Photo-induced reactions—IX : The hydroxylation of phenols by the photo-decomposition of hydrogen peroxide in aqueous media

Abstract

The photolysis of hydrogen peroxide in the presence of phenols in aqueous solution was used for the hydroxylation of various phenols on a preparative scale. Irradiation with the light at 2537 Å resulted in the formation of ortho- and para-dihydroxy compounds as the main products. The ortho-hydroxylation is predominant. Dihydroxybenzenes were also hydroxylated. In the case of p-substituted phenols, only an ortho-hydroxylated derivative was obtained. p-Carboxy- and p-methoxy-phenols gave hydroquinone in addition to the usual catechol derivative. Photolysis under various conditions of p-cresol, gave the best yield of the catechol at pH 3–8. Irradiation of p-cresol in aqueous hydrogen peroxide with the light over 2800 Å at pH ca. 1, in the presence or the absence of uranyl sulphate, resulted in the formation of a mixture of dimers as the main products. The mechanisms, including hydrogen abstraction and addition of hydroxyl radicals, are discussed.