Free radical intermediates in the photoreduction of p-benzoquinone in ethanol solution

To gain deeper insight into the mechanism of photoreduction of p-benzoquinone in ethanol, free radicals from solvent ethanol have been studied by means of e.s.r. using phenyl-N-t-butyl nitrone as a spin trap, and compared with supplementary results for semiquinone intermediates and the final photoreduction products. Ethoxyl radical and p-benzosemiquinone anion have been detected as primary intermediates. The efficiency of their formation can be correlated with the absorption spectrum of the quinone. These results support an ‘anionic mechanism’ for the photoreduction of the quinone to the hydroquinone. The photo-induced one-electron transfer results in the formation of the ethanol positive ion and the semiquinone anion, the former is transformed into the ethoxyl radical by rapid proton transfer, the latter into a dinegative ion by a disproportionation reaction, and this is protonated to give the final product, hydroquinone.