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5.5.3Unit 5. Transition Metals and Organic Nitrogen Chemistry

Aromatic Amines — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 33 of 43 Unit quiz

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Part 1 of 5Preparation of Phenylamine

5.5.3 Aromatic Amines
1
Nitrobenzene is heated under reflux with tin and concentrated hydrochloric acid.
2
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O, though in acid the product is held as the salt C₆H₅NH₃⁺.
3
Excess sodium hydroxide is added to liberate the free phenylamine.
4
The amine is then separated by steam distillation or solvent extraction.

Part 2 of 5Basicity

Basicity
Weaker base
  • Phenylamine is less basic than ethylamine and ammonia.
Reason
  • The nitrogen lone pair overlaps with the delocalised ring, so it is less available to accept a proton.
Alkyl comparison
  • In aliphatic amines the electron-donating alkyl groups increase the density on nitrogen.
Reaction with bromine
  • Bromine water gives a white precipitate of 2,4,6-tribromophenylamine, showing the ring is activated.
Hydrogen bonding
  • Phenylamine is only slightly soluble in water, because the large non-polar ring reduces the effect of hydrogen bonding.

Part 3 of 5Diazotisation and Coupling

Diazotisation
  • C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O, using NaNO₂ and HCl at 0-5 °C.
Why cold
  • Above about 10 °C the diazonium ion decomposes to phenol and nitrogen gas.
Coupling
  • The diazonium ion reacts with phenol dissolved in alkali, or with another activated arene, to give a coloured azo compound.
Azo group
  • The -N=N- link joins two aromatic rings, extending delocalisation so that visible light is absorbed.
Uses
  • Azo compounds are important dyes and food colourings.
Safety
  • Diazonium salts are unstable and must be used in solution immediately, since the dry solids can be explosive.

Part 4 of 5Worked Example

Question State the reagents and conditions for converting phenylamine into benzenediazonium chloride and give the equation.

  1. Use sodium nitrite and hydrochloric acid, which together form nitrous acid.
  2. Keep the temperature between 0 and 5 °C.

Answer C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O

Part 5 of 5Exam Checklist & Tips

5.5.3 – Key Points for the Exam

  • Phenylamine C₆H₅NH₂, an aromatic amine.
  • Diazonium ion An ion containing N₂⁺ attached to a benzene ring.
  • Azo dye A coloured compound containing -N=N- between aromatic rings.
  • Coupling Reaction of a diazonium ion with an activated arene.

Common Mistakes

  • Doing diazotisation at room temperature.
  • Saying that phenylamine is a stronger base than ammonia.
  • Forgetting the NaOH step to obtain the free amine.
  • Omitting alkaline conditions in the coupling reaction with phenol.

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