INFLUENCE OF LIGAND STRUCTURE AND SOLVENT EFFECTS ON COMPLEXATION OF NEUTRAL GUESTS BY SEVERAL CROWN-ETHERS

The formation of complexes between crown ethers and acetonitrile, chlo roform, and nitromethane were investigated in carbon tetrachloride at 25-degrees-C A significant influence of the ring size on the selectivi ty of the host is evident. The host 18-crown-6 forms complexes for whi ch the reaction enthalpy and entropy are quite high. Host molecules wi th benzene side groups form complexes of lower reaction enthalpy and e ntropy and therefore the complexes formed are less stable than that of the analogous crown ethers without aromatic groups. Solvent effects o n the stability constant K, the reaction enthalpy DELTAH, and the reac tion entropy DELTAS were studied for the complexation of malonitrile b y 18-crown-6. The reaction enthalpy and entropy values change in accor dance with the dielectric constant of the solvent used, but no overall effect on complex stability with change in solvent dielectric constan t was observed.