The temperature variation of the HA acidity function for sulphuric acid and the thermodyanamics of protonation of amides

Abstract

The anchoring of the H_A acidity function to 4- and 2-nitroaniline indicators at low acid concentrations has been reexamined and found to be satisfactory. The H_A function has been determined up to 45% sulphuric acid over the temperature range 15–40 °C. As found earlier for the H_O function, the H_A values become less negative with increasing temperature. The thermodynamic functions of protonation for pyrrole-2-carboxamide, 4-methoxybenzamide, and methacrylamide have been derived. The protonations are slightly endothermic and occur with a decrease in entropy. The thermodynamic functions, ΔG° and ΔH°, follow the same correlation as those for the protonation of alkylamines and arylamines, and therefore indicate that the predominant form of amide cations in aqueous acid is the N-protonated form. The entropy changes, ΔS°, indicate that restricted rotation about the C–N bonds in the amide molecules also makes a contribution.