1-chloropropane can be hydrolysed by reacting it with aqueous sodium hydroxide solution. The product is propan-1-ol and a sodium salt. Explain why 1-chloropropane undergoes this reaction more readily than 1-fluoropropane would, and describe what is meant by saying that 1-chloropropane belongs to a homologous series of halogenoalkanes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Halogenoalkanes are formed when a hydrogen atom in an alkane is replaced by a halogen atom. They undergo hydrolysis reactions with aqueous sodium hydroxide solution to form alcohols.
Model answer (5 marks)
1. The C–Cl bond in 1‑chloropropane is weaker than the C–F bond in 1‑fluoropropane, so it requires less energy to break.
2. During hydrolysis the C–Cl bond is cleaved first; the weaker bond means the reaction proceeds more readily.
3. A homologous series is a set of compounds that share the same functional group.
4. Compounds in the series show similar chemical properties.
5. Each successive member differs by a CH₂ group.
2. During hydrolysis the C–Cl bond is cleaved first; the weaker bond means the reaction proceeds more readily.
3. A homologous series is a set of compounds that share the same functional group.
4. Compounds in the series show similar chemical properties.
5. Each successive member differs by a CH₂ group.
Examiner tips
- Mention bond strength difference first to secure the first two marks. Explain the effect on reaction rate to gain the third mark. Define homologous series and note similarity of properties for the fourth mark. State the CH₂ difference for the final mark.
Common mistakes
- Confusing bond strength with bond length. Saying the reaction is ‘faster’ without linking to bond energy. Using ‘hydrolysis’ incorrectly as a generic term rather than SN2 mechanism.
Mark scheme (5 marks)
- The C–Cl bond is weaker than the C–F bond
- Because the C–Cl bond is weaker, it is more easily broken during the reaction / less energy is needed to break it
- A homologous series is a series of compounds with the same functional group
- Members of the homologous series have similar chemical properties
- Each successive member of the homologous series differs by CH₂
Key terms in this question
halogenoalkane · homologous series
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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