*Introduction to Electromeric Effect*

 

Electromeric Effect

*Introduction to Electromeric Effect*

- The *Electromeric Effect* is a temporary effect that occurs in organic compounds when an attacking reagent interacts with the molecule. It leads to the *complete transfer of a pair of π electrons* from one atom to another in the molecule.

- This effect is significant in the presence of multiple bonds (such as C=C or C=O), and is reversed once the attacking reagent is removed.

- It is important to note that the electromeric effect *only persists as long as the attacking reagent* is present. Once the reagent is removed, the molecule reverts back to its original state.

 

Types of Electromeric Effect

There are two main types of electromeric effects:

1. +E Effect
2. -E Effect

1. +E Effect (Positive Electromeric Effect)

   - Occurs when the *pi electrons* of a bond move towards the atom that is under attack by an electrophile.

   - An example of this is the addition of an acid (H⁺) to an alkene. The pi bond shifts toward the more electronegative atom to facilitate the reaction.

   - In this case, the *electrophilic reagent* attaches to the atom that gained an electron pair.

 

2. -E Effect (Negative Electromeric Effect)

   - This occurs when the *shared electron pair* in the π-bond moves away from the atom that is under attack by a nucleophilic reagent.

   - The nucleophile attacks the atom that has lost its shared electrons, forming a bond with it. This is observed when the electron pair is transferred away from the attacking reagent.

 

Example Reactions:

- In the case of *+E effect*, a hydrogen ion (H⁺) reacts with an alkene, where the electrons move towards the more electronegative atom (like a carbon atom with a positive charge), facilitating electrophilic attack.

- In the *-E effect*, a nucleophile such as CN⁻ attacks a carbonyl group (C=O), where the electron pair moves away from the C=O bond towards the oxygen atom, creating a negative charge on the oxygen and making the carbon atom susceptible to nucleophilic attack.

Note: The electromeric effect is more dominant over inductive effect

 Difference between Inductive Effect and Electromeric Effect

*Inductive Effect*	*Electromeric Effect*
It is a *permanent* effect.	It is a *temporary* effect
The presence of multiple bonds (unsaturation) is not essential.	Presence of a multiple bond is essential.
The polarity of the bond is essential.	The polarity of the bond is not essential.
*Partial charge* separation occurs.	*Complete charge* separation occurs.
No ions are formed	Ions are formed.
Attacking reagent is not required	Attacking reagent is required.
The displaced electrons do not leave molecular orbitals	The displaced electrons attain a new position, leaving orbitals.
It involves *mere displacement* of electrons.	It involves *complete transfer* of π-electrons.

*Conclusion*

- The *Electromeric Effect* is an essential concept in understanding reaction mechanisms in organic chemistry, especially in reactions involving alkenes, carbonyls, and other compounds with multiple bonds. Understanding both the *+E and -E effects* provides insight into how reagents interact with organic molecules at the molecular level.