Solvent effects on the activation parameters of the reaction between an alpha-tocopherol analogue and dpph: The role of H-bonded complexes.

The analysis of the activation parameters for the formal H-atom transfer reaction between 2,2,5,7,8-pentamethyl-6-chromanol (ChrOH) and 2,2-diphenyl-1-picrylhydrazyl (dppho) reveals that these parameters are effective probes of the actual reaction mechanism. Indeed, the A factors measured in various polar and apolar solvents are localized in three distinct domains according to whether the reaction occurs via outer-sphere electron transfer (ET) from the anion ChrO- or hydrogen atom transfer (HAT). For instance, A = 5.9 × 105 M-1 s-1 and Ea = 2.5 kcal mol-1 in cyclohexane where the reaction proceeds by HAT, whereas in methanol, ethanol, and their mixtures with water where there is a substantial ET contribution A > 109 M-1s-1 and Ea > 7 kcal mol-1. Interestingly, in nonhydroxylic polar solvents, A ~ 107 M-1s-1 and the Ea values reflect the H-bond accepting ability of the solvent in agreement with the "standard" kinetic solvent effects on HAT reactions. Addition of small quantities of pyridine accelerates the reaction rates in these solvents. This suggests that the H-bonded complex (ChrOH· · ·Py) is able to react via intermolecular ET with dppho. It is known, in fact, that pyridine lowers the oxidation potential of phenols by ~0.5 V and the G ET of ChrOH + dppho consequently decreases by about 10 kcal mol-1.