Molecular Recognition
Crown ethers are perhaps the simplest molecules mimicking the behavior of enzymes, which are essential for life.
Enzymes carry out very selective chemical transformations. The very first step for an enzyme to exert its biological function is an extremely precise molecular recognition of a given molecule (the substrate), discarding other, even very similar, ones.
Crown ethers are able to selectively recognize alkali metal ions thanks to the size of their oxygen-lined cavity, which attracts the positively charged metal by electrostatic forces.
 |  |  |
|---|---|---|
| 12-Crown-4/Li+ | 15-Crown-5/Na+ | 18-Crown-6/K+ |
 |  |  |
The alkali metal ions Li+, Na+ and K+ have quite different ionic radii, each fitting best in a given crown ether. In this way, by using different crown ethers, one could separate the alkaline ions that otherwise would be very difficult to separate from each other.
Sodium and calcium ions are essential for the living cells. But these highly hydrophilic ions cannot get across the highly lipophilic cell membranes unless certain natural crown-ether-like compounds are inserted in them, thus allowing the selective entrance of these ions.
The chelation of the alkali metal ions by crown ethers has an indirect effect on the negative, nucleophilic counterions that always accompany them.
By the use of crown ethers, one can increase the nucleophilicity of nucleophiles because the positive counterion is separated.
The presence of even a small amount of crown ether can make possible, or greatly improve, a reaction that otherwise would not take place.