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 b een studied by titration calorimetry in pyridine (PY) containing 0.1 m ol dm(-3) (C2H5)(4)NClO4 as a constant ionic medium at 25 degrees C, T he calorimetric titration data were well explained in terms of the for mation of the [Na(12-crown-4)](+), [Na(12-crown-4)(2)](+), and [Na(18- crown-6)](+) complexes, and their formation constants, reaction enthal pies, and entropies were determined. The formation of [Na(18-crown-6)] (+) is much pronounced in PY over N,N-dimethylformamide (DMF) and wate r, and the stability order is PY > DMF > water. The enthalpy values fo r the formation of [Na(18-crown-6)](+) are all negative in PY, DMF, an d 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 w ater by forming hydrogen bonds with water molecules. Despite the stron ger electron pair-donating ability of PY than DMF, the complexation is more exothermic in PY than in DME This is ascribed to the different s olvation 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 forme r solvent than in the latter.