THERMODYNAMICS OF COMPLEXATION OF SODIUM-ION WITH 12-CROWN-4 AND 18-CROWN-6 IN PYRIDINE

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)(4)NClO4 as a constant ionic medium at 25 degrees 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 DME This is ascribed to the different solvation number of the sodium ion in PY and DMF, i.e., the sodium ion is coordinated with a smaller number of solvent molecules in the former solvent than in the latter.