Crown ether 1

Crown ether

18-crown-6 coordinating a potassium ion

Crown ethers are cyclic chemical compounds that consist of aring containing several ether groups. The most common crownethers are oligomers of ethylene oxide, the repeating unit beingethyleneoxy, i.e., -CH2CH2O-. Important members of this seriesare the tetramer (n = 4), the pentamer (n = 5), and the hexamer (n= 6). The term "crown" refers to the resemblance between thestructure of a crown ether bound to a cation, and a crown sitting ona person's head. The first number in a crown ether's name refers tothe number of atoms in the cycle, and the second number refers tothe number of those atoms that are oxygen. Crown ethers are muchbroader than the oligomers of ethylene oxide; an important groupare derived from catechol.

Crown ethers strongly bind certain cations, forming complexes.The oxygen atoms are well situated to coordinate with a cationlocated at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often formsalts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis. Thedenticity of the polyether influences the affinity of the crown ether for various cations. For example, 18-crown-6 hashigh affinity for potassium cation, 15-crown-5 for sodium cation, and 12-crown-4 for lithium cation. The highaffinity of 18-crown-6 for potassium ions contributes towards its toxicity.

structures of common crown ethers: 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, and diaza-18-crown-6

Parallels in natureCrown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. Ionophores such asvalinomycin also display a marked preference for the potassium cation over other cations.

History of synthetic crown ethersIn 1967, Charles Pedersen, who was a chemist working at DuPont, discovered a simple method of synthesizing acrown ether when he was trying to prepare a complexing agent for divalent cations.[1][2] His strategy entailed linkingtwo catecholate groups through one hydroxyl on each molecule. This linking defines a polydentate ligand that couldpartially envelop the cation and, by ionization of the phenolic hydroxyls, neutralize the bound dication. He wassurprised to isolate a by-product that strongly complexed potassium cation. Citing earlier work on the dissolution ofpotassium in 16-crown-4,[3][4] he realized that the cyclic polyethers represented a new class of complexing agentsthat were capable of binding alkali metal cations. He proceeded to report systematic studies of the synthesis andbinding properties of crown ethers in a seminal series of papers. The fields of organic synthesis, phase transfercatalysts, and other emerging disciplines benefited from the discovery of crown ethers. Pedersen particularlypopularized the dibenzo crown ethers.[5]

Crown ether 2

Pedersen shared the 1987 Nobel Prize in Chemistry for the discovery of the synthetic routes to, and bindingproperties of, crown ethers.

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

Aza-crowns21- and 18-membered diazacrown ether derivatives exhibit excellent calcium and magnesium selectivities and arewidely used in ion-selective electrodes.[6] Some or all of the oxygen atoms in crown ethers can be replaced bynitrogens to form cryptands. A well-known tetrazacrown is cyclen in which there are no oxygens.[7]

References[1] Pedersen, C. J. (1967). Journal of the American Chemical Society 89 (26): 7017–7036. doi:10.1021/ja01002a035.[2] Pedersen, C. J. (1967). Journal of the American Chemical Society 89 (10): 2495–2496. doi:10.1021/ja00986a052.[3] D. G. Stewart. D. Y. Waddan and E. T. Borrows, GB 785229 (http:/ / worldwide. espacenet. com/ textdoc?DB=EPODOC& IDX=GB785229)

Oct. 23, 1957.[4] J. L. Down, J. Lewis, B. Moore and G. W. Wilkinson, Proc. Chem. Soc., 1959, 209; J. Chem. Soc., 1959, 3767.[5] Charles J. Pedersen (1988), "Macrocyclic Polyethers: Dibenzo-18-Crown-6 Polyether and Dicyclohexyl-18-Crown-6 Polyether" (http:/ /

www. orgsyn. org/ orgsyn/ orgsyn/ prepContent. asp?prep=CV6P0395), Org. Synth., ; Coll. Vol. 6: 395[6] K. Suzuki, K. Watanabe, Y. Matsumoto, M. Kobayashi, S. Sato, D. Siswanta, H. Hisamoto (1995). "Design and Synthesis of Calcium and

Magnesium Ionophores Based on Double-Armed Diazacrown Ether Compounds and Their Application to an Ion Sensing Component for anIon-Selective Electrode". Anal. Chem. 67 (2): 324–334. doi:10.1021/ac00098a016.

[7] Vincent J. Gatto, Steven R. Miller, and George W. Gokel (1988), "4,13-Diaza-18-Crown-6" (http:/ / www. orgsyn. org/ orgsyn/ orgsyn/prepContent. asp?prep=CV8P0152), Org. Synth., ; Coll. Vol. 8: 152

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

Article Sources and Contributors 3

Article Sources and ContributorsCrown ether  Source: http://en.wikipedia.org/w/index.php?oldid=515710961  Contributors: Arcadian, Beetstra, Benjah-bmm27, Bryan Derksen, Chem-awb, DragonflySixtyseven, Edgar181,EllaRess, Grimlock, HappyCamper, IWAN, Ike9898, Ixfd64, Jrockley, Ken2, Kevin Rector, Lamro, Lokionly, M stone, MiPe, Monaco10, Mykhal, Nikevich, Niklo sv, Nuklear, Pelirojopajaro,Pilatus, Prime Entelechy, Qingyusun, RG2, Rg998, Rugby471, Sam Blacketer, Shalom Yechiel, Smokefoot, Stormie, Tomgally, V8rik, Vojtech.dostal, Whoop whoop pull up, 31 ,حسن علي البطanonymous edits

Image Sources, Licenses and ContributorsImage:18-crown-6-potassium-3D-balls-A.png  Source: http://en.wikipedia.org/w/index.php?title=File:18-crown-6-potassium-3D-balls-A.png  License: Public Domain  Contributors: Ben MillsImage:Crowns.png  Source: http://en.wikipedia.org/w/index.php?title=File:Crowns.png  License: Public Domain  Contributors: Smokefoot

LicenseCreative Commons Attribution-Share Alike 3.0 Unported//creativecommons.org/licenses/by-sa/3.0/