Crown ethers and inclusion compounds
- 1. Crown Ethers and Inclusion Compounds SUPERVISED BY PRESENTED BY Prof. Asmita Gajbhiye Deepa Parwani Roll no Y18254013 DEPARTMENT OF PHARMACEUTICAL SCIENCES Dr. Harisingh Gour Central University, Sagar (M.P.)
- 2. Crown Ether Crown ethers are large-ring compounds that contain several oxygen atoms, usually in a regular pattern. These compounds have the property of forming complexes with positive ions, generally metallic ions or ammonium and substituted ammonium ions. Examples of crown ethers • 12-crown-4 • Dicyclohexano-18-crown-6 etc. 18-crown-6
- 3. 12-crown-4 OO O O Where 12 is the size of the ring and 4 represents the number of coordinating atoms, here oxygen
- 4. Dicyclohexano-18-crown-6 O O O O O O Where 18 is size of the ring and 6 represents the number of coordinating atoms.
- 5. The crown ether is called host and the ion is the guest. In most cases, the ion are held tightly in the center of the cavity. Each crown ether binds different ions, depending on the size of the cavity. For example 18-crown-6 coordinating a potassium ion and 12-crown-4 binds lithium ion
- 6. Utility of Crown Ethers Utility of crown ethers in separating mixtures of cations. (The high degree of selectivity enables the crown compounds to identify the guest atom in a solution and wrap around it) Crown ethers are widely used in organic synthesis. Crown ethers are useful in phase transfer catalysis. Crown ethers are most frequently used to complex cations, amines, phenol etc.
- 7. Cryptates Bicyclic molecules can surround the enclosed ion in 3D, binding it even more tightly than the monocyclic crown ethers. Bicyclics and cycles of higher order are called cryptands and the complexes formed are called cryptates. Cryptands (from the greek ‘kryptos’ meaning “hidden”) are compounds that can completely surround a cation with lone pairs of electrons on oxygen and nitrogen atoms. The number of in the name of cryptand is the number of oxygen atoms in each strand of the molecule.
- 8. Cont… For example The potassium complex of 2,2,2,-cryptand, showing how the cation is almost hidden by the cryptand
- 9. Bonding in crown ether complexes is the result of ion- dipole attractions between the heteroatom and the positive ions. The parameters of the host-guest interactions can sometime be measured by NMR. It has been implied that the ability of these host molecule to bind guests is often very specific, often linked to the hydrogen bonding ability of the host, enabling the host to pull just one molecule or ion out of a mixture. This is called molecular recognition.
- 10. In this, the host forms a crystal lattice that has spaces large enough for the guest to fit into. Vander waals forces constitute the only bonding between the host and guest. Types (depending on the shape of the space) Long tunnels or channels Clathrate or cage compounds
- 11. Cont…. In both types, the guest molecule must fit into the space and guests that are too large or too small will not go into the lattice. Example • Hydrogen sulfide forms hexagonal clathrate cages, and a guest molecule is pinacolone
- 12. Cyclodextrins • There is one type of host that can form both channel and cage complexes, this type is called cyclodextrins or cycloamyloses. • The host molecules are made up of six, seven or eight glucose units connected in a large ring, called respectively alpha, beta or gamma cyclodextrin. • As expected for carbohydrate molecules, all of them are soluble in water.
- 13. Since the cavities of the three cyclodextrins are of different sizes, a large variety of guests can be accommodated.
- 14. The cyclodextrins also form channel-type complexes.
- 15. Applications of inclusion compounds Since cyclodextrins are non toxic (they are actually small starch molecules), they are now used industrially to encapsulate foods and drugs. Zeolites are three dimensional, microporous, crystalline solids with well defined structures that contain aluminium, silicon and oxygen in their regular framework , which is used as ion exchangers to convert hard water to soft water.