K. Ohtsu et al., THERMODYNAMICS OF COMPLEXATION OF SODIUM-ION WITH 12-CROWN-4 AND 18-CROWN-6 IN PYRIDINE, Zeitschrift fur Naturforschung. B, A journal of chemical sciences, 50(4), 1995, pp. 529-535
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.