Effect of para-substituents of phenolic antioxidants

Abstract

The effect of substituents on phenolic antioxidants was studied extensively in the 1960s. Based on our recent study, however, there remain some unclear points in the interpretation of the functions of o- and p- substituents. In this work, a phenol having an ortho-alkoxy substituent, in particular methoxy, has been investigated in detail. Interestingly, it has been found that the introduction of a p-substituent such as an alkoxy or alkyl group develops antioxidant activity in o-methoxy phenols, although this phenol itself does not have any antioxidant activity. The p-substituent is considered to have two functions. One is the increase in the electron density on phenolic hydroxyl oxygen by the electron-donating nature of the substituent. Another is to localize the radical electron on the p-position by the inductive effect of the p-substituent. These functions of a p-substituent accelerate hydrogen abstraction by a peroxy radical from the phenol to enhance k inh and decrease the coupling of phenoxy radicals, that is, increase the n value.