Photocatalytic Oxidation of Toluene to Benzaldehyde by Molecular Oxygen

The visible light irradiation of aqueous solutions containing toluene, uranyl(VI) ions, and O2 results in the formation of benzaldehyde as a major product. Small amounts of PhCH2OH are also formed. The yields of benzaldehyde are 3 times greater for toluene-h8 than for toluene-d8, but the kinetic isotope effect for the quenching of the excited state *UO22+ by toluene is negligible (ktoluene-h8/ktoluene-d8 = 1.2). This and other evidence indicate that the quenching takes place in two parallel pathways. The major one involves the aromatic portion of toluene and leads to the recovery of the reactants. The minor, productive path takes place by hydrogen atom abstraction from the methyl group, followed by the oxidation of PhCH2•. Cumene, benzyl alcohol, and benzaldehyde react similarly.