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Learn7405Paper 2 · Paper 2: Organic chemistryAromatic chemistryBenzene and electrophilic substitution
Aromatic chemistry · Subtopic 1 of 2

Benzene and Electrophilic Substitution

Benzene C₆H₆ has a ring of six sp² carbons with a delocalised π system above and below the plane; all C–C bonds are equal (0.139 nm). Evidence against Kekulé's alternating C=C: equal bond lengths, enthalpy of hydrogenation 152 kJ mol⁻¹ less exothermic than expected (delocalisation stability), and resistance to addition. Benzene undergoes electrophilic substitution, keeping the stable ring: nitration (conc. HNO₃ + conc. H₂SO₄, < 55 °C; electrophile NO₂⁺), halogenation (Br₂ with FeBr₃ or AlCl₃ halogen carrier), Friedel–Crafts alkylation/acylation (RCl or RCOCl with AlCl₃). Methylbenzene reacts faster and directs to the 2- and 4-positions; its side chain is oxidised by hot alkaline KMnO₄ to benzoic acid.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 2≈ 55 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe benzene's delocalised structure and the evidence for it.
  • Describe electrophilic substitution mechanisms.
  • Give reagents and conditions for nitration, halogenation and Friedel–Crafts.
  • Describe reactions of methylbenzene.
Pause and think

Cyclohexene decolourises bromine water instantly. Benzene doesn't. Why?

Section 2 of 14

Core idea

Start hereA ring that refuses to add

Benzene looks as if it has three double bonds, yet it ignores bromine water. Its electrons are spread around the whole ring.

The big idea

Benzene's delocalised π system makes it unusually stable, so it reacts by electrophilic substitution, which keeps the ring intact.

1. Evidence for delocalisation

Delocalisation energy
  1. ΔHhyd(cyclohexene)=−120\Delta H_\text{hyd}(\text{cyclohexene})=-120
    One C=C.
  2. Kekuleˊ prediction=3×(−120)=−360\text{Kekulé prediction}=3\times(-120)=-360
    Three C=C.
  3. actual=−208\text{actual}=-208
    Measured for benzene.
  4. 360−208=152 kJ mol−1360-208=152\ \text{kJ mol}^{-1}
    Extra stability from delocalisation.

2. Electrophilic substitution

Nitration mechanism

Generate electrophile, attack, lose H⁺.

Worked exampleYield of nitrobenzene

7.80 g benzene gives 9.84 g nitrobenzene. % yield?

Goal
% yield.
Strategy
1 : 1 ratio.
  1. n=7.8078.0=0.100n=\frac{7.80}{78.0}=0.100
    Moles of benzene.
  2. mtheor=0.100×123=12.3 gm_\text{theor}=0.100\times123=12.3\ \text{g}
    Theoretical.
  3. 9.8412.3×100=80.0%\frac{9.84}{12.3}\times100=80.0\%
    Yield.
Answer80.0%.
✓ CheckLess than 100% ✓.

Summary

  • Delocalised, planar, equal bonds.
  • 152 kJ mol⁻¹ extra stability.
  • Nitration, halogenation, Friedel–Crafts.
  • Methylbenzene 2,4-directing.
Key words
Delocalised
Electrons spread over several atoms, not between two.
Electrophile
Electron-pair acceptor.
Halogen carrier
Lewis acid (FeBr₃, AlCl₃) that generates the electrophile.
HistoryKekulé's dream. August Kekulé said the ring structure of benzene came to him in 1865 in a daydream of a snake biting its own tail.
Fun factTNT. Nitrating methylbenzene three times gives 2,4,6-trinitrotoluene (TNT), the 2,4,6 pattern showing the directing effect of the methyl group.
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