Propene can be produced by cracking. Explain why propene decolourises bromine water but propane does not.

AQA GCSE Chemistry (8462) — 4.7.2 Reactions of alkenes and alcohols (Chem only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Propene is unsaturated – it contains a carbon–carbon double bond. The double bond is a reactive site that can add bromine across it, forming a dibromide and consuming the orange colour of bromine water. Propane is saturated – it has only single C–C bonds and no double bond, so it cannot add bromine. Therefore propane does not decolourise bromine water.

Examiner tips

Mention the unsaturation of propene and the double bond’s reactivity
Explain that bromine adds across the double bond, decolourising the solution
Contrast with propane’s saturation and lack of a reactive double bond
Use the exact terms ‘unsaturated’, ‘double bond’, ‘add bromine’, ‘decolourise’

Common mistakes

Forgetting to state that propene has a double bond
Confusing the reaction with a substitution rather than addition
Claiming propane reacts with bromine or that it is unsaturated

Mark scheme (4 marks)

Propene is unsaturated / has a carbon-carbon double bond
The double bond allows propene to react with / add bromine across the double bond
Propane is saturated / has no carbon-carbon double bond / has only single bonds
Propane cannot react with bromine (water) / propane does not decolourise bromine water because it has no double bond to react with

Key terms in this question

bromine water

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