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A2 · Practice questions · Electrophilic substitution

Attack, then restore.

Six original Cambridge-style questions on electrophilic substitution in arenes.

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.
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Electrophile, intermediate, lose H+.

01
Recall
[2 marks]

Write equations to show how the electrophile is formed in (a) the nitration of benzene and (b) the bromination of benzene using FeBr3.

  • (a) HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4−. ✓
  • (b) Br2 + FeBr3 → Br+ + FeBr4−. ✓
02
Explain
[4 marks]

Describe, with the help of curly arrows in words, the mechanism of the nitration of benzene after the electrophile has formed.

  • A curly arrow goes from the delocalised π electrons of the ring to the N of NO2+, forming a C–N bond. ✓
  • The intermediate is a positive ion: a horseshoe of four delocalised electrons over five carbons, with + inside; the attacked carbon holds H and NO2. ✓
  • A curly arrow goes from the C–H bond back into the ring. ✓
  • H+ is lost, the delocalised ring is restored, and H+ + HSO4− regenerates H2SO4. ✓
03
Explain
[3 marks]

Ethene reacts with bromine water at room temperature, but benzene does not. Benzene does react with pure bromine and FeBr3, by substitution. Explain these differences.

  • In ethene the π electrons are localised, so the electron density is high enough to polarise Br2. ✓
  • In benzene the π electrons are delocalised over six carbons, so the density is lower; FeBr3 is needed to make Br+. ✓
  • Benzene substitutes rather than adds because losing H+ restores the stable delocalised ring, whereas addition would destroy it. ✓
04
Analysis
[3 marks]

Predict the main organic product(s) of mononitration of (a) methylbenzene and (b) benzoic acid. State whether each reacts faster or slower than benzene.

  • (a) 2-nitromethylbenzene and 4-nitromethylbenzene; faster, since –CH3 donates electrons and activates the ring. ✓
  • (b) 3-nitrobenzoic acid; slower, since –COOH withdraws electrons and deactivates the ring. ✓
  • The directing effect follows the group already present: alkyl is 2,4-directing, –COOH is 3-directing. ✓
05
Analysis
[3 marks]

Suggest a two-step synthesis of 1-bromo-3-nitrobenzene from benzene, giving reagents and conditions. Explain the order of the steps.

  • Step 1: nitrate first with conc. HNO3 + conc. H2SO4, 25–60 °C, to give nitrobenzene. ✓
  • Step 2: Br2 with FeBr3 (halogen carrier), giving 1-bromo-3-nitrobenzene. ✓
  • –NO2 is 3-directing; brominating first would put –Br on, which is 2,4-directing, giving the wrong isomers. ✓
06
Explain
[2 marks]

Chlorine is said to be deactivating but 2,4-directing. Explain what each part means.

  • Deactivating: the electronegative Cl withdraws electron density from the ring, so chlorobenzene reacts more slowly than benzene. ✓
  • 2,4-directing: the second substituent goes mainly to the 2- and 4-positions, as it does for –OH, –NH2 and –CH3. ✓

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.