Phenolic antioxidants: electrochemical behavior and the mechanistic elements underlying their anodic oxidation reaction.

Electrochemical analysis has been widely used to assess activities of reductant antioxidants, but the mechanistic factors that determine reducing ability and their corresponding correlations remain to be explored further. In the present paper, the reactivity of a selection of phenolic compounds in anodic oxidation was investigated by cyclic voltammetry (CV). The effects of electron-donation ability, deprotonation equilibrium, stability of radicals and chemical hardness on reducing capacity were studied by computational chemistry methods and multiple linear regression analysis. The results suggested that all these factors made significant contributions to reducing potency although electron-donation ability of the parent molecule plays the most important role in the electrode reaction. With the majority of the compounds examined, the occurrence of multiple electrochemical reactions leading to irreversible anodic oxidation was implied by: (i) the characteristics of the radical cation intermediate; (ii) the propensity of the deprotonation reaction; and (iii) the reactivity of the parent molecule. Upon correlation analysis of oxidation potential and computed physicochemical descriptors, some new light was shed on the mechanism by which phenolic compounds act as antioxidant reductants in free radical reactions. A satisfactory multi-descriptor theoretical QSAR model was derived, which might be of predictive significance in the screening or design of new antioxidants.