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e The cavity in the center of the crown ether molecule is lined with, oxygen atoms and is large enough to be occupied by a cation, such, as K’., , @ The cation is stabilized by interacting with lone pairs of electrons on, the surrounding oxygen atoms. Thus crown ethers solvate cations, inside a hydrophilic cavity, whereas the outer shell, consisting of, C-H bonds, is hydrophobic., , . Affinity for cations, , e Due to the chelate effect and macrocyclic effect, crown ethers, exhibit stronger affinities for cations than their acyclic analogs., Hereby, the cation selectivity for alkali metal ions is mainly, dependent on the size and charge density of the ion and the cavity, size of the crown ether., , , , =, a . 2, Se CN \_/* -on, , LiCN ¢12-crown-4 NaOH ¢15-crown-5 KMn0O, ¢18-crown-6, , , , Comparison of Cavity Size with Effective lon Radii of Alkali metals, , , , , , , , , , , , e All, Crown Ether | Cavity Size/A | Favoured Alkali lon | Effective lon Radii/A, 12-crown-4 0.6-0.75 lig 0.76, 15-crown-5 0.86-0.92 Na* 1.02, 18-crown-6 1.34-1.55 K* 1.38, 21-crown-7 1.7-2.1 Css 1.67, , , , , , , , , , , , , , crown ethers have a central cavity that can accommodate a metal, ion coordinated to the ring of oxygen atoms, and crown ethers with, tings of different sizes prefer to bind metal ions that fit into the, cavity.