Oxidation of hydroxybenzoic acids by ∙OH radical produced radiolytically

• ∙OH radical induces oxidation reactions in hydroxybenzoic acids.
• Radiolysis of o-, m- and p-hydroxybenzoic acid in aqueous solution.
• Distribution of radiolytic products from irradiated hydroxybenzoic acids in aqueous solution.
• Concerted effects of –COOH and –OH groups in the reaction of OH radicals in hydroxybenzoic acids.

The yields of the dihydroxybenzoic acids produced in the radiolytic oxidation of aqueous solutions of the hydroxybenzoic acids were determined to provide information on the concerted effects of –OH and –COOH substituents on the electronic structures of aromatics. Because ∙OH radical is a strong electrophyl it adds preferentially at the electron rich sites of an aromatic ring. The distribution of hydroxylated products, therefore, gives a measure of the charge distribution in the aromatic. In these studies the hydroxylated cyclohexadienyl radicals initially produced were oxidized to the corresponding dihydroxybenzoic acids by ferricyanide present in the irradiated solution. Product analysis was by chromatographic and electrophoretic methods. It is shown that, as in the case of phenol, the hydroxyl substituent has a dominant ortho-para directing effect that is modified somewhat by the presence of a –COOH substituent. Substantial yields for attack of ∙OH at the carboxy substituted sites of 2- and 4-hydroxybenzoic acid are also reported. This observation demonstrates that addition of ∙OH at the ipso positions of carboxylated aromatics can be of considerable importance.