Extraction of alkali metal (Li, Na, K) picrates with benzo-15-crown-5 into various organic solvents. Elucidation of fundamental equilibria determining the extraction-ability and -selectivity

To quantitatively elucidate the effects of the benzo group on the extraction-selectively and -ability of benzo-15-crown-5 (B15C5) for alkali metal ions, the constants of the overall extraction (Kex), the distribution for various diluents having low dielectric constants (K D,MLA), and the aqueous ion-pair formation (KMLA) of B15C5-alkali metal (Li, Na, K) picrate 1:1:1 complexes (MLA) were determined at 25°C. The partition constants of B15C5 were also measured at 25°C. The log KMLA values for Li+, Na+, and K + are -0.32 ± 0.22, 2.66 ± 0.19, 0.71 ± 0.47, respectively. In going from 15-crown-5 (15C5) to B15C5, the benzo group considerably decreases the KMLA value for the same alkali metal ion. The distribution behavior of B15C5 and its 1:1:1 complexes with the alkali metal picrates closely obeys regular solution theory, omitting chloroform. Molar volumes and solubility parameters of B15C5 and the 1:1:1 complexes were determined. For every diluent, the Kex value for B15C5 increases in the order Li+ + +. K D,MLA makes an unfavorable contribution to the Na+ extraction-selectivity of B15C5 because of the smallest molar volume of the Na(B15C5)A complex. The Na+ extraction-selectivity of B15C5 is determined completely by much the highest KNa(B15C5)A value. The extraction-ability and -selectivity of B15C5 for the alkali metal picrates are compared with those of 15C5 on the basis of the underlying equilibrium constants.