EQUILIBRIUM ACIDITIES AND HOMOLYTIC BOND-DISSOCIATION ENERGIES OF THE ACIDIC C-H BONDS IN N-SUBSTITUTED TRIMETHYLAMMONIUM AND PYRIDINIUM CATIONS

Equilibrium acidities (pK(HA)s) of the cations in sixteen N-substituted trimethylammonium salts, one N-phenacylquinuclidinium salt, eight N-substituted pyridinium salts, and N-(ethoxycarbonyl)isoquinolinium bromide, together with the oxidation potentials of their conjugate bases, have been determined in dimethyl sulfoxide (DMSO) solution. The acidifying effects of the alpha-trimethylammonium groups (alpha-Me3N+) and the alpha-pyridinium groups (alpha-PyN+) on the adjacent acidic C-H bonds in these cations were found to average about 10 and 18 pK(HA) units, respectively, in DMSO. The homolytic bond dissociation energies of the acidic C-H bonds in these cations, estimated by the combination of the equilibrium acidities with the oxidation potentials of their corresponding conjugate bases (ylides), show that the alpha-trimethylammonium groups destabilize adjacent radicals by 2-6 kcal/mol, whereas alpha-pyridinium groups stabilize adjacent radicals by 3-6 kcal/mol. The effects of alpha-pyridinium groups on the stabilization energies of the radicals derived from these cations were found to be ca. 4-10 kcal/mol smaller than those of the corresponding phenyl groups, whereas their effects on the equilibrium acidities of the cations were 5.4-13.1 pK(HA) units larger. The pK(HA) value of tetramethylammonium cation (Me4N+) was estimated by extrapolation to be about 42 in DMSO.