Equilibrium Acidities and Homolytic Bond Dissociation Energies of the Acidic C-H Bonds in N-Substituted Trimethylammonium and Pyridinium Cations

Article abstract of DOI:10.1021/jo00063a026

Equilibrium acidities (pK_HAs) 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 α-trimethylammonium groups (α-Me3N⁺) and the α-pyridinium groups (α-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 α-trimethylammonium groups destabilize adjacent radicals by 2-6 kcal/mol, whereas α-pyridinium groups stabilize adjacent radicals by 3-6 kcal/mol.The effects of α-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.