Mesomeric Effect (Resonance Effect)
1. Definition
The mesomeric effect (M-effect or Resonance effect) is a permanent electron displacement in conjugated systems (π-bonds or lone pairs) due to delocalization of electrons through resonance.
It is transmitted through the entire molecule via π-orbitals, unlike inductive effect which dies out after 2–3 carbons.
Key Feature: It operates only in conjugated systems (alternate single and double bonds or p-orbitals).
2. Types of Mesomeric Effect
The mesomeric effect is of two types:
- +M Effect (Positive Mesomeric Effect)
- -M Effect (Negative Mesomeric Effect)
+M effect occurs when the electrons or the pi electrons are transferred from a particular group towards a conjugate system, thus increasing the electron density of the conjugate system. The substituent, in this case, is an electron-donating group.
-M Effect occurs when the pi-bond electrons are transferred from the conjugate system to a particular group, decreasing the electron density of the conjugate system. The substituent, in this case, is an electron-withdrawing group.
+M Effect (Positive Mesomeric)
Group donates electrons through resonance.
Examples: –OH, –OR, –NH₂, –Cl, –Br, –SH
Order: –O⁻ > –COO⁻ > –NH₂ > –OR > –OH > –X (halogens)
-M Effect (Negative Mesomeric)
Group withdraws electrons through resonance.
Examples: –NO₂, –CN, –CHO, –COR, –COOH, –COOR
Order: –NO₂ > –CN > –COOH > –F > –Cl > –Br > –I
3. Examples with Resonance Structures
Example 1: Phenol (–OH shows +M effect)
Explanation: Lone pair on oxygen delocalizes into benzene ring → activates ortho/para positions.
Example 2: Nitrobenzene (–NO₂ shows –M effect)
Explanation: NO₂ pulls electrons → deactivates ring, especially meta position.
4. Comparison with Inductive Effect
| Property | Mesomeric Effect | Inductive Effect |
|---|---|---|
| Nature | Permanent, through π-bonds | Permanent, through σ-bonds |
| Range | Entire conjugated system | Dies after 2–3 carbons |
| Requirement | Conjugation needed | No conjugation needed |
| Direction | Along π-system | Along σ-chain |
| Symbol | Curved arrow (↔) | Straight arrow (→) |
5. Applications in Organic Chemistry
- Reactivity of Aromatics: +M groups activate (phenol), –M deactivate (nitrobenzene)
- Acidity/Basicity: Phenol acidic due to +M stabilization of phenoxide ion
- Stability of Carbocations: Allylic > benzyl > vinyl
- Dipole Moment: Explains abnormal moments in conjugated systems