18-Crown-6

18-Crown-6
Names
	Preferred IUPAC name 1,4,7,10,13,16-Hexaoxacyclooctadecane
Identifiers
CAS Number	17455-13-9 ;
3D model (JSmol)	Interactive image; Interactive image;
Beilstein Reference	1619616
ChEBI	CHEBI:32397 ;
ChEMBL	ChEMBL155204 ;
ChemSpider	26563 ;
ECHA InfoCard	100.037.687
EC Number	241-473-5;
Gmelin Reference	4535
PubChem CID	28557;
UNII	63J177NC5B ;
CompTox Dashboard (EPA)	DTXSID7058626 ;
	InChI InChI=1S/C12H24O6/c1-2-14-5-6-16-9-10-18-12-11-17-8-7-15-4-3-13-1/h1-12H2 Key: XEZNGIUYQVAUSS-UHFFFAOYSA-N ; InChI=1/C12H24O6/c1-2-14-5-6-16-9-10-18-12-11-17-8-7-15-4-3-13-1/h1-12H2 Key: XEZNGIUYQVAUSS-UHFFFAOYAP;
	SMILES O1CCOCCOCCOCCOCCOCC1; C1COCCOCCOCCOCCOCCO1;
Properties
Chemical formula	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)
Solubility in water	75 g/L
Hazards
	GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H302, H315, H319, H335
Precautionary statements	P261, P264, P270, P271, P280, P301+P312, P302+P352, P304+P340, P305+P351+P338, P312, P321, P330, P332+P313, P337+P313, P362, P403+P233, P405, P501
Related compounds
Related compounds	Dibenzo-18-crown-6; Triglyme; Hexaaza-18-crown-6
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

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.

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.[1] 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 ${\ce {[H3O.18-crown-6]+}}$ . For example, interaction of 18-crown-6 with HCl gas in toluene with a little moisture gives an ionic liquid layer with the composition ${\ce {[H3O.18-crown-6]+[HCl2]^{-}.}}3.8{\ce {C6H5Me}}$ , from which the solid ${\ce {[H3O.18-crown-6]+[HCl2]-}}$ can be isolated on standing. Reaction of the ionic liquid layer with two molar equivalents of water gives the crystalline product ${\ce {(H5O2)[H3O.18-crown-6]Cl2}}$ .[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.[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.[1]

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:[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]

References

Steed, Jonathan W.; Atwood, Jerry L. (2009). Supramolecular Chemistry (2nd ed.). Wiley. ISBN 978-0-470-51233-3.
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.
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.
Jilek, Robert E.; Fischer, Paul J.; Ellis, John E. (2014). Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−). Inorganic Syntheses. Vol. 36. pp. 127–134. doi:10.1002/9781118744994.ch24. ISBN 9781118744994.
Dunitz, J. D.; Seiler, P. (1974). "1,4,7,10,13,16-Hexaoxacyclooctadecane". Acta Crystallogr. B30 (11): 2739. doi:10.1107/S0567740874007928.
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 ${\ce {[H3O+.18-crown-6][Cl-H-Cl]}}$ ". Journal of the American Chemical Society. 109 (26): 8100–8101. doi:10.1021/ja00260a033.
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 ${\ce {[K.18-crown-6][Cl-H-Cl]}},$ ${\ce {[Mg.18-crown-6][Cl-H-Cl]2}},$ ${\ce {[H3O.18-crown-6][Cl-H-Cl]}},$ and the related ${\ce {[H3O.18-crown-6][Br-H-Br]}}$ ". Inorganic Chemistry. 29 (3): 467–470. doi:10.1021/ic00328a025.
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.
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.
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.

External links

Sigma Aldrich chemical profile

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[SupraChem-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.

[gokel-3] 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.

[4] Jilek, Robert E.; Fischer, Paul J.; Ellis, John E. (2014). Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−). Inorganic Syntheses. Vol. 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 ${\ce {[H3O+.18-crown-6][Cl-H-Cl]}}$ ". 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 ${\ce {[K.18-crown-6][Cl-H-Cl]}},$ ${\ce {[Mg.18-crown-6][Cl-H-Cl]2}},$ ${\ce {[H3O.18-crown-6][Cl-H-Cl]}},$ and the related ${\ce {[H3O.18-crown-6][Br-H-Br]}}$ ". 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. New York: J. Wiley & Sons. doi:10.1002/047084289X.rc261. ISBN 0471936235.

[9] 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.

[LiottaKCN-10] 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.