Synthesis of thiacrown ether carboxylic acids and their characteristics as extractants for metal ions

Two thiacrown ethers with pendant carboxylic acid groups, 3,6,10,13-tetrathiacyclotetradec-1-oxyacetic acid (TTCTOAA) and 2-(3,6,10,13-tetrathiacyclotetradec-1-oxy)hexanoic acid (TTCTOHA), were synthesized and their proton dissociation constants (pKa) were determined potentiometrically at 25 ± 0.1°C and an ionic strength of 0.1. The pKa values of TTCTOAA and TTCTOHA were 3.97 and 4.05, respectively. The characteristics of thiacrown ether carboxylic acids as extraction reagents for metal ions were examined and compared with those of the neutral thiacrown compound 1,4,8,11-tetrathiacyclotetradecane (TTCT) in the presence of picrate ion for the formation of the ion pair. 1,2-Dichloroethane was used as the extraction solvent. The extractability of metal ions with TTCTOAA was very low. In the case of TTCTOHA, copper(II), silver(I) and copper(I) were extracted into 1,2-dichloroethane. The extractability of silver(I) and copper(I) was lower than that with TTCT and picrate ion. The extraction percentage of silver(I) with TTCTOHA was significantly higher than that of copper(I), whereas hardly any difference in extractability with TTCT was observed between silver (I) and copper(I). Although copper(II), a borderline Lewis acid, was weakly extracted with TTCT and picrate ion, it was extracted well (over 90%) with TTCTOHA into 1,2-dichloroethane. The extraction of copper(II) and silver(I) with TTCTOHA into different organic solvents was examined. The extractability of copper(II) and silver(l) decreased in the order 1-octanol ≈ benzene >; 1,2-dichloroethane and 1-octanol >; 1,2-dichloroethane >; benzene, respectively.