Thermodynamics of proton ionization in dilute aqueous solution. Part XI. pK, ΔH°, and ΔS° values for proton ionization from protonated amines at 25°

Abstract

A calorimetric study has been made of proton ionization from 69 protonated amines in aqueous solution at 25°. The resulting ΔH° values were combined with pK values to calculate ΔS° values. pK Values were obtained from the literature for 51 compounds and new values were determined for 18 compounds. A compilation of pK, ΔH°, and ΔS° values from the present study and the literature is given for proton ionization from 169 protonated amines. The effect of hydrocarbon chain length and branching on ΔH° and ΔS° values for proton ionization from primary and secondary aliphatic protonated amines is described by simple linear equations. Proton ionization for protonated amines was not found to follow the linear relation between ΔG° and ΔS° predicted by the Bjerrum theory of electrostatics. The changes in the ΔH° and ΔS° values from the first to the second step of ionization for 37 protonated diamines have been examined by using the Bjerrum and Kirkwood–Westheimer theories of electrostatic interactions in aqueous solution. A deviation identical to that previously found for dicarboxylic acids was found between the results derived from the experimental data and those predicted by the Kirkwood-Westheimer theory. Possible reasons for this deviation are discussed.