Chapter 4 1st year Book
(Page 5)

Crown complexes of Alkali metals

Alkali metals do not form complexes due to their large size and low positive charge, In 1976 Pedersen discovered that crown ethers are an interesting class of complexing agents for alkali metals. These complexing agents are macro cycle polyethers known as crown ether. Pedersen has synthesized and has given a trivial name to over fifty cyclic polyethers. All these polyether rings contain 9 to 60 atoms. Out of which 3 to 20 atoms are Oxygen.

These crown ethers/complexes may be identified by –

1- Number and kind of substituent group present in the rings.
2- How many atoms are present in the ring.
3- The class name of all such rings is crown.
4. The number of oxygen atoms present in the ring.
Hence, according to Pedersen, the trivial name is given as-Figure 4.04 : Structure of some crown ethers

The dibenzo-18 crown- 6 contains two benzene rings in the compound and 18 atoms make up a crown shape ring with 6 of the ring atoms being oxygen as shown in figure 4.04. These 6 oxygen atoms form complexes with alkali metal ions. The bonding of the metal ion to the polyether is largely electrostatic and there is a close fit between the size of the hole in the center of the polyether and the size of the alkali metal ion. All the cyclic polyethers can have varying sizes of the ring e.g. benzo-12crown-4 has a ring of 12 atoms out of which 4 are O – atoms. This crown ether form complexes especially with Li and Na. Whereas K prefers Di Benzo-18-crown-6. (DB-18 C 6 ). There are a number of crystalline complexes formed by crown ethers with alkali metals but it is important that they dissolve metal salts in organic solvent and they maintain the balance between Na+ and K+ inside and outside the cells. Pedersen prepared fifty complexes using various crown ethers with alkali and alkaline earth metals. If the hole of crown ether is not fit for the size of alkali metal ions then two molecules of it are utilized per metal ion to form a sandwich complex. Following are the complexes formed by alkali metal ions with the crown ethers.

Following are the complexes formed by alkali metal ions with the crown ethers

   Page No. 5

   Previous PageNext Page   

Spread The Love