Crown ethers

These are macrocyclic polyethers containing oxygen or other hetero atoms.

Nomenclature

• The prefix in brackets indicates the total number of atoms constituting the ring system and the number following the hyphen indicates the number of heteroatoms.
• [18]-crown-6 has 18 atoms in the ring including the heteroatoms, which is oxygen in this case and has six oxygen atoms, which are the heteroatoms.
• If a model of this molecule is made it resembles a crown hence the word “crown”.

• These molecules have a hydrophobic exterior and a hydrophilic interior.
• Cations can get into the central cavity of these compounds and are immobilized due to the negative environment of the heteroatoms.
• The cavity size determines which cations can be encapsulated inside the cavity.
• The interactions between the cation and the crown ethers are purely electrostatic, that is through ion dipole attractions.
• Thus each crown ether can selectively capture a cation of a specific size.
• The solvating capacity of crown ethers towards metal ions can be termed as “Host- Guest relationship”.

The Cation size and the Cavity diameter in certain crown ethers

• Presence of groups like cyclohexyl, aryl groups can change the cavity size
• The cavity size or cation size are too different then sandwich complexes or coiled complexes are possible.

• Nature of donor site plays a vital role in selectivity. S will enhance complexes with ions like Ag+ and hamper hard alkali metal ions.
• The heteroatoms include Sulphur and Nitrogen.