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The Effect of Hydrogen Bonding between Methyl-Substituted Phenols and Dipolar Aprotic Solvents on the Rate Constants for Protonation of Anthracene Anion Radical
Nielsen, Merete Folmer,Hammerich, Ole
, p. 883 - 896 (2007/10/02)
The kinetics for the protonation of anthracene anion radical (A-anion radical) by phenol and 13 methyl-substituted phenols (ArOH) have been studied by derivative cyclic voltammery 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 Ar/OH/ArOH/ArO- and the kinetic contributions from protonation of A-anion radical by ArOH dimers.Available data for equilibrium (1) 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-anion radical 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-anion radical.ArOH/solvent ArOH + solvent (I) A-anion radical + ArOH AH-radical + 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 satisfactority accounted for by the influence of the 2,6-methyl substituents on the magnitude of K1.The 'best' values of the substituent constants, ?b2 and ?b2.6, defined by the protonation of A-anion radical were found to be between those defined by equilibrium (1) 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-anion radical.From the relation between kII and ΔG deg it was found that the Broensted α value for the process is close to 0.5.
