The photo-oxidation of 2,6-dimethylphenol and monophenylphenols by uranyl ion in aqueous solution

Abstract

The photo-oxidation of 2,6-dimethylphenol and o-, m- and p-phenylphenols by uranyl ion was investigated in aqueous solution. Steady state and dynamic luminescence quenching studies at pH 0.8 show the rapid dynamic deactivation of excited uranyl ions by the phenols. At the natural pH of uranyl salt solutions (pH 2.3), differences are observed between the steady state and dynamic quenching behaviour, and it is suggested that these differences are due to uranyl hydrolysis. Flash photolysis studies with uranyl ion in the presence of 2,6-dimethylphenol and m- and p-phenylphenols show that the initial photoreaction leads to phenoxyl radical formation. The photolysis products were identified by high performance liquid chromatography (HPLC) and UV absorption spectroscopy. With 2,6-dimethylphenol in aerated solution, both quinone and dimer formation are observed. Kinetic studies show that these processes occur concurrently. In contrast, photolysis of degassed solutions leads to dimer formation only. The quantum yields for these processes are reported. The photo-oxidation of aerated solutions of o-phenylphenol in the presence of uranyl ions leads to the production of two dimers and the quinone, whereas with degassed solutions only the dimers are observed. The photo-oxidation of these substrates by uranyl ion is contrasted with the behaviour of [Co(NH 3) 5N 3] 2+ as photooxidant of the same substrates. With m-phenylphenol, quinone and dimers are observed in aerated solution, whereas only the dimer is observed in deoxygenated solution. With p-phenylphenol, only dimer formation is observed. Possible mechanistic origins of the differences in the selectivity of oxidation by the different metal complexes are discussed.