HYPER CONJUGATION

·   When  sigma electrons are in conjugation with pi- bond then this conjugation is known to be ‘Hyperconjugation’.

·   Generally used to compare stability.

Required Conditions:

1.Compound should have at least one sp2 hybridized carbon atom of alkene or alkyl carbocation or alkyl free radical.

2. Alpha carbon with respect to sp2 hybrid carbon must be sp3 and it should have at least one alpha-Hydrogen.

If both the conditions are fulfilled then only  “Hyperconjugation” occurs otherwise not.

Ex 1:Lets consider an example of 

                  

                

          ·    Alpha-carbon is the carbon attached to the sp2 - hybrid carbon.

          ·     Alpha-Hydrogen is the Hydrogen attached to the alpha-carbon atom.

         Hence,

          This compound has three alpha-hydrogens

       Type 1: Pi-Conjugation

         1.

    

            Two alpha hydrogens in the above compound.

         2.

   

             Five alpha-hydrogens in the above compound.

         Type 2 : Positive Charge conjugation

              

         1.

                                                 

                 Three alpha-hydrogens in the above compound.

         2.   

                 Six alpha-hydrogens in the above compound.

           Type 3: Odd electron conjugation

           

           1.

                    Three alpha-hydrogens in the above compound.

          2.

 

                  Five alpha hydrogens in the above compound.

            Applications:
         
            Hyperconjugation is generally used to compare stability of alkene.
             No. of Hyperconjugative structures = 1 + No. of alpha hydrogens.

             No. of  alpha hydrogens is proportional to the No. of Hyperconjugative
             structures.

                            

                 2. Stability of carbocations

                         

               3.  Stability of free radicals

      

 Reverse Hyperconjugation:-

    
     Only possible if a halogen is in conjugation with pi- bonds.

  

    Three alpha chlorines are in conjugation with the pi bond.whenever 3 halogens are in conjugation with pi bond and hence it is known as reverse conjugation.