18-Crown-6: Difference between revisions
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:(CH<sub>2</sub>OCH<sub>2</sub>CH<sub>2</sub>Cl)<sub>2</sub> + (CH<sub>2</sub>OCH<sub>2</sub>CH<sub>2</sub>OH)<sub>2</sub> + 2 KOH → (CH<sub>2</sub>CH<sub>2</sub>O)<sub>6</sub> + 2 KCl + 2 H<sub>2</sub>O |
:(CH<sub>2</sub>OCH<sub>2</sub>CH<sub>2</sub>Cl)<sub>2</sub> + (CH<sub>2</sub>OCH<sub>2</sub>CH<sub>2</sub>OH)<sub>2</sub> + 2 KOH → (CH<sub>2</sub>CH<sub>2</sub>O)<sub>6</sub> + 2 KCl + 2 H<sub>2</sub>O |
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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.<ref name = gokel/> 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.<ref>{{cite journal|doi=10.1002/9781118744994.ch24|title=Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−) |
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.<ref name = gokel/> 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.<ref>{{cite journal|doi=10.1002/9781118744994.ch24|title=Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−)|volume=36|pages=127–134|series=Inorganic Syntheses|year=2014|last1=Jilek|first1=Robert E.|last2=Fischer|first2=Paul J.|last3=Ellis|first3=John E.|isbn=9781118744994}}</ref> |
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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.<ref>{{cite journal|first=J. D. |last=Dunitz|first2= P. |last2=Seiler |title=1,4,7,10,13,16-Hexaoxacyclooctadecane |journal=Acta Crystallogr. |date=1974 |volume=B30 |page=2739 |doi=10.1107/S0567740874007928}}</ref> The molecule undergoes significant conformational change upon complexation. |
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.<ref>{{cite journal|first=J. D. |last=Dunitz|first2= P. |last2=Seiler |title=1,4,7,10,13,16-Hexaoxacyclooctadecane |journal=Acta Crystallogr. |date=1974 |volume=B30 |issue=11|page=2739 |doi=10.1107/S0567740874007928}}</ref> The molecule undergoes significant conformational change upon complexation. |
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==Reactions== |
==Reactions== |
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==Applications== |
==Applications== |
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:[[Image:18-crown-6-potassium-3D-vdW-A.png|thumb|left|150px|18-crown-6 complex with potassium ion]] |
:[[Image:18-crown-6-potassium-3D-vdW-A.png|thumb|left|150px|18-crown-6 complex with potassium ion]] |
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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 catalyst]]s.<ref>{{cite encyclopedia|last=Liotta|first= C. L.|last2=Berknerin|first2=J. | |
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 catalyst]]s.<ref>{{cite encyclopedia|last=Liotta|first= C. L.|last2=Berknerin|first2=J. |chapter=18-Crown-6|encyclopedia= Encyclopedia of Reagents for Organic Synthesis |editor-first= L. |editor-last=Paquette|date= 2004 |publisher=J. Wiley & Sons|location= New York |doi=10.1002/047084289X.rc261|isbn= 0471936235|title= Encyclopedia of Reagents for Organic Synthesis, 8 Volume Set}}</ref> Salts which are normally insoluble in organic solvents are made soluble by crown ether. 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.<ref name="SupraChem" /> |
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Various substitution reactions are also accelerated in the presence of 18-crown-6, which suppresses ion-pairing.<ref name="LiottaKCN">{{cite journal |last1=Cook |first1=Fred L. |last2=Bowers |first2=Chauncey W. |last3=Liotta |first3=C. L. |title=Chemistry of naked anions. III. Reactions of the 18-crown-6 complex of potassium cyanide with organic substrates in aprotic organic solvents |journal=The Journal of Organic Chemistry |date=November 1974 |volume=39 |issue=23 |pages=3416–3418 |doi=10.1021/jo00937a026}}</ref> The anions thereby become naked [[nucleophiles]]. For example, using 18-crown-6, [[potassium acetate]] is a more powerful nucleophile in organic solvents:<ref name="SupraChem" /> |
Various substitution reactions are also accelerated in the presence of 18-crown-6, which suppresses ion-pairing.<ref name="LiottaKCN">{{cite journal |last1=Cook |first1=Fred L. |last2=Bowers |first2=Chauncey W. |last3=Liotta |first3=C. L. |title=Chemistry of naked anions. III. Reactions of the 18-crown-6 complex of potassium cyanide with organic substrates in aprotic organic solvents |journal=The Journal of Organic Chemistry |date=November 1974 |volume=39 |issue=23 |pages=3416–3418 |doi=10.1021/jo00937a026}}</ref> The anions thereby become naked [[nucleophiles]]. For example, using 18-crown-6, [[potassium acetate]] is a more powerful nucleophile in organic solvents:<ref name="SupraChem" /> |
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Revision as of 07:40, 26 November 2019
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| Names | |
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| IUPAC name
1,4,7,10,13,16-hexaoxacyclooctadecane
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| Identifiers | |
3D model (JSmol)
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| ChEBI | |
| ChEMBL | |
| ChemSpider | |
| ECHA InfoCard | 100.037.687 |
PubChem CID
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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 | |
| Related compounds | |
Related compounds
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Dibenzo-18-crown-6 Triglyme |
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 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] It can be also prepared by the oligomerization of ethylene oxide:[1]
- (CH2OCH2CH2Cl)2 + (CH2OCH2CH2OH)2 + 2 KOH → (CH2CH2O)6 + 2 KCl + 2 H2O
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 . 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]
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. 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.[9] 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
- ^ a b c d e f 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.
- ^ a b Gokel, George W.; Cram, Donald J.; Liotta, Charles L.; Harris, Henry P.; Cook, Fred L. (1988). "18-Crown-6". Organic Syntheses; Collected Volumes, vol. 6, p. 301.
- ^ Jilek, Robert E.; Fischer, Paul J.; Ellis, John E. (2014). "Bis(1,2-Bis(Dimethylphosphano)Ethane)Tricarbonyltitanium(0) and Hexacarbonyltitanate(2−)". Inorganic Syntheses. 36: 127–134. doi:10.1002/9781118744994.ch24. ISBN 9781118744994.
{{cite journal}}: Cite journal requires|journal=(help) - ^ 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 ". 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 and the related ". 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, 8 Volume Set. Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rc261. ISBN 0471936235.
- ^ 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.