CIE A Level Chemistry复习笔记7.6.4 Production & Reactions of Phenylamine

• Phenylamine is an organic compound consisting of a benzene ring and an amine (NH₂) functional group
• It can be produced in a three-step synthesis reaction followed by the separation of phenylamine from the reaction mixture
  • Step 1- Benzene undergoes nitration with concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at 25 to 60 ^oC to form nitrobenzene
  • Step 2 - Nitrobenzene is reduced with hot tin (Sn) and concentrated hydrochloric acid (HCl) under reflux to form an acidic mixture that contains the organic product C₆H₅N⁺H₃
  • Step 3 - Sodium hydroxide (NaOH) is added to the acidic reaction mixture to form phenylamine
  • Step 4 - The phenylamine is separated from the reaction mixture by steam distillation

The overall reaction of formation of phenylamine from benzene

Multi-step synthesis of phenylamine from benzene

• Both the benzene ring as well as the -NH₂ group in phenylamine can take part in chemical reactions
• These reactions include
  • The bromination of phenylamine
  • Formation of a diazonium salt

Bromination of phenylamine

• Phenylamines react in electrophilic substitution reactions in a similar way as phenols
• The lone pair of electrons on the nitrogen atom in phenylamines donate electron density into the benzene ring
  • In phenols, the oxygen atom donates its lone pair of electrons instead
• The delocalisation of the electrons causes an increased electron density in the benzene ring
• The benzene ring, therefore, becomes activated and becomes more readily attacked by electrophiles
• The incoming electrophiles are directed to the 2,4 and 6 positions
• Phenylamines, therefore, react under milder conditions with aqueous bromine at room temperature to form 2,4,6-tribromophenylamine

Bromination of phenylamines gives 2,4,6-tribromophenylamine

Formation of diazonium salt

• Diazonium compounds are very reactive compounds containing an -N₂⁺ group
• The amine (-NH₂) group of phenylamines will react with nitric(III) acid (HNO₂) at a temperature below 10 °C to form diazonium salts
  • Since nitric(III) acid is unstable, it has to be made in the test-tube by reacting sodium nitrite (NaNO₂) and dilute acid (such as HCl)
• These diazonium salts are so unstable that they will upon further warming with water to form a phenol

Phenylamine can form an unreactive diazonium salt which thermally decomposes to a phenol