Halogenoalkanes are formed when alkanes react with halogens. Explain why alkanes react with halogens to form halogenoalkanes, and describe what happens to the halogen atom in the product. Include in your answer the type of reaction taking place and the condition required for the reaction to occur.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane reacts with chlorine to form chloromethane (a halogenoalkane) and hydrogen chloride. The reaction can be represented as: CH₄ + Cl₂ → CH₃Cl + HCl
Model answer (5 marks)
1. Alkanes contain only single C–C bonds and are saturated, so they cannot undergo addition reactions.
2. In a substitution reaction, a hydrogen atom in the alkane is replaced by a halogen atom.
3. The reaction is a radical substitution that requires UV light to initiate.
4. UV light generates chlorine radicals that attack the C–H bond.
5. The halogen atom ends up bonded to the carbon in the halogenoalkane product, replacing the hydrogen.
2. In a substitution reaction, a hydrogen atom in the alkane is replaced by a halogen atom.
3. The reaction is a radical substitution that requires UV light to initiate.
4. UV light generates chlorine radicals that attack the C–H bond.
5. The halogen atom ends up bonded to the carbon in the halogenoalkane product, replacing the hydrogen.
Examiner tips
- Mention saturated nature to rule out addition; state substitution and UV requirement; show hydrogen replaced by halogen; keep answer concise; use correct terminology
Common mistakes
- Confusing addition with substitution; omitting UV light; not stating hydrogen is replaced; using wrong reaction type
Mark scheme (5 marks)
- A hydrogen atom in the alkane is replaced by a halogen atom
- The reaction is called substitution (or a substitution reaction)
- Ultraviolet (UV) light is required as the condition for the reaction
- Alkanes are saturated / contain only single carbon–carbon bonds, so addition reactions cannot occur
- The halogenoalkane product contains a halogen atom bonded to a carbon atom (in place of the hydrogen)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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