Antioxidant activity of some simple phenols present in olive oil

The antioxidant activity of five phenols, i.e. catechol, homovanillyl alcohol, homovanillic acid, gallic acid and syringic acid, present in all olive oils has been investigated. For gallic acid and syringic acid the Bond Dissociation Enthalpy BDE(O-H) has been determined through the EPR (Electron Paramagnetic Resonance spectroscopy) based on radical equilibration technique as 81.0 and 82.1 kcal mol-1, respectively. For both homovanillyl alcohol and homovanillic acid the BDE(O-H) values were assumed to be identical to that of the similar 2-methoxy-4-methylphenol (82.8 kcal mol-1). The inhibition rate constants of the above compounds and of catechol in the autoxidation of cumene (or styrene) were also determined in acetonitrile/chlorobenzene solution and corrected for hydrogen bonding by acetonitrile. The results indicate that in organic solvents gallic acid and catechol are fairly good antioxidants, while homovanillyl alcohol, homovanillic acid and syringic acid, i.e. the three2-methoxy substituted derivatives, are much less efficient antioxidants.