The Effect of Hydrogen Bonding between Methyl-Substituted Phenols and Dipolar Aprotic Solvents on the Rate Constants for Protonation of Anthracene Anion Radical.

Abstract

The kinetics of protonation of anthracene anion radical (A.bul.-) by phenol and 13 methyl-substituted phenols (ArOH) have been studied by deriv. cyclic voltammetry in Me2SO, DMF, propylene carbonate, and MeCN. The rate consts. were cor. for the stoichiometric effects of formation of the homoconjugation complexes ArOH/ArO- and ArOH/ArOH/ArO- and the kinetic contributions from protonation of A.bul.- by ArOH dimers. The unique rate consts. cor. for the effect of ArOH-solvent H bonding were 2-3 orders of magnitude higher than those usually referred to as second-order rate consts. for protonation of A.bul.-. The kinetic effect of Me substituents in the 2,6-positions of the phenol depended on the solvent. These effects could be satisfactorily accounted for by the influence of the 2,6-Me substituents on the ArOH-solvent H bonding equil. const. The best values of the substituent consts., σ2 and σ2,6, defined by the protonation of A.bul.- were between those defined by the H bonding equil. and those defined by the unique equil. acidity of the phenols. This is taken as an indication that the phenol O-H bond is stretched considerably in the transition state for protonation of A.bul.-. A Broensted α of about 0.5 was found. [on SciFinder(R)]