KINETIC SOLVENT EFFECTS ON HYDROXYLIC HYDROGEN-ATOM ABSTRACTIONS ARE INDEPENDENT OF THE NATURE OF THE ABSTRACTING RADICAL - 2 EXTREME TESTS USING VITAMIN-E AND PHENOL

Rate constants have been measured at 25 degrees C in 13 solvents (S) for abstraction of the phenolic hydrogen atom from alpha-tocopherol (TOH) by tert-butoxyl (BO.), k(TOH/BO)(S), and by 2,2-diphenyl-1-picrylhydrazyl (DPPH.), k(TOH/DO)(S), and in eight solvents for abstraction of the phenolic hydrogen atom from phenol by cumyloxyl, k(PhOH/CumO)(S), and DPPH., k(PhOH/DPPH)(S). Over the range of solvents examined k(TOH/BO)(S) and k(TOH/DPPH)(S) vary by a factor of Ca. 65, and k(PhOH/CumO)(S) and k(PhOH/DPPH)(S) vary by a factor of ca. 120. in accordance with a prediction(5) the kinetic solvent effect is essentially identical for the same substrate and is independent of the attacking radical. That is, for almost any pair of solvents, A and B (k(TOH/BO)(A)/k(TOH/BO)(B))/(k(TOH/DPPH)(A)/k(TOH/DPPH)(B)) approximate to 1.0. The same applies with phenol as the substrate. Exceptions to this 1:1 relationship occur when one of the reactions becomes partly diffusion-controlled and in the solvent tert-butyl alcohol in which DPPH. shows a larger reactivity than would be expected. The absolute magnitudes of the alkoxyl and DPPH rate constants in the same solvent differ by a factor of over 1 000 000 (10(6)) for alpha-tocopherol and by 10 000 000 000 (10(10)) for phenol! We have therefore confirmed, under extreme conditions, a new, unifying principle for free radical chemistry in solution.