Radical spectra and product distribution following electrophilic attack by the OH˙ radical on 4-hydroxybenzoic acid and subsequent oxidation

Abstract

The distribution of electrophilic OH˙ radical addition to 4-hydroxybenzoic acid (HBA) has been determined by oxidizing the radical intermediates (substituted hydroxycyclohexadienyl radicals) with quinones and viologens, to yield products. It is deduced from product analysis using high-performance liquid chromatography that the fractions of OH˙ attack at the 1 : 2 : 3 : 4 positions of HBA are 0.16 : 0.04 : 0.65 : 0.15 respectively. Pulse radiolysis studies show that the rate of electron transfer from the radical intermediate formed by OH˙ addition to position 3 of HBA is dependent on the one-electron reduction potential of the oxidant. This electron-transfer process is in competition with the elimination of water by general acid–base catalysis to yield the phenoxyl radical. Catalysis by OH^– proceeds through the formation of the deprotonated species, pK_a= 8.4 ± 0.2, followed by the elimination of water, K_e= 3.0 ± 0.3 × 10⁵ s^–1. The addition of an OH˙ radical to position 3 of HBA gives rise to an absorption band centred at 365 nm.