Molecular Design of Crown Ethers. 1. Effects of Methylene Chain Length: 15- to 17-Crown-5 and 18- to 22-Crown-6

Article abstract of DOI:10.1021/jo00182a017

The ring-enlarged crown ethers, 16- and 17-crown-5 and 19- to 22-crown-6, were synthesized and their cation-binding abilities were evaluated by solvent extraction of aqueous alkali metal picrates.The cation-binding abilities of less symmetrical crown ethers, 3a-e and 4a,b, were generally lower than those of the common symmetrical crown ethers 15-crown-5 (5a) and 18-crown-6 (5b), for which the less symmetrical arrangement of the donor oxygen atoms must be responsible.Compared with 18-crown-6 (5b), the ring-extended crown ethers, 3d, 3e, and 4b, showed a significant shift in cation selectivity, probably due to the enlarged cavity size.The thermodynamic parameters for the extraction of sodium and potassium picrates with 3a, 3c, and 5a,b were calculated from the change of the extraction equilibrium constants (K_ex) between 10-25 deg C.The stability of the cation-crown ether complexes was shown to be governed in general by the enthalpy change.However, a significant contribution of the entropy factor was found in unfavorable size combinations of K⁺ with 3a and Na⁺ with 3c.