Reaction Intermediates

Presented by Aadil Ali Wani

1. Department of Chemistry University of Kashmir Topic:- Reaction intermediates Presented By Aadil Ali Wani

2. Reaction Intermediates In an organic reaction the molecule undergoing change are called reactants and the new molecules formed are called products. e.g. CH₃ + Cl₂→CH₃Cl + HCl In this reaction a C-H bond and a Cl-Cl bond in the reactants have broken,and a new C-C bond and a H Cl bond have formed in the product. There are two possible ways by which a covalent bond can break. Homolytic Cleavage :-The covalent bond between two elements can break in such a way that each element retains an electron of the bonding pair .The resulting fragments are called free radicals.

3. Heterolytic Cleavage:- The bond between two atoms can break in another way in which one element retains the bonding pair and other loses, thus positively and negatively charged ions are formed. Reactions involving heterolytic cleavage are called ionic reactions.

4. These radicals and ions are called intermediates . So these intermediates are short lived (10⁻⁶ to few seconds) and highly reactive fragments formed from reactants or preceeding intermediates and reacts further to give the directly observed products of a chemical reaction. Carbon Radicals :-Those in which the carbon carries one unpaired electron. usually formed by homolytic cleavage at high temp, by UV light or by addition of other radicals .e.g. Carbon radicals are neutral but are extremely reactive. It is supposed to be sp² or sp³ hybridized. They are paramagnetic in character.

5. Stability of carbon radicals:- Increases with the increase in the no of alkyl groups attached to carbon carrying odd electron by hyperconjugation.

6. Carbocations Generally formed in acid catalyzed reactions . An ion in which the carbon carries a positive charge is called carbocation .There are formed by heterolysis .Carbon bearing a positive charge being sp² hybridized . Geometrical shape: -The carbocation takes up the more stable sp ² configuration with the vacant 2p orbital at right angles to the plane of the sp² orbital's .

7. More stable a carbocation, more easily it is formed.

8. Non-Classical Carbocations Carbocations which are stabilized through participation of pi-bonds which are not in conjugation or those of σ -bonds are called non-classical carbocations. e.g.

9. Carbanion Negatively charged organic species in which negative charge resides on the carbon atom. They are formed by heterolysis when a group departs from a molecule without the bonding pair of electrons. Carbon bearing negative charge is sp³ hybridized. Three hybridized orbitals form three covalent bonds with atoms /groups .4 th sp³ hybrid orbital as non bonding pair of electrons. Pyramidal shape similar to ammonia.

10. Stability of carbanions :-stability of carbanions decreases with increase in the no. of alkyl groups.

11. Radical Ions These are reaction intermediates in which both a radical and a charge is present. Generated during Birch reduction

12. Carbenes Carbenes are neutral species having a carbon atom with two bonds and two electrons. Singlet Carbenes : two electrons with opposite spins are paired in one orbital. Carbon atom is sp² hybridized.

13. Triplet Carbenes : electrons have same spin and in different orbital's. Sp hybridized. Triplet carbene is also called biradical. Triplet carbene is relatively more stable than singlet carbene due to inter-electronic repulsions in singlet state

14. Benzynes Highly reactive intermediates generated during course of certain reactions. They have not been isolated. These are generated in certain aromatic compounds in the strong basic conditions.

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