PRIMARY RADICALS IN THE PHOTOOXIDATION OF AROMATICS - REACTIONS OF XYLENOLS WITH (OH)-O-CENTER-DOT, N-3(CENTER-DOT) AND H-CENTER-DOT RADICALS AND FORMATION AND CHARACTERIZATION OF DIMETHYLPHENOXYL, DIHYDROXYDIMETHYLCYCLOHEXADIENYL AND HYDROXYDIMETHYLCYCLOHEXADIENYL RADICALS BY PULSE-RADIOLYSIS

Photo-oxidations of environmental organics in illuminated TiO2 dispers ions have implicated surface-bound (OH)-O-. radicals and/or valence ba nd holes. To explore the implications of the former oxidizing entity, six isomeric xylenols (dimethylphenols) were examined by pulsed (nanos econds to milliseconds) radiolysis methods. The spectral and kinetic c haracteristics of formation and decay of the transients formed by the reaction of N-3 ., (OH)-O-. and H-. radicals with these xylenols were assessed in buffered (pH 4, 10(-3) M phosphate) aqueous media, where t he xylenols exist in their protonated form (pK approximate to 10.19-10 .65). The products from the reaction of N-3(.) with 2,6- and 3,4-xylen ol were exclusively the corresponding dimethylphenoxyl radicals, forme d via electron transfer followed by deprotonation. In contrast, except with 3,4-xylenol, the principal radical intermediates formed initiall y upon reaction with (OH)-O-. were the corresponding (OH)-O-. adducts, the dihydroxydimethylcyclohexadienyl radicals. 3,4-Xylenol was examin ed in the pH range 4-10. At pH less than or equal to 8 the initial (OH )-O-. adduct (dihydroxy-3,4-dimethylcyclohexadienyl radical) was subse quently transformed (about 20%-40%) via water elimination into the dim ethylphenoxyl radical. In contrast, at pH 9 and 10 the (OH)-O-. adduct and the dimethylphenoxyl radical were formed concurrently (about 60% (OH)-O-. adduct and about 40% dimethylphenoxyl species), the latter th rough an inner-sphere electron transfer pathway. The switch in behavio ur from pH 8 to pH 9 suggests that the pK(a) of the dihydroxy-3,4-dime thylcyclohexadienyl radical is about 8-9, about 2 pK units below the p K(a) of the parent substrate (10.4). A mechanism for the conversion of the (OH)-O-. adduct to the dimethylphenoxyl radical is proposed. Reac tion of 2,6-xylenol with H-. radicals gave exclusively the H-. adduct (hydroxycyclohexadienyl radical), whose spectral characteristics are s imilar to those of the related (OH)-O-. adduct.