Kinetic Solvent Effects on Hydroxylic Hydrogen Atom Abstractions Are Independent of the Nature of the Abstracting Radical. Two Extreme Tests Using Vitamin E and Phenol

Abstract

Rate constants have been measured at 25°C in 13 solvents (S) for abstraction of the phenolic hydrogen atom from α-tocopherol (TOH) by tert-butoxyl (BO.) k(TOH/BO)(S), and by 2,2-diphenyl-1-picrylhydrazyl (DPPH.), k(TOH/DPPH)(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, 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)) ~ 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 1000000 (106) for α-tocopherol and by 10000000000 (1010) for phenol! We have therefore confirmed, under extreme conditions, a new, unifying principle for free radical chemistry in solution.