Pulse radiolysis studies of aqueous phenol. Water elimination from dihydroxycyclohexadienyl radicals to form phenoxyl

Abstract

The reactions of e–aq, H· and OH· with phenol have been studied. The corresponding rate constants were 1.8 × 107, 1.8 × 109 and 1.4 × 1010 M–1 sec–1, respectively. The product detected after e–aq and H· attack was the H adduct (ε3300Å= 3800 M–1 cm–1). The initial product of OH· attack was the OH adduct (ε3300Å= 4400 M–1 cm–1). Both H and OH adducts are probably mixtures of isomers. OH adducts subsequently undergo either unimolecular elimination of water to form the more resonance-stabilized phenoxyl radical, ϕO·(ε4000Å= 2200 M–1 cm–1), or bimolecular radical-radical reactions. The elimination is catalyzed by H+, OH– and HPO2–4 ions. There are indications of different rates of catalysis for different isomers of the OH adduct. The uncatalyzed rate of elimination is ⩽103 sec–1. The elimination was also detected from OH adducts of p-cresol (to give p-CH3C6H4O-, ε4050Å= 2400 M–1 cm–1) and tyrosine. The ability to eliminate water appears to be a general property of OH adducts of phenols.