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