18-Crown-6

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.[1] Like other crown ethers, 18-crown-6 functions as a ligand for some metal cations with a particular affinity for potassium cations (binding constant in methanol: 106 M−1). The point group of 18-crown-6 is S6. The dipole moment of 18-crown-6 varies in different solvent and under different temperature. Under 25 °C, the dipole moment of 18-crown-6 is 2.76 ± 0.06 D in cyclohexane and 2.73 ± 0.02 in benzene.[2] The synthesis of the crown ethers led to the awarding of the Nobel Prize in Chemistry to Charles J. Pedersen.

See also: Dibenzo-18-crown-6
18-Crown-6
Names
IUPAC name
1,4,7,10,13,16-hexaoxacyclooctadecane
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.037.687
UNII
Properties
C12H24O6
Molar mass 264.315 g/mol
Density 1.237 g/cm3
Melting point 37 to 40 °C (99 to 104 °F; 310 to 313 K)
Boiling point 116 °C (241 °F; 389 K) (0.2 Torr)
75 g/L
Related compounds
Related compounds
 Dibenzo-18-crown-6
Triglyme
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Synthesis

This compound is prepared by a modified Williamson ether synthesis in the presence of a templating cation:[3] It can be also prepared by the oligomerization of ethylene oxide:[1]

(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.[3] 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.[4]

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.[5] The molecule undergoes significant conformational change upon complexation.

Reactions
The complex of H3O+ with 18-crown-6

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 . For example, interaction of 18-crown-6 with HCl gas in toluene with a little moisture gives an ionic liquid layer with the composition , from which the solid can be isolated on standing. Reaction of the ionic liquid layer with two molar equivalents of water gives the crystalline product .[1][6][7]

Applications
18-crown-6 complex with potassium ion

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 phase transfer catalysts.[8] Salts which are normally insoluble in organic solvents are made soluble by crown ether. For example, potassium permanganate 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.[1]

Various substitution reactions are also accelerated in the presence of 18-crown-6, which suppresses ion-pairing.[9] The anions thereby become naked nucleophiles. For example, using 18-crown-6, potassium acetate is a more powerful nucleophile in organic solvents:[1]

[K(18-crown-6)+]OAc− + C6H5CH2Cl → C6H5CH2OAc + [K(18-crown-6)+]Cl−

The first electride salt to be examined with X-ray crystallography, [Cs(18-crown-6)2]+·e−, 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.[1]

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References
  1. Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3.
  2. 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.
  3. Gokel, George W.; Cram, Donald J.; Liotta, Charles L.; Harris, Henry P.; Cook, Fred L. (1988). "18-Crown-6". Organic Syntheses.; Collective Volume, 6, p. 301
  4. Jilek, Robert E.; Fischer, Paul J.; Ellis, John E. (2014). Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−). Inorganic Syntheses. 36. pp. 127–134. doi:10.1002/9781118744994.ch24. ISBN 9781118744994.
  5. Dunitz, J. D.; Seiler, P. (1974). "1,4,7,10,13,16-Hexaoxacyclooctadecane". Acta Crystallogr. B30 (11): 2739. doi:10.1107/S0567740874007928.
  6. 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 ". Journal of the American Chemical Society. 109 (26): 8100–8101. doi:10.1021/ja00260a033.
  7. 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 and the related ". Inorganic Chemistry. 29 (3): 467–470. doi:10.1021/ic00328a025.
  8. Liotta, C. L.; Berknerin, J. (2004). "18-Crown-6". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis, 8 Volume Set. Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rc261. ISBN 0471936235.
  9. 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.
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