Propene (C₃H₆) is an alkene. Explain why propene is described as an unsaturated hydrocarbon and describe what happens when propene undergoes an addition reaction with hydrogen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Propene contains a carbon‑to‑carbon double bond (C=C). Unsaturated means the molecule is not joined to the maximum number of hydrogen atoms / does not contain only single bonds. In the addition reaction, hydrogen adds across the double bond. The double bond becomes a single bond / the product is saturated. The product formed is propane (C₃H₈).
Examiner tips
- Use the term "unsaturated" to explain the double bond; mention the lack of maximum H atoms.
- Show that H₂ adds across the C=C to give a single bond.
- State the product as propane and note it is saturated.
Common mistakes
- Calling propene a saturated hydrocarbon.
- Forgetting to mention that the double bond becomes a single bond.
- Not naming the product as propane or describing it as saturated.
Mark scheme (5 marks)
- Propene contains a carbon-to-carbon double bond (C=C)
- Unsaturated means the molecule is not joined to the maximum number of hydrogen atoms / does not contain only single bonds
- In the addition reaction, hydrogen adds across the double bond
- The double bond becomes a single bond / the product is saturated
- The product formed is propane (C₃H₈)
Key terms in this question
unsaturated · addition reaction · alkene
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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