Photochemical behaviour of 2,6-dimethylbenzoquinone in the absence and in the presence of iron(III)

The photochemical behaviour of 2,6-dimethylbenzoquinone (DMBQ) in aqueous solution was investigated in the absence and in the presence of iron(III). Upon excitation at 365 nm, the photodegradation of DMBQ leads to the formation of 2,6-dimethylhydroquinone (DMHQ) and of 3-hydroxy-2,6-dimethylbenzoquinone (OHDMBQ) with a quantum yield of 0.47. When irradiated in the presence of iron(III), even in a large excess, the major route of DMBQ degradation remains the direct photolysis but subsequent thermal oxidation of DMHQ by iron(III) occurs which gives back the starting DMBQ. In this case, OHDMBQ appears to be the only photoproduct. However, traces of a second photoproduct are detected. Its formation results from the attack of OHDMBQ by 'OH radicals arising from Fe(OH)2+∗.