EXTRAORDINARY KINETIC-BEHAVIOR OF THE ALPHA-TOCOPHEROXYL (VITAMIN-E) RADICAL

Rate constants which have been reported for the bimolecular self-reaction of alpha-tocopheroxyl radicals vary by about 5 orders of magnitude. We have found that the observed bimolecular rate constant can vary by about a factor of 7 during a single, but typical experiment, e.g., in chlorobenzene at 37 degrees C from ca. 7 x 10(3) M(-1) s(-1) initially to ca. 1 x 10(3) M(-1) s(-1) finally. The overall reaction involves a disproportionation with the transfer of a hydrogen atom from the 5-methyl group of one radical to the phenoxyl oxygen atom of the other radical forming alpha-tocopherol and an o-quinone methide. In the slow regime (which corresponds to the true reaction of two alpha-tocopheroxyl radicals) this disproportionation has a deuterium kinetic isotope effect of 3.7. The bizarre kinetic behavior observed with alpha-tocopheroxyl radicals has been traced to a very minor impurity which will be present in any normal sample of alpha-tocopherol. The impurity in question is a bisphenol in which two alpha-tocopherol moieties have become linked through their 5-methyl carbon atoms. This bisphenol is a ''natural'' impurity in alpha-tocopherol since it will be formed upon exposure of alpha-tocopherol to air. The coupling of two o-quinone methide molecules yields a spiro-dimer which is then reduced to the bisphenol, probably by unoxidized alpha-tocopherol.