ChemistryMediumClass 12

Electromeric Effect

GOC (General Organic Chemistry)

JEE Qs

Hard

min

Always remember that the Electromeric effect is a temporary, reaction-dependent electron displacement in multiple bonds, triggered by an attacking reagent.

✅ Key Points for JEE

1The Electromeric effect (E effect) is a temporary effect involving the complete transfer of shared pi (π) electrons of a multiple bond (double or triple) to one of the atoms linked by the multiple bond.
2This effect occurs only in the presence of an attacking reagent. Once the attacking reagent is removed, the molecule reverts to its original electronic state.
3It is observed exclusively in compounds containing multiple bonds (alkenes, alkynes, carbonyl compounds) during reactions like addition reactions.
4The effect is classified into two types based on the direction of electron transfer relative to the attacking reagent: +E effect and -E effect.
5+E effect occurs when the π electrons are transferred to the atom to which the attacking electrophile attaches. Example: Attack of H⁺ on an alkene.
6-E effect occurs when the π electrons are transferred to an atom *away* from the attacking nucleophile, or towards the nucleophile itself in specific cases. Example: Attack of CN⁻ on the carbonyl carbon of an aldehyde/ketone.

⚠️ Common Mistakes

✕Confusing the Electromeric effect with permanent electronic effects like Inductive effect or Resonance (Mesomeric effect). Electromeric effect is strictly temporary.
✕Incorrectly identifying +E vs. -E effects, especially in complex reactions, by misjudging the direction of pi electron shift relative to the attacking species.
✕Applying the Electromeric effect in systems without multiple bonds or in the absence of an attacking reagent.

NCERT Chapters

Class 11 Chemistry Part 2, Chapter 12: Organic Chemistry - Some Basic Principles and Techniques