The concurrence of photoreduction and bromination of 1,4-benzoquinone in aqueous solution

The 1,4-benzoquinone–bromine photoreaction is investigated with UV/Vis spectroscopy, mass spectrometry, and 1H NMR methods. The absorption and mass spectra suggest that 1,4-benzoquinone in aqueous solution reacts with bromine only in the presence of light, which leads to the production of brominated-1,4-hydroquinones. Parallel experiments performed with illuminated and unilluminated 1,4-hydroquinone–bromine reactions indicate that the bromination of the benzene ring takes place during rather than after the photoreduction of 1,4-benzoquinone. The results provide an explanation for the subtle photosensitivity seen in a number of acidic bromate–aromatic compound reactions.