Editing Crown ether (section)

{{Short description|Ring molecules with several ether (–O–) groups}}
[[Image:18-crown-6-potassium-3D-balls-A.png|thumb|right|200px|[[18-crown-6]] coordinating to a [[potassium]] ion]]

In [[organic chemistry]], '''crown ethers''' are cyclic [[chemical compound]]s that consist of a [[Ring (chemistry)|ring]] containing several [[ether]] groups ({{chem2|R\sO\sR’}}).  The most common crown ethers are cyclic [[oligomer]]s of [[ethylene oxide]], the repeating unit being ethyleneoxy, i.e., {{chem2|\sCH2CH2O\s}}.  Important members of this series are the tetramer (''n''&nbsp;=&nbsp;4), the pentamer (''n''&nbsp;=&nbsp;5), and the hexamer (''n''&nbsp;=&nbsp;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 [[oligomer]]s 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 to its toxicity.  The smallest crown ether still capable of binding cations is 8-crown-4,<ref>{{Cite journal|last1=van der Ham|first1=Alex|last2=Hansen|first2=Thomas|last3=Lodder|first3=Gerrit|last4=Codée|first4=Jeroen D. C.|last5=Hamlin|first5=Trevor A.|last6=Filippov|first6=Dmitri V.|date=2019|title=Computational and NMR Studies on the Complexation of Lithium Ion to 8-Crown-4|journal=ChemPhysChem|language=en|volume=20|issue=16|pages=2103–2109|doi=10.1002/cphc.201900496|issn=1439-7641|pmc=6772996|pmid=31282054}}</ref> with the largest experimentally confirmed crown ether being 81-crown-27.<ref>{{cite journal |last1=Yang |first1=Zhao |last2=Yu |first2=Ga-Er |last3=Cooke |first3=Jennifer |last4=Ali-Abid |first4=Ziad |last5=Viras |first5=Kyriakos |last6=Matsuura |first6=Hiroatsu |last7=Ryan |first7=Anthony J |last8=Booth |first8=Colin |title=Preparation and crystallinity of a large unsubstituted crown ether, cyclic heptacosa(oxyethy1ene) (cyc2o=E2, 81-crown-27), studied by Raman spectroscopy, X-ray scattering and differential scanning calorimetry |journal=J. Chem. Soc., Faraday Trans. |date=1996 |volume=92 |issue=17 |pages=3173–3182 |doi=10.1039/FT9969203173 |url=https://pubs.rsc.org/en/content/articlelanding/1996/FT/ft9969203173|url-access=subscription }}</ref> 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.

Crown ethers have been shown to coordinate to [[Lewis acids]] through electrostatic, σ-hole (see [[halogen bond]]) interactions, between the Lewis basic oxygen atoms of the crown ether and the electrophilic Lewis acid center.<ref>{{cite journal |last1=Marczenko |first1=K. M. |last2=Mercier |first2=H. P. A. |last3=Schrobilgen |first3=G. J. |title=A Stable Crown-Ether Complex with a Noble-gas Compound |journal=Angew. Chem. Int. Ed. |volume=57 |issue=38 |pages=12448–12452 |doi=10.1002/anie.201806640|pmid=29953704 |year=2018 |s2cid=49589053 }}</ref><ref>{{cite journal |last1=Lipkowski |first1=J. |title=Antimony(III) fluoride: Inclusion complexes with crown ethers |last2=Fonari |first2=M. S. |last3=Kravtsov |first3=V. C. |last4=Simonov |first4=Y. A. |last5=Ganin |first5=E. V. |last6=Gemboldt |first6=V. O. |journal=J. Chem. Crystallogr. |date=1996 |volume=26 |issue=12 |page=823|doi=10.1007/BF01670315|bibcode=1996JCCry..26..823L |s2cid=93153773 }}</ref>

:[[Image:Various crown ethers (molecular diagrams).svg|thumb|center|700px|Structures of common crown ethers: [[12-crown-4]], [[15-crown-5]], [[18-crown-6]], [[dibenzo-18-crown-6]], and an [[aza-crown ether]]]]