18-Crown-6 Explained

See also: Dibenzo-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 is solvent- and temperature-dependent. Below 25 °C, the dipole moment of 18-crown-6 is in cyclohexane and in benzene.^[2] The synthesis of the crown ethers led to the awarding of the Nobel Prize in Chemistry to Charles J. Pedersen.

Synthesis

This compound is prepared by a modified Williamson ether synthesis in the presence of a templating cation:^[3] (CH₂OCH₂CH₂Cl)₂ + (CH₂OCH₂CH₂OH)₂ + 2 KOH → (CH₂CH₂O)₆ + 2 KCl + 2 H₂O

It can be also prepared by the oligomerization of ethylene oxide. 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.^[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

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 $\backslash ce3.8\backslash ce$, 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.^{[6] [7]}

Applications

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.^[9] 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.

Various substitution reactions are also accelerated in the presence of 18-crown-6, which suppresses ion-pairing.^[10] 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 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.

External links

• Sigma Aldrich chemical profile

Notes and References

1. Book: Supramolecular Chemistry . 2nd. Jonathan W. . Steed. Jerry L. . Atwood. Wiley. 2009. 978-0-470-51233-3.
2. Caswell. Lyman R.. Savannunt. Diana S.. Temperature and solvent effects on the experimental dipole moments of three crown ethers. J. Heterocyclic Chem.. January 1988. 25. 1. 73–79. 10.1002/jhet.5570250111.
3. Gokel. George W.. Cram. Donald J.. Donald J. Cram. Liotta. Charles L.. Harris. Henry P.. Cook. Fred L.. 1977. 18-Crown-6. Org. Synth.. 57. 30. 10.15227/orgsyn.057.0030.
4. Book: 10.1002/9781118744994.ch24. Inorganic Syntheses: Volume 36. 36. 127–134. 2014. Jilek. Robert E.. Fischer. Paul J.. Ellis. John E.. Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−) . 9781118744994.
5. J. D. . Dunitz. P. . Seiler . 1,4,7,10,13,16-Hexaoxacyclooctadecane . Acta Crystallogr. . 1974 . B30 . 11. 2739 . 10.1107/S0567740874007928.
6. Atwood . Jerry L. . Bott . Simon G. . Coleman . Anthony W. . Robinson . Kerry D. . Whetstone . Stephen B. . Means . C. Mitchell . The oxonium cation in aromatic solvents. Synthesis, structure, and solution behavior of $\backslash ce$ . Journal of the American Chemical Society . December 1987 . 109 . 26 . 8100–8101 . 10.1021/ja00260a033.
7. Atwood . Jerry L. . Bott . Simon G. . Means . C. Mitchell . Coleman . Anthony W. . Zhang . Hongming . May . Michael T. . Synthesis of salts of the hydrogen dichloride anion in aromatic solvents. 2. Syntheses and crystal structures of $\backslash ce,$ $\backslash ce,$ $\backslash ce,$ and the related $\backslash ce$ . Inorganic Chemistry . February 1990 . 29 . 3 . 467–470 . 10.1021/ic00328a025.
8. Encyclopedia: Liotta. C. L.. Berknerin. J. . 18-Crown-6. Encyclopedia of Reagents for Organic Synthesis . L. . Paquette. 2004 . J. Wiley & Sons. New York . 10.1002/047084289X.rc261. 0471936235.
9. Wynn . etal . David . The Solubility of Alkali-Metal Fluorides in Non-Aqueous Solvents With and Without Crown Ethers... . Talanta . 1984 . 31 . 11 . 1036–1040 . 10.1016/0039-9140(84)80244-1. 18963717 .
10. Cook . Fred L. . Bowers . Chauncey W. . Liotta . C. L. . 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 . November 1974 . 39 . 23 . 3416–3418 . 10.1021/jo00937a026.