MCQ Detailed View

The electromeric effect (E-effect) is an important concept in organic chemistry that explains how electrons behave in molecules containing multiple bonds when an attacking reagent approaches. It is different from permanent electronic effects like the inductive effect or resonance effect, because it exists only during a reaction and disappears when the reagent is removed.

Definition:

The electromeric effect refers to the complete transfer of a shared electron pair from a multiple bond (such as C=C, C=O, or C≡N) to one of the bonded atoms under the influence of an attacking reagent.

Key Characteristics:

1. 
   

   Temporary in nature: It operates only during the approach of a reagent and stops immediately when the reagent is removed.

   
   


2. 
   

   Seen in multiple bonds: This effect is most common in alkenes, carbonyl compounds, and nitriles where electron pairs are easily shifted.

   
   


3. 
   

   Reagent-dependent: It is triggered by either electrophiles (E⁺) or nucleophiles (Nu⁻).

   
   

Types of Electromeric Effect:

1. 
   

   +E Effect (Positive Electromeric Effect):
   When the electron pair is transferred towards the attacking reagent (e.g., in the reaction of alkene with acids like H⁺).

   
   


2. 
   

   –E Effect (Negative Electromeric Effect):
   When the electron pair is shifted away from the attacking reagent (e.g., in the addition of CN⁻ to a carbonyl group).

   
   

Importance:

• 
  

  It helps explain the mechanism of electrophilic addition and nucleophilic addition reactions.

  
  


• 
  

  Plays a role in determining reactivity of alkenes and carbonyls.

  
  


• 
  

  Useful in understanding reaction intermediates like carbocations and carbanions.

  
  

Why other options are incorrect:

• 
  

  Option 1 (Permanent effect): Wrong, because it exists only in the presence of a reagent.

  
  


• 
  

  Option 2 (Operates due to electronegativity difference): This describes the inductive effect, not electromeric.

  
  


• 
  

  Option 4 (None of the above): Incorrect, because option 3 is correct.

  
  

Key Point:

The electromeric effect is a temporary effect, involving complete transfer of electrons in a multiple bond during the attack of a reagent, and is crucial in explaining many addition reactions in organic chemistry.