Heat capacity changes in proton addition to the nitrogen of saturated organic molecules in water. Effects of solvation

Abstract

The ΔC^°_p values for proton addition to some aminoalcohols, aminoethers and diamines in aqueous solution have been determined at 25 and 40°C, using partial molal heat capacity data for the neutral molecules and their hydrochlorides. A comparison is made between ΔC^°_p, ΔS° and ΔV° values and the corresponding values for monofunctional amines, and the following features emerge for proton addition to the nitrogen atom of saturated amines in water: (1) Either in mono- or in bifunctional amines the thermodynamic reaction parameters, ΔC^°_p and ΔS°, change systematically in going from primary, to secondary, to tertiary amines; the same is observed for ΔV°, but only in the case of monofunctional amines. (2) The ΔC^°_p and ΔS° values of protonation of bifunctional amines are higher than those of monofunctional amines of the same type.

Interpretation of this behaviour is made in terms of two concurrent solvation effects: (i) the charged nitrogen–water interaction which is strongly dependent on the number of N-bonded hydrogens; (ii) the hydration of alkyl residues which is less extensive when they are bound to a charged nitrogen than when the nitrogen is neutral.