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Names | |
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Preferred IUPAC name
1,4,7,10,13,16-Hexaoxacyclooctadecane | |
Identifiers | |
3D model (
JSmol)
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1619616 | |
ChEBI | |
ChEMBL | |
ChemSpider | |
ECHA InfoCard | 100.037.687 |
EC Number |
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4535 | |
PubChem
CID
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UNII | |
CompTox Dashboard (
EPA)
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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 | |
Hazards | |
GHS labelling: | |
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Warning | |
H302, H315, H319, H335 | |
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
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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).
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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.
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.
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]
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]
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]