18-Crown-6

18-Crown-6 is an organic compound with the formula [C2H4O]6 and the IUPAC name of 1,4,7,10,13,16-hexaoxacyclooctadecane. The compound is a crown ether. Crown ethers coordinate some metal ions in their central cavity, and 18-crown-6 displays an affinity for potassium ions.

Applications
Crown ethers are useful as phase transfer catalysts. In the presence of 18-crown-6 potassium permanganate dissolves in benzene, although this application is eclipsed by the use of quaternary ammonium/phosphonium salts.
 * [[Image:18-crown-6-potassium.png|200px|18-crown-6 coordinating a potassium ion]]

Crown ethers can also be used to change the properties of inorganic compounds in organic solution. For example, in the presence of 18-crown-6, potassium acetate is a more powerful nucleophile in organic solvents than potassium acetate alone.

It can be purified by distillation, where its tendency to supercool becomes evident. 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. Other ligands for alkali metal cations include cryptands and acyclic polyethers.

Synthesis
The synthesis of the crown ethers led to the Nobel Prize in Chemistry to Charles J. Pedersen. Several methods exist: Williamson ether synthesis, oligomerization of ethylene oxide in the presence of a template.

Related compounds
A related and generally superior complexant for alkali metal cations is dibenzo-18-crown-6.
 * [[Image:Dibenzo-18-crown-6.png|200px|Dibenzo-18-crown-6]]