Friedel-Crafts Reactions: The Power and The Limitations
Master alkylation and acylation mechanisms with carbocation rearrangements, polyalkylation issues, and deactivating group limitations.
Why Friedel-Crafts Reactions Matter in JEE
Friedel-Crafts reactions are fundamental electrophilic aromatic substitution reactions that appear frequently in JEE examinations. Understanding their mechanisms and limitations is crucial for:
- Mechanism-based questions (JEE Main & Advanced)
- Predicting reaction products with rearrangements
- Understanding limitations for synthetic planning
- Differentiating between alkylation and acylation
Friedel-Crafts Alkylation
Ar-H + R-X + AlCl₃ → Ar-R + HX
(Where R-X is alkyl halide, Ar-H is aromatic compound)
⚗️ Mechanism Steps:
Step 1: Generation of Electrophile
R-X + AlCl₃ → R⁺ + [AlCl₄]⁻
Lewis acid AlCl₃ polarizes R-X bond, creating carbocation
Step 2: Electrophilic Attack
Ar-H + R⁺ → Ar-R-H⁺ (arenium ion intermediate)
Step 3: Deprotonation
Ar-R-H⁺ + [AlCl₄]⁻ → Ar-R + AlCl₃ + HCl
Base removes proton to restore aromaticity
🎯 JEE Application Example:
Problem: Predict the product when benzene reacts with CH₃-CH₂-Cl in presence of AlCl₃
Solution:
Benzene + CH₃CH₂Cl + AlCl₃ → Ethylbenzene
Friedel-Crafts Acylation
Ar-H + R-CO-X + AlCl₃ → Ar-CO-R + HX
(Where R-CO-X is acid chloride, forms ketone product)
⚗️ Mechanism Steps:
Step 1: Acylium Ion Formation
R-CO-Cl + AlCl₃ → R-C≡O⁺ + [AlCl₄]⁻
Forms stable acylium ion resonance structure
Step 2: Electrophilic Substitution
Ar-H + R-C≡O⁺ → Ar-CO-R-H⁺ (arenium ion)
Step 3: Regeneration of Catalyst
Ar-CO-R-H⁺ + [AlCl₄]⁻ → Ar-CO-R + AlCl₃ + HCl
🎯 JEE Application Example:
Problem: What product forms when benzene reacts with CH₃COCl in presence of AlCl₃?
Solution:
Benzene + CH₃COCl + AlCl₃ → Acetophenone
Major Limitations of Friedel-Crafts Reactions
🚫 1. Carbocation Rearrangement
Problem: Primary carbocations can rearrange to more stable secondary/tertiary carbocations
Example: 1-Chloropropane with benzene gives isopropyl benzene instead of n-propyl benzene
CH₃-CH₂-CH₂⁺ → CH₃-CH⁺-CH₃ (more stable 2° carbocation)
Results in unexpected product formation
🚫 2. Polyalkylation
Problem: Alkylated products are more reactive than starting material
Result: Multiple substitutions occur, giving mixture of mono-, di-, and tri-substituted products
Benzene → Toluene → Xylene → Mesitylene
Electron-donating alkyl groups activate ring for further substitution
🚫 3. Deactivating Groups Prevent Reaction
Problem: Strong electron-withdrawing groups deactivate benzene ring
Groups that prevent reaction: -NO₂, -CN, -SO₃H, -COOH, -CHO, -COR
Nitrobenzene does not undergo Friedel-Crafts reaction
-NO₂ group strongly deactivates ring towards electrophilic substitution
🚀 Problem-Solving Strategies
For Alkylation vs Acylation:
- Acylation doesn't have rearrangement issues
- Acylation gives only mono-substituted product
- Alkylation can use alcohols and alkenes too
- Remember the catalyst (AlCl₃) is required
JEE Exam Tips:
- Always check for possible rearrangements
- Identify deactivating groups first
- Predict polyalkylation products when relevant
- Practice mechanism writing for both reactions
Advanced Applications Available
Includes Clemmensen reduction, Gattermann-Koch reaction, and JEE Advanced level problems
📝 Quick Self-Test
Try these JEE-level problems to test your understanding:
1. Predict the major product when benzene reacts with CH₃-CH₂-CH₂-Cl + AlCl₃
2. Why does nitrobenzene not undergo Friedel-Crafts reaction?
3. Compare and contrast Friedel-Crafts alkylation and acylation
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