18-Crown-6
Chemical compound
From Wikipedia, the free encyclopedia
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.
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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 |
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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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
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 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]
![{\displaystyle {[\mathrm {K} \,{\cdot }\,18{\text{-}}\mathrm {crown} {\text{-}}6][\mathrm {Cl} {-}\mathrm {H} {-}\mathrm {Cl} ]},}](http://wikimedia.org/api/rest_v1/media/math/render/svg/033a571d20719af08d7a5198f57da59afc23a226)
![{\displaystyle {[\mathrm {Mg} \,{\cdot }\,18{\text{-}}\mathrm {crown} {\text{-}}6][\mathrm {Cl} {-}\mathrm {H} {-}\mathrm {Cl} ]{\vphantom {A}}_{\smash[{t}]{2}}},}](http://wikimedia.org/api/rest_v1/media/math/render/svg/abf6c19ea8d8195eea1d4679f2a696d69feb2f72)
![{\displaystyle {[\mathrm {H} {\vphantom {A}}_{\smash[{t}]{3}}\mathrm {O} \,{\cdot }\,18{\text{-}}\mathrm {crown} {\text{-}}6][\mathrm {Cl} {-}\mathrm {H} {-}\mathrm {Cl} ]},}](http://wikimedia.org/api/rest_v1/media/math/render/svg/53c829dc937cd5208e3ccb616bdddf618c8b6332)
![{\displaystyle {[\mathrm {H} {\vphantom {A}}_{\smash[{t}]{3}}\mathrm {O} \,{\cdot }\,18{\text{-}}\mathrm {crown} {\text{-}}6][\mathrm {Br} {-}\mathrm {H} {-}\mathrm {Br} ]}}](http://wikimedia.org/api/rest_v1/media/math/render/svg/d1dbd94a5bd7bbec553563db4f4df5d9496b9cf1)