Precipitation of strong uncharged organic acids with various crown ethers or with acyclic polyethers from aqueous medium. Characterization of adducts in the solid state

It was found that substituted picric acids form slightly soluble 1 : 2 : 2 adducts with several simple crown ethers or with tri‐, tetra‐, or pentaethylene glycol dimethyl ether (glymes 4, 5 and 6) in water, whereas 2,4,6‐trinitrobenzenesulfonic acid forms a 1 : 1 : 1 adduct with dicyclohexano‐18‐crown‐6 ( cis ‐ syn ‐ cis ). The ability of a crown ether to precipitate a given acid follows the order dicyclohexano‐18‐crown‐6 ( cis ‐ syn ‐ cis ) > benzo‐15‐crown‐5 > dicyclohexano‐24‐crown‐8 > 18‐crown‐6 > 21‐crown‐7, 15‐crown‐5 ≫ 12‐crown‐4, roughly in the order of its base strength. Precipitation can occur even when the concentration(s) of one or more of the components are < 10 −4 M. The solubility products of the 18‐crown‐6 complexes with dichloro‐ or dimethylpicric acids were estimated. In the solid state the 1 : 2 : 2 dicyclohexano‐18‐crown‐6 ( cis ‐ anti ‐ cis )–dichloropicric acid–water complex, which reportedly has two pseudohydronium ions lacking C 3 v symmetry of H 3 O + , exhibits an IR spectrum in the OH stretching region characteristic of an aqua complex.