abstract

Photo-oxidations of environmental organics in illuminated TiO 2 dispersions have implicated surface-bound •OH radicals and/or valence band holes. To explore the implications of the former oxidizing entity, six isomeric xylenols (dimethylphenols) were examined by pulsed (nanoseconds to milliseconds) radiolysis methods. The spectral and kinetic characteristics of formation and decay of the transients formed by the reaction of N 3 •, •OH and H • radicals with these xylenols were assessed in buffered (pH 4, 10 −3 M phosphate) aqueous media, where the xylenols exist in their protonated form (p K ≈ 10.19–10.65). The products from the reaction of N 3 • with 2,6- and 3,4-xylenol were exclusively the corresponding dimethylphenoxyl radicals, formed via electron transfer followed by deprotonation. In contrast, except with 3,4-xylenol, the principal radical intermediates formed initially upon reaction with •OH were the corresponding •OH adducts, the dihydroxydimethylcyclohexadienyl radicals. 3,4-Xylenol was examined in the pH range 4–10. At pH ⩽ 8 the initial •OH adduct (dihydroxy-3,4-dimethylcyclohexadienyl radical) was subsequently transformed (about 20%–40%) via water elimination into the dimethylphenoxyl radical. In contrast, at pH 9 and 10 the •OH adduct and the dimethylphenoxyl radical were formed concurrently (about 60% •OH adduct and about 40% dimethylphenoxyl species), the latter through an inner-sphere electron transfer pathway. The switch in behaviour from pH 8 to pH 9 suggests that the p K a of the dihydroxy-3,4-dimethylcyclohexadienyl radical is about 8–9, about 2 p K units below the p K a of the parent substrate (10.4). A mechanism for the conversion of the •OH adduct to the dimethylphenoxyl radical is proposed. Reaction of 2,6-xylenol with H • radicals gave exclusively the H • adduct (hydroxycyclohexadienyl radical), whose spectral characteristics are similar to those of the related •OH adduct.