The basicities of substituted N-trimethylammoniophenylacetamidates and N-trimethylammoniocinnamamidates. The hammett correlations and the thermodynamics of protonation

Abstract

The pK_a values of a number of substituted N-trimethylammoniophenylacetamidates and N-trimethylammoniocinnamamidates have been determined over a range of temperature and the thermodynamic functions of protonation (ΔH° and ΔS°) derived. Both reaction series obey the Hammett equation very well, the ρ values at 25 °C being 0.639 ± 0.026 and 0.709 ± 0.049, respectively. It is found that the ratio of these ρ values to the ρ values for the ionization of the corresponding carboxylic acids is constant at 1.50 ± 0.04, regardless of the structure of the rest of the molecule, and is thus a characteristic of the amidate group. An analysis of the pK_a data for these acid–base series and comparison with the pK_a data for the corresponding carboxylic acid series leads to the conclusion that mesomeric electron donation by the benzene ring to the basic groups (a) and (b) is more important in the carboxylates than in the amidates, consistent with the greater overall electronegativity of the carboxylate group. The thermodynamic functions of protonation, ΔH° and ΔS°, broadly show a similar behaviour to that of the N-trimethylammoniobenzamidate series, although the entropies of protonation are more positive.