Electromeric Effect in Organic Chemistry | chemca

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Electromeric Effect (E-Effect)

The temporary displacement of pi ($\pi$) electrons in the presence of an attacking reagent.

By chemca Team • Updated Jan 2026

The Electromeric Effect is a temporary electronic effect observed in organic compounds containing multiple bonds (double or triple bonds). It involves the complete transfer of a shared pair of $\pi$-electrons to one of the bonded atoms, but only in the presence of an attacking reagent.

1. Key Characteristics

• Temporary Effect: It vanishes as soon as the attacking reagent is removed.
• Pi ($\pi$) Electrons Only: It operates only in compounds with multiple bonds ($C=C, C=O, C \equiv N$, etc.).
• Complete Transfer: The electron pair completely shifts to one atom, creating distinct positive and negative centers.
• Reagent Dependent: Unlike the Inductive effect, this effect remains dormant until a reagent approaches.

2. Types of Electromeric Effect

The direction of electron transfer depends on the nature of the attacking reagent (Electrophile or Nucleophile).

+E Effect (Positive)

The $\pi$-electrons are transferred to the same atom to which the attacking reagent gets attached.

Common Condition:

Occurs when the reagent is an Electrophile ($H^+$).

$$ >C=C< + H^+ \rightarrow >\overset{+}{C}-\underset{H}{\underset{|}{C}}< $$

Example: Addition of acid to alkene.

-E Effect (Negative)

The $\pi$-electrons are transferred to the atom other than the one to which the attacking reagent gets attached.

Common Condition:

Occurs when the reagent is a Nucleophile ($CN^-$).

$$ >C=O + CN^- \rightarrow >\underset{CN}{\underset{|}{C}}-O^- $$

Example: Addition of cyanide to carbonyl.

3. Comparison: Inductive vs Electromeric

Inductive Effect (I-Effect)	Electromeric Effect (E-Effect)
Permanent effect. Always present.	Temporary effect. Only present with reagent.
Involves displacement of Sigma ($\sigma$) electrons.	Involves transfer of Pi ($\pi$) electrons.
Partial charge separation ($\delta+, \delta-$).	Complete charge separation ($+, -$).
No attacking reagent required.	Attacking reagent is mandatory.

4. Note on Direction

When both Inductive and Electromeric effects operate in opposite directions, the Electromeric Effect usually predominates because it involves a complete transfer of loose $\pi$-electrons, whereas the Inductive effect is a mere displacement of $\sigma$-electrons.

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