Electromeric Effect in Organic Chemistry
Definition, Types, Examples, Difference from Inductive Effect, and Applications
🔹 Introduction
Organic chemistry is full of fascinating electron shifts and reaction mechanisms. One of the most important temporary effects is the Electromeric Effect. This effect plays a vital role in understanding how electrophiles and nucleophiles attack molecules during reactions such as addition and substitution.
📌 Definition of Electromeric Effect
The Electromeric Effect (E-effect) is a temporary effect observed in compounds with double or triple bonds. When an attacking reagent approaches the molecule, the shared pair of π-electrons is completely transferred towards one atom, leading to temporary polarity in the molecule.
🔹 Types of Electromeric Effect
There are two types of electromeric effects based on the direction of electron transfer:
1. +E Effect (Positive Electromeric Effect)
In this effect, the π-electrons are shifted towards the atom to which the reagent attaches.
Example: In the attack of H⁺ ion on an alkene (C=C), the electron pair shifts towards the carbon atom where H⁺ attaches.
2. –E Effect (Negative Electromeric Effect)
In this effect, the π-electrons are shifted away from the atom to which the reagent attaches.
Example: When CN⁻ attacks a carbonyl group (C=O), the electrons shift towards oxygen, away from carbon.
🔹 Difference between Electromeric and Inductive Effect
| Feature | Electromeric Effect | Inductive Effect |
|---|---|---|
| Nature | Temporary | Permanent |
| Cause | Presence of attacking reagent | Difference in electronegativity |
| Bonds involved | π-bonds (double/triple bonds) | σ-bonds |
| Reversibility | Reversible when reagent is removed | Non-reversible |
🔹 Applications of Electromeric Effect
- Explains addition reactions in alkenes and alkynes.
- Helps to understand nucleophilic attack on carbonyl compounds.
- Important in predicting reaction intermediates such as carbocations and carbanions.
- Used to explain reaction mechanisms in organic chemistry.
✅ Key Points to Remember
- Electromeric Effect = Temporary shift of π-electrons.
- Occurs only when a reagent attacks.
- Two types: +E (towards reagent) and –E (away from reagent).
- Differs from inductive effect which is permanent.
🔹 Conclusion
The Electromeric Effect is an essential concept in organic chemistry that explains how molecules temporarily adjust their electron distribution during a reaction. By understanding this effect, students can easily predict the mechanism of many important organic reactions.
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