The oxidation potentials of phenolic and amino antioxidants

Abstract

AbstractA simple and rapid polarographic method of measuring the relative oxidation potentials of organic compounds has been developed, and values for a large number of phenolic and amino compounds have been determined to find whether any relationship exists between oxidation potential and antioxidant activity. Buffered electrolytes at various pH levels were used and when the determined potentials are plotted against the pH of the electrolyte, most monohydric phenols give straight‐line graphs which correspond closely to the theoretical equation for oxidations in which hydrogen ions are involved in the electrode processE = E0 – 0·058 pH at 20° where E0 is the standard oxidation potential. Other phenolic compounds give graphs showing a ‘break point’ at about pH 6, and have different slopes above and below this value.Most of the hydroxyanisoles and amines exhibit slight curves when potential is plotted against pH.Sufficient alkylated phenols have been examined to establish definite trends in the effects of substitution; thus ortho and para substituents lower the oxidation potential by about 0·08 v while meta substituents have a much smaller effect. These effects are largely additive and the oxidation potentials of polyalkylated phenols can be predicted from a knowledge of the effects of the separate substitutents. With certain 2: 4: 6‐compounds containing large groups, the measured oxidation potential is higher than that predicted and this is attributed to steric hindrance.In general, as the oxidation potential decreases, the antioxidant activity increases. The relationship is not invariably close and, with certain substituted hydroxyanisoles, the antioxidant activities measured are even higher than would be anticipated from their low potentials. No compound of high antioxidant activity has been found to possess a potential higher than 0·70 v and hence the polarographic method should prove to be valuable for a rapid preliminary assessment of possible anti‐oxidants.