No abstract

The extraction of water by several crown ethers into chloroform + carbon tetrachloride mixtures has been investigated using a proton NMR technique. The equilibrium is well described by formation of a 1:1 water−crown complex in rapid exchange with uncomplexed ligand and water. The fraction (k) of crown ether complexed with water increases with crown cavity size, varying from (15 ±1)% for 12-crown-4 to (97 ± 5)% for 18-crown-6. Addition of carbon tetrachloride to chloroform lowers the k value for all crown ethers in equilibrium with water, and the value is close to zero in pure CCl4. The partition coefficient follows the opposite trend:  the amount of crown ether in the organic phase increases with the percentage of CCl4 in this phase. The chemical shifts of free and complexed water also vary with solvent composition. Interaction of water with crown ether depends on solvation environment and may play a significant role in liquid−liquid extraction of metal ions using macrocyclic polyethers as extractants.

In the presence of 18-crown-6, D2O forms a 1:1 complex with the macrocyclic molecule in supercritical fluid CO2 with two different configurations. The D2O molecule can be bonded to two oxygen atoms of the crown cavity in a bridged configuration that is characterized by a broad peak at 2590 cm-1. The D2O molecule can also form one hydrogen bond with an oxygen atom of the crown cavity that can be characterized by two peaks at 2679 and 2733 cm-1, with the former assigned to the hydrogen-bonded O−D stretching and the latter the unbonded O−D stretching. The equilibrium constants of the two configurations in supercritical CO2 have been calculated. The enthalpy of formation is −12 ± 2 kJ mol-1 for the single-hydrogen-bond complex and −38 ± 3 kJ mol-1 for the bridged configuration complex. At high 18-crown-6 to D2O ratios, the formation of another complex in supercritical CO2 that involves one D2O molecule hydrogen bonded to two 18-crown-6 molecules becomes possible.