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Learn9701Paper 4 · A Level Structured QuestionsNitrogen compoundsPhenylamine, amides and azo dyes
Nitrogen compounds · Subtopic 1 of 2

Phenylamine, Amides and Azo Dyes

Base strength depends on how available the N lone pair is: ethylamine > ammonia > phenylamine. Alkyl groups push electrons onto N; in phenylamine the lone pair delocalises into the ring, so it's a weaker base. Phenylamine is made by reducing nitrobenzene with Sn and conc. HCl (reflux), then NaOH. Its ring is activated: bromine water gives white 2,4,6-tribromophenylamine. Diazotisation: phenylamine + HNO₂ (NaNO₂ + HCl) below 10 °C gives benzenediazonium chloride, which couples with phenol in alkali to form an orange azo dye (–N=N–). Amides (RCONH₂) are neutral (lone pair delocalised onto C=O); they hydrolyse with acid or alkali and are reduced by LiAlH₄ to amines.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 2≈ 50 minRevision 1 · updated 2026-11-01
Section 1 of 14

Learning objectives

  • Explain the relative basicity of amines.
  • Describe the preparation and reactions of phenylamine.
  • Describe diazotisation and azo coupling.
  • Describe amide hydrolysis and reduction.
Pause and think

Why must diazotisation be kept below 10 °C?

Section 2 of 14

Core idea

Start hereThe chemistry of colour

Orange food colouring, methyl orange indicator and many fabric dyes all share one link: –N=N–, made from phenylamine.

The big idea

Where the nitrogen lone pair can go decides how basic a compound is, and the diazonium ion made from phenylamine couples with phenols to make azo dyes.

1. Basicity

Lone pair availability

More available lone pair, stronger base.

2. Making an azo dye

Benzene to azo dye
  1. C6H6→C6H5NO2\mathrm{C_6H_6\rightarrow C_6H_5NO_2}
    Conc. HNO₃/H₂SO₄, below 55 °C.
  2. C6H5NO2→C6H5NH2\mathrm{C_6H_5NO_2\rightarrow C_6H_5NH_2}
    Sn/conc. HCl, reflux; then NaOH.
  3. C6H5NH2→C6H5N2+\mathrm{C_6H_5NH_2\rightarrow C_6H_5N_2^+}
    NaNO₂/HCl, below 10 °C.
  4. C6H5N2++C6H5OH→C6H5N=NC6H4OH\mathrm{C_6H_5N_2^++C_6H_5OH\rightarrow C_6H_5N{=}NC_6H_4OH}
    Phenol in NaOH(aq): orange dye.
Worked examplePhenylamine from nitrobenzene

12.3 g nitrobenzene (MrM_r 123). Mass of phenylamine (MrM_r 93.0) at 100% yield?

Goal
Mass.
Strategy
1 : 1.
  1. n=12.3123=0.100n=\frac{12.3}{123}=0.100
    Moles.
  2. m=0.100×93.0=9.30 gm=0.100\times93.0=9.30\ \text{g}
    Mass.
Answer9.30 g.
✓ CheckSmaller Mr product, smaller mass ✓.

Summary

  • Ethylamine > NH₃ > phenylamine.
  • Diazotise below 10 °C.
  • Couple with phenol → azo dye.
  • Amides neutral.
Key words
Diazonium ion
C₆H₅N₂⁺, unstable above 10 °C.
Azo dye
Coloured compound containing –N=N– between aromatic rings.
HistoryThe dye industry. Azo dyes, first made in the 1860s, created the German chemical industry. Many companies still around today began as dye makers.
Fun factIndicators are azo dyes. Methyl orange is an azo compound: protonating the N=N group shifts its colour from yellow to red.
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