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

The ΔCpo 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 po, ΔSo and ΔVo 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, ΔCpo and ΔSo, change systematically in going from primary, to secondary, to tertiary amines; the same is observed for ΔVo, but only in the case of monofunctional amines. (2) The ΔCpo and ΔSo 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.