Alkenes are a homologous series of hydrocarbons. Explain how bromine water can be used to distinguish between an alkane and an alkene, and explain why alkenes undergo this reaction more readily than alkanes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkenes contain a carbon-carbon double bond (C=C). Alkanes contain only carbon-carbon single bonds. Both are hydrocarbons.
Model answer (5 marks)
Bromine water (Br₂ in water) is orange–brown. When it is added to an alkene it turns colourless, showing that the bromine has reacted.
When bromine water is added to an alkane the solution remains orange–brown, indicating no reaction.
The alkene reacts because it is unsaturated and contains a C=C double bond. The double bond is a site of high electron density.
The reaction is an addition: Br₂ adds across the double bond to give a dibromoalkane.
Alkanes are saturated, having only C–C single bonds, so they cannot undergo addition reactions with bromine water.
When bromine water is added to an alkane the solution remains orange–brown, indicating no reaction.
The alkene reacts because it is unsaturated and contains a C=C double bond. The double bond is a site of high electron density.
The reaction is an addition: Br₂ adds across the double bond to give a dibromoalkane.
Alkanes are saturated, having only C–C single bonds, so they cannot undergo addition reactions with bromine water.
Examiner tips
- Show the colour change for each compound; use the word ‘colourless’ for alkene, ‘orange‑brown’ for alkane.
- Explain that the double bond is unsaturated and allows addition, whereas single bonds do not.
- Mention the addition mechanism briefly to demonstrate understanding of why alkenes react.
- Use the exact terms ‘unsaturated’ and ‘saturated’ as the mark scheme expects.
Common mistakes
- Writing that bromine water turns colourless for both alkane and alkene.
- Forgetting to mention that the alkane does not react.
- Using ‘oxidation’ instead of ‘addition’ to describe the reaction with bromine.
Mark scheme (5 marks)
- Bromine water changes from orange/brown to colourless with the alkene
- Bromine water remains orange/brown with the alkane (no reaction)
- The alkene reacts with bromine water because it is unsaturated / contains a C=C double bond
- The alkene undergoes an addition reaction where bromine adds across the double bond
- The alkane is saturated / has only single bonds and so cannot undergo addition reactions with bromine water
Key terms in this question
alkene · alkane · bromine water
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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