Basicity of N-substituted anilines and pyridine in dimethylsulfoxide

Abstract

The ionization constants of the conjugated acids BH+ of aniline, N-methyl- and N,N-dimethylaniline, and pyridine have been determined potentiometrically in Me2SO. The heats of neutralization of the amines B and their heats of solution have been obtained by calorimetry. The pK's and enthalpies of ionization of these BH+ ions and alkylammonium ions are linearly related. Comparisons with gas phase data show that the abnormal basicity order of the amines B in Me2SO is mainly due to the decreasing exothermicity of the heat of solution of the BH+ ions with the number of N—H protons available for H-bonding to Me2SO. The large increase in basicity difference between equally substituted aromatic and alkylamines, when passing from the gas phase to Me2SO, is linked to the larger heat of vaporization of the aromatic amines.