ABSOLUTE KINETICS OF HYDROGEN ABSTRACTION FROM ALPHA-TOCOPHEROL BY SEVERAL REACTIVE SPECIES INCLUDING AN ALKYL RADICAL

Laser flash photolysis and competitive techniques have been employed to study the reactions of alpha-tocopherol with various radicals and ketone triplets in solution. For example benzophenone triplets abstract hydrogen with rate constants of 5.1 x 10(9) and 3.7 x 10(9) M-1 s-1 in benzene and benzene/1.3 M methanol. Similar near-diffusion-controlled values were obtained for several other ketone triplets, as well as tert-butoxyl and 4-methoxybenzoyloxyl radicals. Deuterium kinetic isotope effects are frequently very small, reflecting the expected lack of selectivity of fast reactions. The reactivity of the 5-hexenyl radical toward alpha-tocopherol was examined by studying the competition of this process with the radical cyclization to the cyclopentylmethyl radical. The value of (1.7 +/- 0.2) x 10(6) M-1 s-1 (at 70-degrees-C in benzene) for this hydrogen atom abstraction from alpha-tocopherol makes this process exceptionally fast in comparison with the limited available rate data for reactions of carbon-centered radicals with other phenols.