MCQ Detailed View

Hyperconjugation is a very important electronic effect in organic chemistry. It explains the stability of carbocations, alkenes, and free radicals by showing how electrons can delocalize beyond the normal bonding framework.

Definition:

Hyperconjugation is the delocalization of electrons of a sigma bond (C–H or C–C) with an adjacent empty p-orbital, π-orbital, or antibonding orbital. Because of this delocalization, it is often called “no bond resonance.”

Why Called "No Bond Resonance"?

In resonance, π-electrons or lone pairs are delocalized, whereas in hyperconjugation, even the electrons in a sigma bond (C–H) can participate. When this happens, it seems as if the C–H bond no longer exists, hence the term “no bond resonance.”

Examples:

1. 
   

   Carbocations:
   In a carbocation like the ethyl carbocation (CH₃–CH₂⁺), the C–H bonds on the carbon adjacent to the positively charged carbon can overlap with the vacant p-orbital, delocalizing charge and stabilizing the carbocation.

   
   


2. 
   

   Alkenes:
   In alkenes, hyperconjugation explains why more substituted alkenes (like 2-butene) are more stable than less substituted ones (like 1-butene). More alkyl groups mean more C–H bonds available for hyperconjugation, leading to greater stability.

   
   


3. 
   

   Free radicals:
   Similar stabilization occurs when free radicals are present on a carbon adjacent to C–H bonds.

   
   

Importance of Hyperconjugation:

• 
  

  Explains the relative stability of carbocations (tertiary > secondary > primary > methyl).

  
  


• 
  

  Explains the order of stability in alkenes (tetra-substituted > tri-substituted > di-substituted > mono-substituted).

  
  


• 
  

  Plays a role in the orientation of electrophilic substitution reactions in aromatic compounds.

  
  

Why other options are incorrect:

• 
  

  Option 1 (Reasonable): Not related to hyperconjugation.

  
  


• 
  

  Option 3 (Inductive effect): Involves polarization of sigma bonds due to electronegativity difference, not delocalization.

  
  


• 
  

  Option 4 (Electromeric effect): A temporary effect in the presence of a reagent, not a permanent delocalization like hyperconjugation.

  
  

Key Point:

Hyperconjugation is also known as no bond resonance, because it involves delocalization of sigma bond electrons without the need for a conventional π-bond resonance structure.