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A2 · Practice questions · Structure of benzene

Prove the ring is delocalised.

Six original Cambridge-style questions on the structure of benzene.

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

sp2, planar, delocalised.

01
Explain
[4 marks]

Describe the bonding in a benzene molecule in terms of orbital overlap.

  • Each carbon is sp2 hybridised and forms three sigma bonds: two to C and one to H, at 120° in a plane. ✓
  • Each carbon has one remaining p orbital, at right angles to the ring. ✓
  • The p orbitals overlap sideways with both neighbours all round the ring. ✓
  • This gives a delocalised pi system of six electrons above and below the plane. ✓
02
Calculation
[2 marks]

The enthalpy of hydrogenation of cyclohexene is −120 kJ mol−1 and of benzene is −208 kJ mol−1. Calculate the stabilisation energy of benzene.

  • Kekulé prediction = 3 × (−120) = −360 kJ mol−1. ✓
  • Stabilisation = −208 − (−360) = 152 kJ mol−1; benzene is more stable by this amount. ✓
03
Explain
[3 marks]

Explain how C–C bond length data support the delocalised model of benzene.

  • Kekulé predicts three short C=C (0.134 nm) and three long C–C (0.154 nm) bonds. ✓
  • X-ray diffraction shows all six bonds are 0.139 nm. ✓
  • Equal, intermediate lengths mean the pi electrons are shared equally round the ring. ✓
04
Analysis
[2 marks]

Cyclohexene decolourises bromine water at room temperature but benzene does not. Explain.

  • Cyclohexene has a localised C=C with a high electron density, so it attacks Br2 and undergoes addition. ✓
  • Benzene's pi electrons are delocalised (lower density) and addition would destroy the stable ring, so no reaction without a catalyst. ✓
05
Recall
[2 marks]

State the shape of a benzene molecule and the H–C–C bond angle.

  • Planar (a flat regular hexagon, all 12 atoms in one plane). ✓
  • 120°. ✓
06
Analysis
[3 marks]

Cyclohexa-1,3-diene has two C=C bonds and a measured enthalpy of hydrogenation of −232 kJ mol−1. Using −120 kJ mol−1 for one C=C, calculate its stabilisation energy and compare it with that of benzene.

  • Predicted value for the diene = 2 × (−120) = −240 kJ mol−1, so its stabilisation is only 8 kJ mol−1. ✓
  • Benzene: predicted −360, measured −208, stabilisation 152 kJ mol−1. ✓
  • The diene's two C=C bonds overlap only slightly; benzene's six p electrons are fully delocalised round a closed ring, giving far greater stability. ✓

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.