THE EFFECT OF HYDROGEN-BONDING BETWEEN METHYL-SUBSTITUTED PHENOLS AND DIPOLAR APROTIC-SOLVENTS ON THE RATE CONSTANTS FOR PROTONATION OF ANTHRACENE ANION RADICAL

The kinetics for the protonation of anthracene anion radical (A-.) by phenol and 13 methyl-substituted phenols (ArOH) have been studied by derivative cyclic voltammetry in dimethyl sulfoxide, N,N-dimethylformamide, propylene carbonate and acetonitrile. The observed rate constants were corrected for the stoichiometric effects of formation of the homoconjugation complexes, ArOH/ArO- and ArOH/ArOH/ArO- and the kinetic contributions from protonation of A-. by ArOH dimers. Available data for equilibrium (I) show that ArOH exists in solution predominantly as the ArOH/solvent hydrogen bond complex with only minor amounts of the 'free' ArOH present. It is suggested that the protonation of A-. involves only the free ArOH in the proton transfer step (II). The unique rate constants corrected for the effect of equilibrium (I) were between two and three orders of magnitude higher than those usually referred to as the second-order rate constants for protonation of A-.. ArOH/solvent K(I) reversible ArOH + solvent (I) A-. + ArOH k(II) --> AH. + ArO- (II) The kinetic effect of methyl substituents in the 2,6-positions of the phenol was observed to depend on the solvent. It was found that these effects could be satisfactorily accounted for by the influence of the 2,6-methyl substituents on the magnitude of K(I). The 'best' values of the substituent constants, sigma-2b and sigma-2,6b, defined by the protonation of A-. were found to be between those defined by equilibrium (I) and those defined by the unique equilibrium 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-.. From the relation between k(II) and DELTA-G-degrees it was found that the Bronsted alpha value for the process is close to 0.5.