18-Crown-6

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 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.

18-Crown-6

Skeletal formula
Ball-and-stick model
sample
Names
Preferred IUPAC name 1,4,7,10,13,16-Hexaoxacyclooctadecane
Identifiers
CAS Number	17455-13-9 Y
3D model (JSmol)	Interactive image Interactive image
Beilstein Reference	1619616
ChEBI	CHEBI:32397 Y
ChEMBL	ChEMBL155204 Y
ChemSpider	26563 Y
ECHA InfoCard	100.037.687
EC Number	241-473-5
Gmelin Reference	4535
PubChem CID	28557
UNII	63J177NC5B Y
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 Y Key: XEZNGIUYQVAUSS-UHFFFAOYSA-N Y 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). Y verify (what is YN ?) Infobox references

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 [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]⁻·3.8C₆H₅Me, 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.^[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)]⁺AcO⁻ + 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]