A2 · Practice questions · Phenylamine and azo dyes

Keep it cold.

Six original Cambridge-style questions on phenylamine and azo dyes.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Reduce, diazotise, couple.

Nitrobenzene: Sn + conc. HCl, reflux, then NaOH.
Br₂(aq): white 2,4,6-tribromophenylamine, no catalyst.
HNO₂: NaNO₂ + HCl, below 10 °C.
Couple with phenol in NaOH(aq): –N=N– dye.

Recall

[3 marks]

Describe how phenylamine is made from nitrobenzene. Include reagents, conditions and an equation using [H].

Heat nitrobenzene under reflux with tin and concentrated hydrochloric acid. ✓
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O. ✓
Add excess NaOH(aq) to release phenylamine from its salt, C₆H₅NH₃⁺Cl⁻. ✓

Explain

[3 marks]

Phenylamine reacts with bromine water at room temperature, but benzene does not. Explain, and give the equation.

The N lone pair is partly delocalised into the ring, increasing its electron density (especially at positions 2, 4 and 6). ✓
The ring polarises Br₂ and is attacked without a halogen carrier; benzene's ring is not electron-rich enough. ✓
C₆H₅NH₂ + 3Br₂ → C₆H₂Br₃NH₂ + 3HBr (white precipitate). ✓

Recall

[3 marks]

Give the reagents and conditions, and an equation, for converting phenylamine into benzenediazonium chloride.

NaNO₂ and HCl(aq), which make nitrous acid in situ. ✓
Keep the temperature below 10 °C (ice bath). ✓
C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O. ✓

Explain

[2 marks]

A student warms the benzenediazonium chloride solution to 30 °C. State what is observed and write an equation.

Bubbles of nitrogen gas are seen as the diazonium ion decomposes. ✓
C₆H₅N₂⁺ + H₂O → C₆H₅OH + N₂ + H⁺. ✓

Calculation

[2 marks]

0.930 g of phenylamine (M_r 93.0) is converted into the azo dye C₆H₅N=NC₆H₄OH (M_r 198.0). 1.58 g of dye is collected. Calculate the percentage yield.

n(phenylamine) = 0.930 ÷ 93.0 = 0.0100 mol, so maximum dye = 0.0100 × 198.0 = 1.98 g. ✓
Yield = 1.58 ÷ 1.98 × 100 = 79.8%. ✓

Explain

[2 marks]

Explain why the azo dye is coloured and give one use of azo compounds.

The –N=N– group joins two benzene rings into one extended delocalised system, so the gap between energy levels is small enough to absorb visible light. ✓
Uses: dyes for textiles or food colourings, and acid-base indicators such as methyl orange. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.