Quantitative elucidation of macrocyclic effect on extraction-ability and -selectivity. Benzo-18-crown-6 and its open-ring analog 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene-alkali metal picrate systems

The constants of overall extraction equilibrium (Kex) and partition (KD,MLA) for various diluents and aqueous ion-pair formation (KMLA) of alkali metal (Li-Cs) picrate (MA) 1:1:1 complexes (MLA) with benzo-18-crown-6 (B18C6) and its open-ring analogue 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene (AC·B18C6) were determined at 25 °C, L being B18C6 or AC·B18C6; the distribution constants of AC·B18C6 were measured at 25 °C. The partition behavior of AC·B18C6 and the MLA complexes closely obeys regular solution theory, omitting chloroform; the molar volumes and the solubility parameters of AC·B18C6 and the MLA complexes were determined. The extraction selectivities of AC·B18C6 and B18C6 for the alkali metal picrates increase in the order Li < Na < Cs < K or Rb < or ≈ Rb or K and Li ≪ Na < Cs < Rb < K, respectively. The extraction selectivity of B18C6 for neighboring alkali metal ions in the periodic table is superior to that of AC·B18C6. The macrocyclic effect on the extraction-ability and -selectivity was quantitatively elucidated by the four fundamental equilibrium constants.