Electrophilic Aromatic Substitution of Benzene – Complete Guide for JEE & NEET
Electrophilic Aromatic Substitution (EAS) is one of the most important reaction mechanisms in organic chemistry. It explains how benzene and its derivatives undergo substitution reactions without losing aromaticity. This topic is a core pillar of Class 11–12 NCERT organic chemistry and is frequently tested in JEE Main, JEE Advanced, NEET, and board examinations.
Understanding EAS is essential for mastering reactions of benzene, toluene, phenol, aniline, nitrobenzene, and many other aromatic compounds.
Why Benzene Undergoes Substitution and Not Addition
Benzene is an aromatic compound with:
- A planar hexagonal structure
- Delocalized Ο electrons
- Extra stability due to aromaticity
Addition reactions would destroy aromaticity, which is energetically unfavorable. Therefore, benzene prefers substitution reactions, where one hydrogen atom is replaced by another atom or group while retaining the aromatic ring.
What Is Electrophilic Aromatic Substitution?
Electrophilic Aromatic Substitution is a reaction in which:
- An electrophile (E⁺) attacks the benzene ring
- One hydrogen atom is substituted
- Aromaticity is restored at the end of the reaction
General reaction:
Benzene + Electrophile → Substituted benzene + H⁺
General Mechanism of Electrophilic Aromatic Substitution
The mechanism proceeds in three key steps, common to all EAS reactions.
Step 1: Generation of Electrophile (Slow or Fast)
A strong electrophile is generated using:
- Lewis acids (AlCl₃, FeCl₃)
- Acids (H₂SO₄, HNO₃)
Example:
- Cl₂ + FeCl₃ → Cl⁺ (electrophile)
π This step varies depending on the reaction.
Step 2: Formation of Sigma Complex (Rate-Determining Step)
- Electrophile attacks the benzene ring
- A Ο-complex (arenium ion) is formed
- Aromaticity is temporarily lost
π This is the slowest and most important step
π Stability of sigma complex controls reaction rate
Step 3: Deprotonation and Restoration of Aromaticity
- A proton (H⁺) is removed
- Aromaticity is restored
- Substituted benzene is formed
Important Electrophilic Aromatic Substitution Reactions
1. Nitration of Benzene
Reagents:
- Concentrated HNO₃ + Concentrated H₂SO₄
Electrophile:
- NO₂⁺ (Nitronium ion)
Product:
- Nitrobenzene
π Very important for JEE & NEET
2. Sulphonation of Benzene
Reagents:
- Fuming H₂SO₄ (oleum)
Electrophile:
- SO₃
Product:
- Benzene sulphonic acid
π Reversible reaction (temperature dependent)
3. Halogenation of Benzene
Reagents:
- Cl₂ / Br₂ in presence of FeCl₃ / FeBr₃
Electrophile:
- Cl⁺ / Br⁺
Product:
- Chlorobenzene / Bromobenzene
π Fluorination and iodination require special conditions
4. Friedel–Crafts Alkylation
Reagents:
- Alkyl halide + AlCl₃
Product:
- Alkylbenzene
π Limitations:
- Carbocation rearrangement
- Polyalkylation
- Fails with strongly deactivated rings
5. Friedel–Crafts Acylation
Reagents:
- Acyl chloride + AlCl₃
Product:
- Acylbenzene
π No rearrangement, monoacylation only
π Preferred over alkylation
Orientation and Activating/Deactivating Groups (Brief Insight)
Substituents already present on the benzene ring affect:
- Rate of reaction
- Position of substitution (ortho/meta/para)
| Group Type | Effect | Orientation |
|---|---|---|
| Activating | Increase rate | Ortho/Para |
| Deactivating | Decrease rate | Meta (mostly) |
π This concept is heavily tested in JEE Advanced.
Energy Profile of EAS Reaction
- Formation of sigma complex = highest energy point
- Substitution is favored because aromaticity is restored
- More stable sigma complex → faster reaction
NCERT & Exam-Oriented Key Points
✔ Benzene undergoes substitution, not addition
✔ Sigma complex formation is rate-determining
✔ Lewis acids help generate electrophiles
✔ Aromaticity is restored in the final step
✔ Friedel–Crafts reactions have limitations
Important Questions for JEE, NEET & Boards
Very Short Answer
- Why does benzene prefer substitution reactions?
- What is a sigma complex?
- Name the electrophile in nitration.
Short Answer
- Explain the mechanism of electrophilic aromatic substitution.
- Why is Friedel–Crafts acylation preferred over alkylation?
- Why is nitration considered an EAS reaction?
Long Answer
- Explain nitration of benzene with mechanism.
- Discuss Friedel–Crafts reactions and their limitations.
- Draw energy profile diagram for EAS.
Conclusion
Electrophilic Aromatic Substitution is the foundation of aromatic chemistry. A strong grip on its mechanism, electrophile generation, and reaction conditions makes it easier to understand reactions of substituted benzenes and advanced organic synthesis problems.
For competitive exams like JEE and NEET, mastering EAS can significantly improve accuracy and confidence in organic chemistry.
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