Crown ether
From Wikipedia, the free encyclopedia
Crown ethers are heterocyclic chemical compounds that, in their simplest form, are cyclic oligomers of ethylene oxide. 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 because the molecules formed when this special group of heterocycles binds to cations resemble a crown sitting on a head in structure.
The crown ethers are notable for their ability to strongly solvate cations. In other words, the equilibrium is strongly towards the complex. The oxygen atoms are ideally situated to coordinate with a cation in the interior of the ring, whereas the exterior of the ring is hydrophobic. The result is that the complexed cation is soluble in nonpolar solvents. The size of the interior of the crown ether determines the size of the cation it can solvate. Therefore, 18-crown-6 has high affinity for potassium cation, 15-crown-5 for sodium cation and 12-crown-4 for lithium cation. The high affinity of 18-crown-6 for potassium ions contributes towards its toxicity in humans.
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| 12-crown-4 | 15-crown-5 | 18-crown-6 |
Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. Ionophores such as nonactin and valinomycin also display a marked preference for the potassium cation over other cations.
Early reports of crown ethers concentrated on synthetic methods for their production; only later were their properties and the fundamental theoretical implications thereof realized.
In 1967, Charles Pedersen, who was a chemist working at DuPont, discovered a simple method of synthesizing a crown ether when he was trying to prepare a complexing agent for divalent cations [1]. 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. Citing earlier work on the dissolution of potassium in 16-crown-4 [2] [3], he realized that the cyclic polyethers represented a new class of complexing agents that were capable of binding alkali metal cations. He proceeded to report systematic studies of the synthesis and binding properties of crown ethers in a seminal series of papers. The fields of anionic synthetic reagents, phase transfer catalysts, biological ion transfer, and other emerging disciplines benefited profoundly from the discovery of crown ether.
Pedersen received a Nobel Prize in 1987 for the discovery of the synthetic routes to and binding properties of crown ethers (along with Donald Cram and Jean-Marie Lehn for their contribution to supramolecular chemistry).
Aza analogues of crown ethers exist as well notably cyclen.
[edit] References
- ^ C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 7017.
- ^ D. G. Stewart. D. Y. Waddan and E. T. Borrows, British Patent 785,229, Oct. 23, 1957.
- ^ J. L. Down, J. Lewis, B. Moore and G. W. Wilkinson, Proc. Chem. Soc., 1959, 209; J. Chem. Soc., 1959, 3767.
