Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

5.4.3Unit 5. Transition Metals and Organic Nitrogen Chemistry

Benzene – Electrophilic Substitution — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 29 of 43 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 6General Mechanism

5.4.3 Benzene - Electrophilic Substitution
1
A strong electrophile E⁺ is generated first, with a catalyst or acid mixture.
2
The electrophile accepts a pair of electrons from the delocalised ring and forms a bond to a carbon atom.
3
This gives a positively charged intermediate in which the delocalisation is disrupted over part of the ring.
4
The carbon bearing E loses its hydrogen as H⁺, and the electrons return to the ring.
5
The aromatic ring is restored, and the H⁺ lost regenerates the catalyst.

Part 2 of 6Generating the Electrophile

Nitration
  • HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻, which produces the nitronium ion.
Bromination
  • Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻, the catalyst polarising Br₂ so that it behaves as a source of Br⁺.
Acylation
  • CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻.
Alkylation
  • CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻.
Regeneration
  • The catalyst returns when H⁺ combines with FeBr₄⁻ or AlCl₄⁻, giving HBr or HCl and the original halide.

Part 3 of 6Products and Uses

Products and Uses
Nitrobenzene
  • C₆H₅NO₂ is reduced to phenylamine, an intermediate in dyes.
Bromobenzene
  • C₆H₅Br is used in further synthesis.
Methylbenzene
  • C₆H₅CH₃ is made by alkylation with CH₃Cl and AlCl₃.
Phenylethanone
  • C₆H₅COCH₃ forms by acylation with ethanoyl chloride and AlCl₃.

Part 4 of 6Practical Conditions

Practical Conditions
Temperature control
  • Nitration is kept below about 55 °C to prevent several nitro groups from being added.
Dry conditions
  • AlCl₃ and FeBr₃ react with water, so the reagents and apparatus must be dry.
Mixed acids
  • The sulfuric acid acts as an acid, protonating nitric acid, and is regenerated at the end of the reaction.

Part 5 of 6Worked Example

Question Write the equation for the nitration of benzene and state the conditions.

  1. Replace one H of C₆H₆ by NO₂ and form water.
  2. Use a mixture of concentrated nitric and concentrated sulfuric acids at about 50 °C.

Answer C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

Part 6 of 6Exam Checklist & Tips

5.4.3 – Key Points for the Exam

  • Electrophile An electron-pair acceptor.
  • Nitronium ion NO₂⁺, the electrophile in nitration.
  • Intermediate A positively charged species formed between steps.
  • Friedel-Crafts reaction Alkylation or acylation of an arene using AlCl₃.

Common Mistakes

  • Drawing the curly arrow from the electrophile to the ring instead of from the ring to the electrophile.
  • Forgetting that the catalyst is regenerated.
  • Showing the intermediate with a complete circle.
  • Using bromine water for substitution.

Last updated:

\n