The origin of base catalysis in the •OH oxidation of phenols in water

Time-resolved Raman and transient absorption studies on hydroxyl (•OH) radical reactions with aqueous 4-carboxyphenol (-O2C−PhOH), the substrate in p-hydroxybenzoate hydroxylase, have led to the first direct observation of the basic form of a dihydroxycyclohexadienyl (•OH-adduct of phenolate anion) radical and hydration-induced intramolecular electron transfer in this species. The base-catalyzed phenoxyl radical formation in this model system has been quantitatively described in terms of pKa of the phenolic proton (9.05) of the •OH-adduct and the rate of OH- elimination (2.9 × 106 s-1) from its basic form. A small fraction, 12(+2)%, of the phenoxyl radical is formed via water elimination from the •OH-adduct at the ipso position of the hydroxyl group (rate > 107s-1). About 3% •OH addition is seen at the carboxylic ipso position in basic solutions, which produces the p-benzosemiquinone radical anion (rate ∼ 106s-1). This work provides spectroscopic and kinetic evidence of the early chemical steps in the phenoxyl radical formation by •OH oxidation and establishes the precise relationship between the formation rate and pH. A relationship between the rates of OH- elimination from the •OH adducts of phenolate anions and pKa of the corresponding phenols is given.