34.2 Phenylamine and Azo Compounds

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe how phenylamine is prepared from benzene (nitration → reduction → NaOH treatment)
  2. Describe the reactions of phenylamine with (a) bromine water and (b) HNO₂/NaNO₂ and dilute acid to form a diazonium salt, and what happens when that salt is warmed with water
  3. Describe and explain the relative basicities of aqueous ammonia, ethylamine, and phenylamine
  4. Describe the formation of azo compounds, identify the azo group, and state their uses as dyes

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1. Preparing Phenylamine from Benzene

Phenylamine (also called aniline) is an organic compound with the formula C₆H₅NH₂. It has a benzene ring (a ring of six carbon atoms with special, stable bonding) attached to an –NH₂ group (called an amino group). Making phenylamine from benzene is a two-stage process.

Stage 1 — Nitration: Making Nitrobenzene

First, benzene is converted into nitrobenzene (C₆H₅NO₂) through a reaction called nitration.

  • Benzene is mixed with a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄), called the nitrating mixture.
  • The reaction is carried out at around 50°C (kept below 55°C to avoid further unwanted reactions).
  • The sulfuric acid acts as a catalyst (something that speeds up the reaction without being used up).
  • The –NO₂ (nitro) group replaces one hydrogen atom on the benzene ring.

Word equation:

Benzene + concentrated HNO₃ → Nitrobenzene + H₂O (in the presence of concentrated H₂SO₄, ~50°C)

Structural equation:

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

Nitrobenzene is a pale yellow oily liquid.

Stage 2 — Reduction: Converting Nitrobenzene to Phenylamine

Reduction means adding hydrogen (or removing oxygen) from a molecule. Here, the –NO₂ group is reduced to –NH₂.

  • Nitrobenzene is heated with tin (Sn) and concentrated hydrochloric acid (HCl) under reflux (this means heating in a sealed flask so no vapour escapes).
  • These conditions are described as hot Sn / concentrated HCl.
  • This reduces the –NO₂ group to –NH₂, but initially produces a salt (phenylammonium chloride) in the acidic solution.

To free the phenylamine from the salt:

  • Sodium hydroxide solution (NaOH(aq)) is then added.
  • NaOH is an alkali (a base dissolved in water). It reacts with the salt and releases phenylamine as the free amine.

Overall reduction equation:

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

(The [H] represents hydrogen atoms supplied by the Sn/HCl system.)

Summary of the full preparation:

StepWhat happensReagents & Conditions
1Benzene → NitrobenzeneConc. HNO₃ + conc. H₂SO₄, ~50°C
2Nitrobenzene → PhenylamineHot Sn + conc. HCl, then NaOH(aq)

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