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18-Crown-6
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<h2 id="synthesis">Synthesis</h2> <p>This compound is prepared by a modified <a href="/facts/Williamson_ether_synthesis/QLmp8Qcw">Williamson ether synthesis</a> in the presence of a templating cation:<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p> (CH2OCH2CH2Cl)2 + (CH2OCH2CH2OH)2 + 2 KOH → (CH2CH2O)6 + 2 KCl + 2 H2O <p>It can be also prepared by the oligomerization of <a href="/facts/Ethylene_oxide/4WDVKX2r">ethylene oxide</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> It can be purified by <a href="/facts/Distillation/plDvUMxn">distillation</a>, where its tendency to supercool becomes evident. 18-Crown-6 can also be purified by recrystallisation from hot <a href="/facts/Acetonitrile/fNpbvRp6">acetonitrile</a>. It initially forms an insoluble solvate.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Rigorously dry material can be made by dissolving the compound in <a href="/facts/THF/P5t6SkMK">THF</a> followed by the addition of <a href="/facts/NaK/09HKRf44">NaK</a> to give [K(18-crown-6)]Na, an <a href="/facts/Alkalide/6lw9cCFv">alkalide</a> salt.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p><p>Crystallographic analysis reveals a relatively flat molecule but one where the oxygen centres are not oriented in the idealized 6-fold symmetric geometry usually shown.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> The molecule undergoes significant conformational change upon complexation. </p> <h2 id="reactions">Reactions</h2> <p>18-Crown-6 has a high affinity for the <a href="/facts/Hydronium/HzE3BGYJ">hydronium</a> ion H3O+, as it can fit inside the crown ether. Thus, reaction of 18-crown-6 with strong acids gives the cation [H3O·18-crown-6]+. For example, interaction of 18-crown-6 with HCl gas in <a href="/facts/Toluene/Pzdt8dwD">toluene</a> with a little moisture gives an <a href="/facts/Ionic_liquid/Ypse8taj">ionic liquid</a> layer with the composition [H3O·18-crown-6]+[HCl2]−·3.8C6H5Me, from which the solid [H3O·18-crown-6]+[HCl2]− can be isolated on standing. Reaction of the ionic liquid layer with two molar equivalents of water gives the crystalline product (H5O2)[H3O·18-crown-6]Cl2.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p> <h2 id="applications">Applications</h2> <p>18-Crown-6 binds to a variety of small cations, using all six oxygens as donor atoms. Crown ethers can be used in the laboratory as <a href="/facts/Phase_transfer_catalyst/hvVU9bc6">phase transfer catalysts</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Salts which are normally insoluble in organic solvents are made soluble by crown ether.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> For example, <a href="/facts/Potassium_permanganate/nyjjuDlo">potassium permanganate</a> dissolves in benzene in the presence of 18-crown-6, giving the so-called "purple benzene", which can be used to oxidize diverse organic compounds.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p><p>Various substitution reactions are also accelerated in the presence of 18-crown-6, which suppresses ion-pairing.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> The anions thereby become naked <a href="/facts/Nucleophiles/xYccoaJQ">nucleophiles</a>. For example, using 18-crown-6, <a href="/facts/Potassium_acetate/IWfDK0HV">potassium acetate</a> is a more powerful nucleophile in organic solvents:<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p> [K·(18-crown-6)]+AcO− + C6H5CH2Cl → C6H5CH2OAc + [K·(18-crown-6)]+Cl− <p>The first <a href="/facts/Electride/2Lp8eUIK">electride</a> salt to be examined with <a href="/facts/X-ray_crystallography/WtI0F5DN">X-ray crystallography</a>, [Cs(18-crown-6)2]+·<i>e</i>−, was synthesized in 1983. This highly air- and moisture-sensitive solid has a sandwich molecular structure, where the electron is trapped within nearly spherical lattice cavities. However, the shortest electron-electron distance is too long (8.68 Å) to make this material a conductor of electricity.