crown ether

Crown ethers are heterocyclic chemical compounds that, in their simplest form, are cyclic oligomers of dioxane. The essential repeating unit of any simple crown ether is ethyleneoxy, i.e., -CH₂CH₂O-, which repeats twice in dioxane and six times in 18-crown-6. The nine-membered ring 1,4,7-trioxonane (9-crown-3) is often called a crown and can interact with cations. Macrocycles of the (-CH₂CH₂O-)_n type in which n ≥ 4 are generally referred to as crown ethers rather than by their systematic names. This is partly because they comprise a special group of heterocycles that bind cations.

width="120" align="center" \| 12-crown-4 chemical fourmula	width="150" align="center" \| 15-crown-5 chemical fourmula	width="150" align="center" \| 18-crown-6 chemical fourmula
align = "center" \| 12-crown-4	align = "center" \| 15-crown-5	align = "center" \| 18-crown-6

In the mid-1960s, Charles Pedersen, who was a chemist working at Dupont, discovered crown ether when he was trying to prepare a complexing agent for divalent cations. His strategy was to link two catechols through one hydroxyl on each molecule. This would give him a compound that could partially envelop the cation and, by ionization of the phenolic hydroxyls, neutralize the bound dication. He was surprised to isolate a by-product that bound or complexed with potassium cation but had no ionizable hydroxyl group. He realized that the polyethers represented a new class of complexing agents that were capable of binding alkali metal cations. The fields of anionic synthetic reagents, phase-transfer catalysis, biological ion transfer, and other emerging disciplines benefited profoundly from the discovery of crown ether. The discovery of crown ethers led to a Nobel Prize in 1987 for Charles Pedersen (with Donald Cram and Jean-Marie Lehn for their contribution to supramolecular chemistry).

Crown Ether

See Also