Pulse radiolysis studies on redox reactions of gallic acid: one electron oxidation of gallic acid by gallic acid–OH adduct

Abstract

Using the pulse radiolysis technique, studies on reactions of 3,4,5-trihydroxybenzoic acid [gallic acid (GA)] with radical species generated in water are reported. At pH 6.8 and 9.7, OH radicals react with GA to give an adduct initially with rate constants of the order of 1×10¹⁰ d mol^-1 s^-1. This adduct then reacts with parent GA molecules with rate constants of the order 5×10⁸ d mol^-1 s^-1 to give phenoxyl type radical species having absorption maxima in the 350 nm region. At pH 12 and 13.6, OH/O^- radicals directly bring about oxidation of GA. Specific oxidants like azidyl radical bring about one electron oxidation at neutral and alkaline pHs with almost diffusion controlled rate constants. Rate constants for the reaction of Br₂^- radicals are found to be lower than those for OH radicals by an order of magnitude. At pH 0, both OH and Cl₂^- radicals react with GA to give phenoxyl type radicals. The phenoxyl radicals formed are quite stable at higher pHs, which make GA a good antioxidant. Rate constants for the reactions of e_aq^- with different protolytic forms of GA are determined.