View source for Crown ether

[[Image:18-crown-6-potassium-3D-balls-A.png|thumb|right|200px|[[18-crown-6]] coordinating a potassium ion]]
'''Crown ethers''' are cyclic [[chemical compound]]s that consist of a ring containing several [[ether]] groups.  The most common crown ethers are [[oligomer]]s of ethylene oxide, the repeating unit being ethyleneoxy, i.e., -CH<sub>2</sub>CH<sub>2</sub>O-.  Important members of this series are the tetramer (n = 4), the pentamer (n = 5), and the hexamer (n = 6).  The term "crown" refers to the resemblance between the structure of a crown ether bound to a [[cation]], and a [[crown (headgear)|crown]] sitting on a person's head. The first number in a crown ether's name refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are [[oxygen]].  Crown ethers are much broader than the oligomers of ethylene oxide; an important group are derived from catechol.

Crown ethers strongly bind certain cations, forming [[complex (chemistry)|complexes]]. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic.  The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in [[phase transfer catalysis]].  The [[ligand|denticity]] of the polyether influences the affinity of the crown ether for various cations.  For example, 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.

::[[Image:Crowns.png|thumb|center|700px|structures of common crown ethers: [[12-crown-4]], [[15-crown-5]], [[18-crown-6]], [[dibenzo-18-crown-6]], and [[diaza-18-crown-6]] ]]
==Crown ethers in nature==
Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. [[Ionophore]]s such as [[valinomycin]] also display a marked preference for the potassium cation over other cations.

==History of synthetic crown ethers==
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 cation]]s.<ref>{{Cite doi|10.1021/ja01002a035}}</ref><ref>{{Cite doi|10.1021/ja00986a052}}</ref> His strategy entailed linking two [[catechol]]ate groups through one [[hydroxyl]] on each molecule. This linking defines a polydentate ligand 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 strongly complexed [[potassium]] cation. Citing earlier work on the dissolution of [[potassium]] in 16-crown-4,<ref name="stewart">D. G. Stewart. D. Y. Waddan and E. T. Borrows, {{Patent|GB|785229}} Oct. 23, '''1957'''.</ref><ref name="down">J. L. Down, J. Lewis, B. Moore and G. W. Wilkinson, Proc. Chem. Soc., 1959, 209; J. Chem. Soc., '''1959''', 3767.</ref> he realized that the cyclic [[polyether]]s 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 [[organic synthesis]], [[phase transfer catalyst]]s, and other emerging disciplines benefited from the discovery of crown ethers.  Pedersen particularly popularized the dibenzo crown ethers.<ref>{{OrgSynth | author = Charles J. Pedersen | title = Macrocyclic Polyethers: Dibenzo-18-Crown-6 Polyether and Dicyclohexyl-18-Crown-6 Polyether | collvol = 6 | collvolpages = 395 | year = 1988 | prep = CV6P0395}}</ref>

Pedersen shared the 1987 [[Nobel Prize in Chemistry]] for the discovery of the synthetic routes to, and binding properties of, crown ethers.

==Affinity for cations==
Apart from its high affinity for potassium cations, [[18-crown-6]] can also bind to protonated amines and form very stable complexes in both solution and the gas phase. Some [[amino acids]], such as [[lysine]], contain a primary [[amine]] on their side chains. Those protonated amino groups can bind to the cavity of 18-crown-6 and form stable complexes in the gas phase. Hydrogen-bonds are formed between the three hydrogen atoms of protonated amines and three oxygen atoms of 18-crown-6. These hydrogen-bonds make the complex a stable adduct.

==Aza-crowns==
21- and 18-membered diazacrown ether derivatives exhibit excellent [[Calcium]] and [[Magnesium]] selectivities and are widely used in [[ion-selective electrode]]s.<ref>{{cite journal |author = K. Suzuki, K. Watanabe, Y. Matsumoto, M. Kobayashi, S. Sato, D. Siswanta, H. Hisamoto |title = Design and Synthesis of Calcium and Magnesium Ionophores Based on Double-Armed Diazacrown Ether Compounds and Their Application to an Ion Sensing Component for an Ion-Selective Electrode |journal = Anal. Chem. |year = 1995 |volume = 67 |pages = 324–334 |doi = 10.1021/ac00098a016}}</ref> Some or all of the oxygen atoms in crown ethers can be replaced by nitrogens to form [[cryptand]]s.   A well-known tetrazacrown is [[cyclen]] in which there are no oxygens.  <ref>{{OrgSynth | author = Vincent J. Gatto, Steven R. Miller, and George W. Gokel | title = 4,13-Diaza-18-Crown-6 | collvol = 8 | collvolpages = 152| year = 1988 | prep = CV8P0152}}</ref>

==See also==
* [[Cryptand]]
* [[Metallacrown]]

==References==
<div class="references-small"><references/></div>

==External links ==
* [http://nobelprize.org/chemistry/laureates/1987/pedersen-lecture.pdf Charles Pedersen's Nobel Lecture]
* [http://www.org-chem.org/yuuki/crown/crown_en.html molecular crown]

[[Category:Crown ethers| ]]
[[Category:supramolecular chemistry]]
[[Category:Chelating agents]]
[[Category:Macrocycles]]

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