Thermodynamic study on solvent extraction of 18-crown-6-alkali metal ion complexes and tetraalkylammonium ions with picrate anions into benzene

Enthalpy and entropy changes for ion-pair extractions of tetraalkylammonium ions (Me4N+, Et4N+, n-Pr4N+, n-Bu4N+) with picrate anions, overall extractions of alkali metal (Na, K, Rb, Cs) picrates with 18-crown-6 (18C6), and distribution of 18C6 itself were determined between benzene and water, Me, Et, rc-Pr, and n-Bu denoting methyl, ethyl, propyl, and butyl groups, respectively. All the extracted 18C6 complexes were 1:1:1 complexes (alkali metal ion: 18C6:picrate anion). The values of enthalpy and entropy changes are negative for overall extractions of all the alkali metal picrates with 18C6. Plots of thermodynamic quantities for ion-pair extractions of R4NA vs. the number of carbon atoms of R4N+ show a linear relationship, R4N+ and A~ being tetraalkylammonium ion and picrate anion, respectively. Enthalpy and entropy changes were calculated for ion-pair extractions of 18C6-alkali metal ion complexes with picrate anions. The thermodynamic parameters for ion-pair extractions of 18C6-alkali metal picrate complexes were compared with those of R4NA, and were discussed in detail from the standpoint of molecular grounds. It was found from these data that the 18C6-K+ complex acts as a structure breaker in water. © 1987 The Chemical Society of Japan.