Crown ether

Crown ethers are cyclic ethers whose schematic structure in the sequence of ethyleneoxy units (–CH ₂ –CH ₂ –O–) is reminiscent of a crown. They were discovered by Charles Pedersen , a chemist at DuPont (Pedersen 1967). For this discovery he received the Nobel Prize in Chemistry in 1987 together with Jean-Marie Lehn and Donald J. Cram .

construction

Crown ethers with m ring members and n heteroatoms are designated as follows: [ m ] crown n .

• Examples of crown ethers
• 

  [12] Crown-4

• 

  [15] Crown-5

• 

  [18] Crown-6

• 

  [21] Crown-7

• 

  [24] Crown-8

• 

  Diaza- [18] crown-6

As in the case of dicyclohexano [18] crown-6 below, the names of additional substituents are prefixed as prefixes. A more precise IUPAC nomenclature exists, but it is much more complex.

meaning

Dicyclohexano- [18] crown-6

In addition to the aesthetic appeal of the structures, their ability to complex cations with the formation of so-called coronates is of great interest. In the case of metal cations, complexation is achieved through the attractive interaction of the negatively polarized oxygen (or other heteroatoms, such as nitrogen or sulfur) with the cations. In the case of ammonium ions, hydrogen bonds are formed with the protons. Matches with the structures and bonding properties of e.g. B. Valinomycin or Nonactin are striking. This property makes crown ethers an important link between organic chemistry, inorganic chemistry and biochemistry. The size of the opening is determined by the number of sub-units. If there is a good match between the inner diameter and the cation radius, a remarkable selectivity can be achieved. For example, dicyclohexano [18] crown-6 binds potassium ions around a hundred times better than sodium ions .

Chemical properties

Due to the balanced hydrophilicity - hydrophobicity balance, crown ethers are soluble in most organic solvents as well as in water. Their ability to convert ionic, hydrophilic compounds into organic phases is based on this. Potassium permanganate, for example, becomes soluble in benzene with a violet color when [18] crown-6 is added . With these and other crown ether-mediated reactions, otherwise hardly possible reactions succeed (Lehn 1975; Liotta 1974). With chiral crown ethers, almost 100% enantiomeric excess can be achieved in Michael reactions (Cram 1981). In addition to synthesis, crown ethers are also important in analytical chemistry (Kolthoff 1979). The use for ion-selective electrodes is of particular importance (Koryta 1980). After the cation in the cryptate (complex) has been fixed, the activity of the unbound anion increases, which becomes strongly nucleophilic. For this reason, when crown ethers are added, the hydrolysis of esters with potassium hydroxide solution gives much faster results.

synthesis

Crown ethers are produced by intramolecular ether synthesis according to Alexander William Williamson , the so-called Williamson ether synthesis . To suppress intermolecular linkages, it is necessary to work in a very dilute solution.

Cryptands

[2.2.2] Cryptand

By bridging the opening, so-called cryptands (so called von Lehn , 1978 ) are obtained, which can form even stronger and more selective complexes (the cryptates ), especially with alkali and alkaline earth metal ions . The “bridgehead” is formed by two nitrogen atoms which, like crown ethers, are linked by three bridges made up of ethyleneoxy units (–CH ₂ –CH ₂ –O–). For the simplified nomenclature, the number of oxygen atoms in the first, second and third bridge is separated from each other by dots in front of the word "cryptand" in square brackets: in the example shown [2.2.2] cryptand . Like crown ethers, cryptands are also readily soluble in water and many organic solvents .

The cryptand synthesis is much more difficult and expensive than the normal crown ether synthesis.

literature

• C. Pedersen: J. Am. Chem. Soc. 1967 , 89 , 7017.
• CL Liotta, HP Harris, J. Am. Chem. Soc. 1974 , 96 , 2250.
• DJ Cram, Chem. Comm. 1981 , 625.
• IM Kolthoff, Anal. Chem. 1979 , 51 , 1R.
• J. Koryta: Ion-Selective Electrodes . Wiley, 1980 .
• J.-M. Lehn, B. Dietrich, Tetrahedron. Lett. 1973 , 1225; doi : 10.1016 / S0040-4039 (01) 95803-4 .
• J.-M. Lehn, Acc. Chem. Res. 1978 , 11 , 49; doi : 10.1021 / ar50122a001 .
• Overview: Fritz Vögtle : Supramolekulare Chemie . Teubner Study Books , 1989, p. 39ff., ISBN 3519035022 .

Web links

Commons : Kronenether  - collection of pictures, videos and audio files

• Information from the Nobel Foundation on the award ceremony in 1987 to Charles Pedersen, Jean-Marie Lehn and Donald J. Cram (English)

Individual evidence

1. ↑ External identifiers from or database links to [21] Krone-7 : CAS number: 33089-36-0, EC number: 251-373-3, ECHA InfoCard: 100.046.689 , PubChem : 93160 , Wikidata : Q55176433 .
2. ↑ External identifiers from or database links to [24] Krone-8 : CAS number: 33089-37-1, EC number: 251-374-9, ECHA InfoCard: 100.046.690 , PubChem : 10893545 , ChemSpider : 9068808 , Wikidata : Q82934081 .
3. ↑ External identifiers of or database links to Diaza- [18] Krone-6 : CAS number: 23978-55-4, EC number: 245-965-0, ECHA InfoCard: 100.041.772 , PubChem : 72805 , ChemSpider : 65641 , Wikidata : Q72507855 .
4. ↑ Beyer, Walter; Organic Chemistry Textbook; 2004; Vol. 24, p. 324.
5. ^ Vögtle, Fritz: Supramolekulare Chemie, Stuttgart 1989, p. 336.
6. ↑ Dietrich, B. "Cryptands" in Comprehensive Supramolecular Chemistry; Gokel, GW Ed; Elsevier: Oxford, 1996; Vol. 1, pp. 153-211. ISBN 0080406106 .