The basicities of N-trimethylammonioacetamidate and of substituted N-trimethylammoniobenzamidates. The Hammett correlation and the thermodynamics of protonation

Abstract

The pK_a values of N-trimethylammonioacetamidate and a number of substituted N-trimethylammoniobenzamidates have been determined over the temperature range 15–55 °C. The pK_a, values show a correlation with Hammett σ constants with ρ 1.51 ± 0.03 at 25 °C. These facts are consistent with an N-protonated cation structure, i.e. the cations are N-trimethylammoniobenzamides, as deduced earlier from spectroscopic studies. The thermodynamic functions of protonation have been compared with those for tertiary amines and a close similarity of the entropies of protonation has been observed. The variation of the entropies of ionisation of N-trimethylammoniobenzamides with change of substituent is analogous to that in the benzoic acid series, except that solvation effects are smaller. The substantial difference in the enthalpies of ionisation and their variation with substitution in the two reaction series is largely attributable to that difference in solvation.