Antioxidant properties of phenols

Abstract

The current understanding of the antioxidant properties of phenols (in homogeneous solutions) is reviewed, with particular emphasis on the role of the solvent. Phenols (ArOH) are known to reduce the rates of oxidation of organic matter by transferring a H atom (from their OH groups) to the chain-carrying ROO• radicals, a mechanism that most likely involves a concerted transfer of the hydrogen as a proton and of one electron between the two oxygen atoms, O-H•••O• (proton-coupled electron transfer mechanism). The antioxidant capabilities of phenols are strongly reduced by hydrogen-bond accepting solvents since the hydrogen-bonded molecules ArOH•••S are virtually unreactive toward ROO• radicals. The magnitude of these kinetic solvent effects is determined by the solute acidity α2H of ArOH (range 0 to 1) and solvent basicity β2H (range 0 to 1). Hydroxyl solvents (alcohols) have a double effect on ArOH. On the one hand, they act as hydrogen-bond accepting solvents and reduce the conventional rates of the ArOH + ROO• reaction. On the other hand, these solvents favour the ionization of ArOH into their phenoxide anions ArO−, which may react with ROO• very rapidly by electron transfer (sequential proton loss electron transfer mechanism). The overall effect is therefore determined by the ionization degree of ArOH. Other aspects of the kinetics and thermodynamics of ArOH + ROO• are also discussed.