Crown ethers.pptx

crown ethers are cyclic chemical compounds that consist of a ring containing several ether compounds.
they are cyclic poly-ethers that contain 4 or more oxygen atoms separated by 2 or 3 carbon atoms.

1. Crown ethers Submitted by: Anupama.cp Second semester M- Pharm 1

2. CROWN ETHERS Crown ethers are cyclic chemical compounds that consist of a ring containing several ether compounds. They are cyclic polyethers that contain four or more oxygen atoms separated by two or three carbon atoms. Most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit is ethyleneoxy ie , -CH 2 CH 2 O- 2

3. General formula is (OCH 2 CH 2 )n. For eg . 18 crown 6 is a cyclic oligomer of ethylene glycol. They form stable complexes with a number of metal ions, particularly the alkali metal ions. Crown ethers are named as X- CROWN- Y. X is total number of atoms in the ring and Y is the number of oxygen atoms. In places of oxygen other hetero atoms like sulphur and nitrogen may also present. 3

4. Complexation is mainly from electrostatic ion- dipole attraction between the metal ion situated in the cavity of the ring and the oxygen atom surrounding it. All crown ethers have a central cavity that can accommodate a metal ion coordinated to the ring of oxygen atoms. The size of metal must match the size of crown to form strong attractions. 4

5. Structure of typical crown ether 5

6. Metal ions are not soluble in low polarity solvent, the crown ether complexes to metal cation results soluble complexes. Crown ethers are used to help ionic salts needed for organic reactions to dissolve in organic solvents. 6

7. General synthesis of crown ethers Cyclic materials may be formed from compound that is nucleophilic at the one end, and electrophilic at the other end by reacting with itself. Second possibility is the nucleophilic or negative end of one molecule may find the electrophilic or positive end of another molecule . 7

8. AZA CROWN : macrocycles containing nitrogen are called as aza crown ethers. CRYPTAND : A molecular entity comprising a cyclic or poly cyclic assembly of binding sites held together by covalent bonds. 8

9. Applications SYNTHETIC APPLICATIONS: Esterification Saponification Alkylation Anhydride formation Aromatic substitution reactions Elimination reaction Displacement reaction Generation of carbenes 9

10. ANALYTICAL APPLICATION: Determination of gold in geological samples. Supercritical fluid extraction of trace metal from solid or liquid materials. Oxidation and determination of aldehydes . Used as phase transfer catalyst. OTHER APPLICATIONS: It is used in photocynation . Resolution of racemic mixture Benzoin condensation 10

11. References Vogels textbook of quantitative chemical analysis by J. Mendham ,2019 edition , pg.no . 207-208. Practical high performance liquid chromatography by Veronika .R. Mayer 4 th edition pg.no . 307-308. 11