Extraction by Crown Ethers:

A new class of reagents, namely the crown compounds, is fast emerging as potential extractants. Initially, the importance of these compounds was highlighted because of the fact that they are good extractants for alkaline and alkali earth metals and thus, the separation of these metals by solvent extraction became possible. These are multidentate macrocyclic ligands with O, N or S as donor atoms. The binding of the metal is determined by the compatability of the cavity size of the crown compound and the radius of the metal ion.

The first few compounds of this series were crown ethers which consist of cyclic (-CH₂-O-CH₂-) groups to form a cyclic compound having 12-24 ring oxygen atoms.

Following the synthesis of crown ethers, several new compounds have been prepared and investigated. These compounds differ in properties owing to the differences in ring atoms and the number of donor atoms. The variation in the ring size leads to the changes in the size and hence, the size of the ion which can be accommodated. By introduction of the linking group, the donor strength of the oxygen can be changed. Also, by introducing S or N atoms instead of oxygen, crown compound of different specificity are synthesized. The introduction of oligoether chains in the classical crown ethers leads to the formation of a new bicyclic type of ligands. These ligands are known as cryptands and find considerable application in analytical chemistry. It may be worth mentioning that the discovery of macrocyclic polyether was a chance but the synthesis of macrobicyclic polyethers is carried out in a planned way.