1-iodopropane can react with ammonia solution to form an organic product. Describe what happens during this reaction and explain why halogenoalkanes are useful starting materials for making a range of new organic compounds.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1‑iodopropane reacts with aqueous ammonia in a nucleophilic substitution (S_N2) reaction.
1. The lone pair on NH₃ attacks the electrophilic carbon bearing the iodine.
2. The C–I bond breaks, iodine leaves as I⁻.
3. The new C–NH₂ bond is formed, giving propylamine (1‑aminopropane).
Halogenoalkanes are useful starting materials because they contain a reactive C–X bond. This bond can be cleaved and replaced by a variety of nucleophiles or reagents, allowing different functional groups to be introduced. Consequently, a single halogenoalkane can be converted into many different organic compounds.
1. The lone pair on NH₃ attacks the electrophilic carbon bearing the iodine.
2. The C–I bond breaks, iodine leaves as I⁻.
3. The new C–NH₂ bond is formed, giving propylamine (1‑aminopropane).
Halogenoalkanes are useful starting materials because they contain a reactive C–X bond. This bond can be cleaved and replaced by a variety of nucleophiles or reagents, allowing different functional groups to be introduced. Consequently, a single halogenoalkane can be converted into many different organic compounds.
Examiner tips
- Use the word ‘substitution’ and specify S_N2. Mention C–I bond cleavage and formation of C–NH₂. Explain that the C–X bond is reactive and can be replaced by many nucleophiles. Show the product name (propylamine).
Common mistakes
- Failing to state that the reaction is S_N2. Not recognising the C–I bond breaking. Calling the product ‘amine’ without the correct name. Overlooking the general reactivity of C–X bonds.
Mark scheme (5 marks)
- The iodine atom (halogen/leaving group) is replaced by an ammonia/amine group (NH₂) — identifying this as a substitution reaction
- The C–I (carbon–halogen) bond is broken during the reaction
- An amine (propylamine / 1-aminopropane) is formed as the organic product
- Halogenoalkanes contain a reactive C–X bond (carbon–halogen bond) which can be broken and replaced by different nucleophiles/reagents
- This allows different functional groups to be introduced, so a variety of different organic compounds/products can be made from the same halogenoalkane starting material
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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