|Jmol-3D images||Image 1
|Molar mass||264.315 g/mol|
|Melting point||37–40 °C|
|Boiling point||116 °C (0.2 Torr)|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
18-Crown-6 is an organic compound with the formula [C2H4O]6 and the IUPAC name of 1,4,7,10,13,16-hexaoxacyclooctadecane. It is a white, hygroscopic crystalline solid with a low melting point. Like other crown ethers, 18-crown-6 functions is a ligand for some metal cations with a particular affinity for potassium cations (binding constant in methanol: 106 M−1). The synthesis of the crown ethers led to the awarding of the Nobel Prize in Chemistry to Charles J. Pedersen.
- (CH2OCH2CH2Cl)2 + (CH2OCH2CH2OH)2 + 2 KOH → (CH2CH2O)6 + 2 KCl + 2 H2O
It can be purified by distillation, where its tendency to supercool becomes evident. 18-Crown-6 can also be purified by recrystallisation from hot acetonitrile. It initially forms an insoluble solvate. Rigorously dry material can be made by dissolving the compound in THF followed by the addition of NaK to give [K(18-crown-6)]Na, an alkalide salt. 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. The molecule undergoes significant conformational change upon complexation.
18-Crown-6 binds to a variety of small cations. It wraps around metal cations, providing an octahedral cavity.
18-Crown-6 has a high affinity for the hydronium 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 toluene with a little moisture gives an ionic liquid layer with the composition [H3O⊂18-crown-6]+[HCl2]-•3.8 C6H5Me, 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.
Crown ethers are used in the laboratory as phase transfer catalysts. In general however it is not widely used; cheaper and more versatile phase transfer catalysts are known. In the presence of 18-crown-6, potassium permanganate dissolves in benzene giving the so-called "purple benzene", which can be used to oxidize diverse organic compounds.
Various substitution reactions are also accelerated in the presence of 18-crown-6, which suppresses ion-pairing. The anions thereby become naked nucleophiles. For example, using 18-crown-6, potassium acetate is a more powerful nucleophile in organic solvents:
- [K(18-crown-6)+]OAc- + C6H5CH2Cl → C6H5CH2OAc + [K(18-crown-6)+]Cl-
The first electride salt using 18-crown-6 that has been synthesized and examined with X-ray crystallography is [Cs(18-crown-6)2]+•e- 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 related and generally superior complexant for alkali metal cations is dibenzo-18-crown-6.
- Jonathan W. Steed; Jerry L. Atwood (2009). Supramolecular Chemistry, 2nd edition. Wiley. ISBN 978-0-470-51233-3.
- George W. Gokel, Donald J. Cram, Charles L. Liotta, Henry P. Harris, and Fred L. Cook (1988), 18-Crown-6, Org. Synth.; Coll. Vol. 6: 301
- J. D. Dunitz, P. Seiler "1,4,7,10,13,16-Hexaoxacyclooctadecane" Acta Cryst. (1974). B30, 2739. doi:10.1107/S0567740874007928.
- Liotta, C. L.; Berknerin, J. "18-Crown-6" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.rc261