Phenolic Antioxidants. Effect of o-Alkyl Substituents.

It is widely recognized that, concerning phenolic antioxidants, there is no room to improve their activities nor to be replaced by new and more effective phenols, since the function of substituents is explained theoretically and the mechanism of action of phenols seems established. The authors, however, doubted the presently accepted interpretation of effect of o-substituents of phenols. In this study, the effect of o-alkyl substituents, such as methyl, ethyl, isopropyl, and tert-butyl, is investigated in detail. Interestingly, it is found that the former three substituents were found to be completely different, in terms of the effect on the activities of phenols, from the last one. That is, those can stabilize phenoxy radicals derived from phenols, and give their hydrogen atom to the phenoxy radical to regenerate a phenol. This hydrogen atom is derived from α-hydrogen of o-substituents, and a substituent having a lower energy of α-C-H bond dissociation. For example isopropyl enhances activities of phenols to a greater extent. In particular, the use of 2, 6-diisopropyl-4-methyl-phenol can be reduced to about half in amount, compared with that of BHT which has been used as a standard antioxidant, to get approximately the same effect.