Thermodynamics of Complexation of Sodium Ion with 12-Crown-4 and 18-Crown-6 in Pyridine

Abstract

The complexation of sodium ion with 1,4,7,10-tetraoxacyclododecane (12-crown-4) and 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) has been studied by titration calorimetry in pyridine (PY) containing 0.1 mol dm-3 (C2H5)4NClO4 as a constant ionic medium at 25 °C. The calorimetric titration data were well explained in terms of the formation of the [Na(12-crown-4)]+, [Na(12-crown-4)2]+, and [Na(18-crown-6)]+ complexes, and their formation constants, reaction enthalpies, and entropies were determined. The formation of [Na(18-crown-6)]+ is much pronounced in PY over N,N-dimethylformamide (DMF) and water, and the stability order is PY > DMF > water. The enthalpy values for the formation of [Na(18-crown-6)]+ are all negative in PY, DMF, and water, and increase in the order PY < DMF < water. The complexation is the least exothermic in water, though sodium ion is the most weakly solvated in water. This is because 18-crown-6 is much stabilized in water by forming hydrogen bonds with water molecules. Despite the stronger electron pair-donating ability of PY than DMF, the complexation is more exothermic in PY than in DMF. This is ascribed to the different solvation number of the sodium ion in P Y and DMF, i.e., the sodium ion is coordinated with a smaller number of solvent molecules in the former solvent than in the latter.