Crown ether
Crown ethers, cyclic ethers, whose schematic structure in the sequence of ethylene oxy units ( -CH2-CH2 -O- ) to a crown are remembered. They were obtained from Charles Pedersen, a chemist at DuPont discovered ( Pedersen 1967). For this discovery he was awarded in 1987 with Jean- Marie Lehn and Donald J. Cram received the Nobel Prize for Chemistry.
Construction
Crown ethers with m n ring members and oxygen atoms are denoted as follows: [m ] crown -n.
- Examples of crown ethers
Crown-5
Crown-6
Crown -7
Crown -8
Diaza -crown-6
The names of additional substituents are prefixed as in the case below the DicyclohexanoKrone -6 as prefixes. A more precise nomenclature exists, but is much more complex.
Importance
In addition to the aesthetic appeal of the structures is their ability to complex cations to form so-called Coronate of great interest. The complexation in the case of metal cations achieved by the attractive interaction between the negatively polarized oxygen atoms with the cations. In the case of ammonium ion hydrogen bonds are formed to the protons. Similarities with the structures and binding properties of, eg, valinomycin or nonactin are striking. This property makes crown ethers to an important link between organic chemistry, inorganic chemistry and biochemistry. The width of the opening is determined by the number of sub-units. In good agreement with an internal diameter and radius of the cation is a remarkable selectivity can be achieved. So DicyclohexanoKrone -6 binds potassium ions around a hundred times better than sodium ions.
Chemical Properties
The balanced hydrophilicity - hydrophobicity balance of crown ethers are soluble in most organic solvents and in water. This is the basis of their ability to convert ionic, hydrophilic compounds into organic phases. Thus, potassium permanganate is soluble on addition vonKrone -6 in benzene with a violet color. With these and other crown ether -mediated reactions otherwise succeed hardly possible reactions ( Lehn, 1975; Liotta 1974). The use of chiral crown ethers almost 100 % enantiomeric excess leave at Michael reactions achieve ( Cram 1981). In addition to the synthesis of crown ethers are also of importance in analytical chemistry ( Kolthoff 1979). Of particular importance is the use of ion-selective electrodes ( Koryta 1980).
Synthesis
Crown ethers are intramolecular ether synthesis by Alexander William Williamson, the so-called Williamson ether produced. To suppress intermolecular linkages working in highly dilute solution is necessary.
Cryptands
By bridging of the opening is obtained so-called cryptand ( as indicated by Lehn, 1978), which may form ( cryptates ) especially with alkali metal and alkaline earth metal ions, and even more selective complexes. The " bridgehead" form two nitrogen atoms such as crown ethers linked by three constructed from ethylene oxy units ( -CH2-CH2 -O-) bridges. For the simplified nomenclature, the number of oxygen atoms is prefixed in the first, second and third bridge separated by dots before the word " cryptand " in square brackets: in the example shown [2.2.2 ] cryptand. As crown ethers are also cryptands in water and many organic solvents soluble.
The cryptand synthesis turns out much harder and more expensive than the normal Kronenethersynthese.