11.7 : Crown Ethers

Crown ethers are cyclic compounds derived from ethylene glycol and its substituents, with multiple ether linkages.

First discovered by Charles Pederson in 1967, crown ethers are so-called because their molecular shape resembles a crown.

The naming of crown ethers uses the general formula x-crown-y, where x indicates the total number of atoms in the ring and y represents the number of oxygen atoms. For example, 18-crown-6 is an 18-membered ring containing six ether oxygen atoms.

Crown ethers are known to bind metal cations in their internal cavity lined by regularly spaced oxygen atoms. For example, 18-crown-6 strongly binds potassium ion.

The lone pairs of electrons on the oxygen atoms lining the inner cavity, make the cavity polar and coordinate with the metal ion effectively.

The selectivity for the cation varies with the size of the cavity. Crown ethers primarily bind alkali metal ions whose diameters are comparable to the diameter of the ether cavity.

For example, 18-crown-6 binds potassium ion effectively but not lithium ion because the diameter of the ether cavity approximates the diameter of the potassium ion. In contrast, 12-crown-4, with a smaller cavity, binds lithium ion more strongly.

The outer surface of crown ethers resembles a hydrocarbon and is nonpolar. As a result, crown ethers can solubilize inorganic salts in nonpolar organic solvents by solvating the cation.

For example, potassium permanganate, an inorganic salt, does not dissolve in benzene, an organic solvent, by itself. However, upon the addition of 18-crown-6, the salt dissolves, giving benzene a purple color.

18-crown-6 forms a complex with potassium ion, which dissolves in benzene. The unsolvated permanganate ion imparts a characteristic purple color to the benzene solution. The resulting "purple benzene" is a useful reagent for oxidation reactions.

Similarly, crown ethers facilitate the availability of otherwise inaccessible anions in a nonpolar solvent and enhance their nucleophilicity.