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> </p> <h2 id="external-links">External links</h2> <ul><li>[<a href="https://www.sigmaaldrich.com/US/en/search/18-crown-6?focus=products&page=1&perpage=30&sort=relevance&term=18-crown-6&type=product">https://www.sigmaaldrich.com/US/en/search/18-crown-6?focus=products&page=1&perpage=30&sort=relevance&term=18-crown-6&type=product</a></li> <li><a href="https://www.researchgate.net/publication/386397904">[1]</a>ISBNcrowndocx</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3. <a href="978-0-470-51233-3" target="_blank">978-0-470-51233-3</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Caswell, Lyman R.; Savannunt, Diana S. (January 1988). "Temperature and solvent effects on the experimental dipole moments of three crown ethers". J. Heterocyclic Chem. 25 (1): 73–79. doi:10.1002/jhet.5570250111. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Gokel, George W.; Cram, Donald J.; Liotta, Charles L.; Harris, Henry P.; Cook, Fred L. (1977). "18-Crown-6". Org. Synth. 57: 30. doi:10.15227/orgsyn.057.0030. <a href="/wiki/Donald_J._Cram" target="_blank">/wiki/Donald_J._Cram</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3. <a href="978-0-470-51233-3" target="_blank">978-0-470-51233-3</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Gokel, George W.; Cram, Donald J.; Liotta, Charles L.; Harris, Henry P.; Cook, Fred L. (1977). "18-Crown-6". Org. Synth. 57: 30. doi:10.15227/orgsyn.057.0030. <a href="/wiki/Donald_J._Cram" target="_blank">/wiki/Donald_J._Cram</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Jilek, Robert E.; Fischer, Paul J.; Ellis, John E. (2014). "Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−)". Inorganic Syntheses: Volume 36. Vol. 36. pp. 127–134. doi:10.1002/9781118744994.ch24. ISBN 9781118744994. <a href="9781118744994" target="_blank">9781118744994</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Dunitz, J. D.; Seiler, P. (1974). "1,4,7,10,13,16-Hexaoxacyclooctadecane". Acta Crystallogr. B30 (11): 2739. doi:10.1107/S0567740874007928. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3. <a href="978-0-470-51233-3" target="_blank">978-0-470-51233-3</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Atwood, Jerry L.; Bott, Simon G.; Coleman, Anthony W.; Robinson, Kerry D.; Whetstone, Stephen B.; Means, C. Mitchell (December 1987). "The oxonium cation in aromatic solvents. Synthesis, structure, and solution behavior of [H3O+·18-crown-6][Cl-H-Cl]". Journal of the American Chemical Society. 109 (26): 8100–8101. doi:10.1021/ja00260a033. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Atwood, Jerry L.; Bott, Simon G.; Means, C. Mitchell; Coleman, Anthony W.; Zhang, Hongming; May, Michael T. (February 1990). "Synthesis of salts of the hydrogen dichloride anion in aromatic solvents. 2. Syntheses and crystal structures of [ K ⋅ 18 − crown − 6 ] [ Cl − H − Cl ] , {\displaystyle {\ce {[K.18-crown-6][Cl-H-Cl]}},} [ Mg ⋅ 18 − crown − 6 ] [ Cl − H − Cl ] 2 , {\displaystyle {\ce {[Mg.18-crown-6][Cl-H-Cl]2}},} [ H 3 O ⋅ 18 − crown − 6 ] [ Cl − H − Cl ] , {\displaystyle {\ce {[H3O.18-crown-6][Cl-H-Cl]}},} and the related [ H 3 O ⋅ 18 − crown − 6 ] [ Br − H − Br ] {\displaystyle {\ce {[H3O.18-crown-6][Br-H-Br]}}} ". Inorganic Chemistry. 29 (3): 467–470. doi:10.1021/ic00328a025. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Liotta, C. L.; Berknerin, J. (2004). "18-Crown-6". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rc261. ISBN 0471936235. <a href="0471936235" target="_blank">0471936235</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Wynn, David; et al. (1984). "The Solubility of Alkali-Metal Fluorides in Non-Aqueous Solvents With and Without Crown Ethers...". Talanta. 31 (11): 1036–1040. doi:10.1016/0039-9140(84)80244-1. PMID 18963717. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3. <a href="978-0-470-51233-3" target="_blank">978-0-470-51233-3</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Cook, Fred L.; Bowers, Chauncey W.; Liotta, C. L. (November 1974). "Chemistry of naked anions. III. Reactions of the 18-crown-6 complex of potassium cyanide with organic substrates in aprotic organic solvents". The Journal of Organic Chemistry. 39 (23): 3416–3418. doi:10.1021/jo00937a026. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3. <a href="978-0-470-51233-3" target="_blank">978-0-470-51233-3</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3. <a href="978-0-470-51233-3" target="_blank">978-0-470-51233-3</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> </ol>