Alkenes are unsaturated hydrocarbons that contain a carbon-to-carbon double bond. Describe what is meant by the terms 'unsaturated' and 'homologous series', and explain why alkenes are more reactive than alkanes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethene (C₂H₄) and propene (C₃H₆) are the first two members of the alkene homologous series. Ethane (C₂H₆) is the corresponding alkane with the same number of carbon atoms as ethene.
Model answer (5 marks)
Unsaturated means the molecule contains at least one carbon‑to‑carbon double bond, so not all C–C bonds are single.
A homologous series is a group of compounds that share the same general formula and functional group, and each successive member differs by a CH₂ unit.
Alkenes are more reactive than alkanes because the C=C double bond is a π‑bond that can be broken and can accept two new atoms or groups in addition reactions. Alkanes have only C–C single bonds, which are σ‑bonds that are harder to break and do not allow addition reactions.
A homologous series is a group of compounds that share the same general formula and functional group, and each successive member differs by a CH₂ unit.
Alkenes are more reactive than alkanes because the C=C double bond is a π‑bond that can be broken and can accept two new atoms or groups in addition reactions. Alkanes have only C–C single bonds, which are σ‑bonds that are harder to break and do not allow addition reactions.
Examiner tips
- Use the exact definitions of unsaturated and homologous series. Explain the CH₂ difference clearly. State that alkenes undergo addition reactions because of the π‑bond. Mention that alkanes cannot undergo addition reactions because they lack a double bond.
Common mistakes
- Confusing unsaturated with unsaturated meaning ‘not saturated with hydrogen’. Forgetting to mention the CH₂ increment. Saying alkanes are reactive because they have many bonds, rather than explaining the lack of a π‑bond.
Mark scheme (5 marks)
- Definition of 'unsaturated': contains a carbon-to-carbon double bond (C=C) / not all bonds are single bonds
- Definition of 'homologous series': a family/group of compounds with the same general formula / same functional group / similar chemical properties
- Each successive member of a homologous series differs by CH₂ (from the previous member)
- Alkenes are more reactive than alkanes because the C=C double bond can break / the double bond is available for reactions (addition reactions)
- Alkanes only have C–C single bonds which are harder to break / alkanes are saturated so cannot undergo addition reactions
Key terms in this question
unsaturated · homologous series · alkene · alkane
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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