Extraordinary kinetic behavior of the \u3b1-tocopheroxyl (vitamin E) radical

Rate constants which have been reported for the bimolecular self-reaction of \u3b1-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 \ub0C from ca. 7 x 103 M-1 s-1 initially to ca. 1 x 103 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 \u3b1-tocopherol and an o-quinone methide. In the slow regime (which corresponds to the true reaction of two \u3b1-tocopheroxyl radicals) this disproportionation has a deuterium kinetic isotope effect of 3.7. The bizarre kinetic behavior observed with \u3b1-tocopheroxyl radicals has been traced to a very minor impurity which will be present in any normal sample of \u3b1-tocopherol. The impurity in question is a bisphenol in which two \u3b1-tocopherol moieties have become linked through their 5-methyl carbon atoms. This bisphenol is a "natural" impurity in \u3b1-tocopherol since it will be formed upon exposure of \u3b1-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 \u3b1-tocopherol. \ua9 1995 American Chemical Society.Peer reviewed: YesNRC publication: Ye