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7.4Advanced Organic Chemistry

Aromatic Chemistry — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 12

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Aromatic Chemistry: Benzene, Nitration and Friedel–Crafts Acylation — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Explain the structure and stability of benzene and describe electrophilic substitution reactions including nitration and Friedel–Crafts acylation.In everyday langua…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Delocalisation
  • Electrophile

Part 1 of 5Structure of benzene

In everyday language, the word aromatic is used to describe pleasant or fragrant smells; in chemistry it refers to compounds containing a benzene ring, C6H6.

1
Benzene is a planar ring of six carbon atoms with all C-C bond lengths equal (0.140 nm), between the lengths of single (0.154 nm) and double (0.134 nm) bonds.
2
Each carbon uses three sigma bonds and one p-orbital; the six p-orbitals overlap sideways to form a ring of delocalised π electrons above and below the plane.
3
The Kekulé model (alternating single and double bonds) is therefore wrong.
4
Evidence includes the equal bond lengths, the lack of reaction with bromine water (no localised C=C),
  • and thermochemical data: the hydrogenation enthalpy of benzene (-208 kJ mol-1) is much less exothermic than the -360 kJ mol-1 predicted for three C=C bonds, showing about 152 kJ mol-1 of extra stability from delocalisation.

Part 2 of 5Electrophilic substitution

The electron-rich delocalised ring attracts electrophiles. Benzene undergoes electrophilic substitution rather than addition, because addition would destroy the delocalised ring and its stability. General mechanism: the electrophile E+ accepts a pair of electrons from the ring to form a positively charged intermediate; then H+ is lost, restoring the delocalised ring, and the H+ regenerates the catalyst.

Part 3 of 5Nitration of benzene

Benzene is warmed (about 50 °C, to limit multiple substitution) with a mixture of concentrated nitric and sulfuric acids.

1
The sulfuric acid is a catalyst that generates the electrophile, the nitronium ion: HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+ (or HNO3 + H2SO4 → NO2+ + HSO4- + H2O).
2
Then C6H6 + NO2+ → C6H5NO2 + H+, and H+ + HSO4- → H2SO4.
3
Nitrobenzene can be reduced (Sn/conc.
4
HCl, then NaOH) to phenylamine, the route to dyes.
5
Nitro compounds are also used in explosives such as TNT.

Part 4 of 5Friedel–Crafts acylation

Benzene reacts with an acyl chloride in the presence of an anhydrous aluminium chloride catalyst (halogen carrier), under reflux and dry conditions, to give a phenyl ketone: C6H6 + CH3COCl → C6H5COCH3 + HCl (phenylethanone).

Friedel–Crafts acylation
The AlCl3 accepts a lone pair from the chlorine to generate the acylium ion electrophile: CH3COCl + AlCl3 → CH3CO+ + AlCl4-.
After attack on the ring, H+ is lost and combines with AlCl4- to regenerate AlCl3 and HCl.
The same reaction with an alkyl halide (alkylation) adds an alkyl group.

Part 5 of 5Worked examples

  1. Give two pieces of evidence that benzene does not contain three localised C=C bonds. [2]2 marks

  2. Write equations showing the formation of the electrophile in nitration. [2]2 marks

  3. Why is the temperature kept below about 50 °C in nitration? [1]1 mark

  4. Write an equation for the Friedel–Crafts acylation of benzene with propanoyl chloride. [2]2 marks

Key terms

TermMeaning
DelocalisationSharing of π electrons over several atoms.
ElectrophileSpecies that accepts an electron pair.
Electrophilic substitutionReplacement of H on a ring by an electrophile.
Nitronium ionNO2+, the electrophile in nitration.
Acylium ionRCO+, the electrophile in Friedel–Crafts acylation.

Avoid these mistakes

1
Drawing benzene with alternating double and single bonds and saying it reacts like an alkene.
2
Writing addition rather than substitution.
3
Forgetting that H+ is lost to regenerate the catalyst.
4
Using aqueous AlCl3 - it must be anhydrous.

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