Nucleophilic Aromatic Substitution
The $S_NAr$ (Addition-Elimination) Mechanism and factors affecting reactivity.
By chemca Team • Updated Jan 2026
Unlike aliphatic halides, aryl halides are generally inert to nucleophilic substitution due to resonance stabilization of the C-X bond. However, substitution can occur under drastic conditions or if strong Electron Withdrawing Groups (EWG) are present.
1. Why are Aryl Halides Inert?
- Resonance Effect: Lone pairs on the halogen delocalize into the benzene ring, giving the Carbon-Halogen bond partial double bond character. This makes bond cleavage difficult.
- Hybridization: The carbon atom is $sp^2$ hybridized (more electronegative, shorter bond) compared to $sp^3$ in alkyl halides, making the bond stronger.
- Instability of Phenyl Cation: $S_N1$ mechanism is ruled out because the phenyl cation is highly unstable.
- Electronic Repulsion: The electron-rich nucleophile is repelled by the electron-rich benzene ring.
2. $S_NAr$ Mechanism (Addition-Elimination)
Bimolecular Process
This mechanism operates when strong Electron Withdrawing Groups (like $-NO_2$) are present at ortho or para positions.
Step 1: Addition (Slow/RDS)
The nucleophile attacks the carbon bearing the halogen, disrupting aromaticity and forming a resonance-stabilized carbanion called the Meisenheimer Complex.
The nucleophile attacks the carbon bearing the halogen, disrupting aromaticity and forming a resonance-stabilized carbanion called the Meisenheimer Complex.
$$ Ar-X + Nu^- \xrightarrow{\text{Slow}} [Ar(X)(Nu)]^- \text{ (Meisenheimer Complex)} $$
Step 2: Elimination (Fast)
The leaving group ($X^-$) departs, and aromaticity is restored.
The leaving group ($X^-$) departs, and aromaticity is restored.
$$ [Ar(X)(Nu)]^- \xrightarrow{\text{Fast}} Ar-Nu + X^- $$
3. Effect of Substituents
Role of Nitro Group ($-NO_2$)
Electron Withdrawing Groups stabilize the anionic Meisenheimer complex by dispersing the negative charge, lowering the activation energy.
Ortho/Para vs Meta:
The negative charge in the intermediate appears at ortho and para positions relative to the attack site. An EWG at these positions can withdraw charge effectively via resonance (-R).
Meta-isomer: The negative charge does not land on the carbon bearing the EWG, so stabilization is only via Inductive effect (-I), which is weaker.
The negative charge in the intermediate appears at ortho and para positions relative to the attack site. An EWG at these positions can withdraw charge effectively via resonance (-R).
Meta-isomer: The negative charge does not land on the carbon bearing the EWG, so stabilization is only via Inductive effect (-I), which is weaker.
Reactivity Order:
2,4,6-Trinitrochlorobenzene > 2,4-Dinitro > 4-Nitro > Chlorobenzene
Reaction Conditions Example:
- Chlorobenzene: NaOH, 623 K, 300 atm (Dow's Process).
- 4-Nitrochlorobenzene: NaOH, 443 K.
- 2,4-Dinitrochlorobenzene: NaOH, 368 K.
- 2,4,6-Trinitrochlorobenzene: Warm water (forms Picric Acid).
4. Comparison: $S_NAr$ vs Benzyne
| Feature | $S_NAr$ (Addition-Elimination) | Benzyne (Elimination-Addition) |
|---|---|---|
| Condition | Requires Strong EWG ($NO_2$) | Requires Strong Base ($NaNH_2$) |
| Intermediate | Meisenheimer Complex (Anion) | Benzyne (Neutral) |
| Mechanism | Nucleophile adds first | Leaving group leaves first |
| Product | Direct Substitution (Ipso) | Mixture of Direct + Cine |
| Effect of Halogen | $F \gg Cl \approx Br \approx I$ | $I > Br > Cl > F$ |
Why Fluorine is fastest in $S_NAr$? Fluorine is the most electronegative. It makes the C-F carbon highly positive (electrophilic), speeding up the rate-determining nucleophilic attack (Step 1). The bond strength (C-F) matters less because bond breaking happens in the fast Step 2.
Knowledge Check
Test your understanding of SNAr
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