Ethanol can be converted into chloroethane by reacting it with phosphorus pentachloride. Explain why chloroethane is classified as a halogenoalkane and describe two chemical properties of halogenoalkanes that differ from those of alcohols.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanol has the molecular formula C₂H₅OH. Chloroethane has the molecular formula C₂H₅Cl.
Model answer (5 marks)
Chloroethane is classified as a halogenoalkane because it contains a chlorine atom bonded to a carbon chain. It therefore has the general formula R–Cl rather than an –OH group.
Two properties that differ from alcohols:
1. Halogenoalkanes do not burn cleanly in excess oxygen; they give incomplete combustion products (CO, CO₂, HCl) rather than the CO₂ and H₂O that alcohols give.
2. Halogenoalkanes undergo nucleophilic substitution (e.g. hydrolysis) to give alcohols, whereas alcohols do not react with water or hydroxide ions in this way.
(Alternatively, halogenoalkanes do not react with sodium to release H₂, unlike alcohols.)
Two properties that differ from alcohols:
1. Halogenoalkanes do not burn cleanly in excess oxygen; they give incomplete combustion products (CO, CO₂, HCl) rather than the CO₂ and H₂O that alcohols give.
2. Halogenoalkanes undergo nucleophilic substitution (e.g. hydrolysis) to give alcohols, whereas alcohols do not react with water or hydroxide ions in this way.
(Alternatively, halogenoalkanes do not react with sodium to release H₂, unlike alcohols.)
Examiner tips
- Use the word ‘halogenoalkane’ and mention the chlorine atom; explain the lack of an –OH group. Show two contrasting reactions – combustion and substitution – and give the products. Keep the answer concise and use correct terminology.
- Remember to state the reaction type (substitution) and the products (alcohol).
- Use the word ‘halogenoalkane’ exactly as in the mark scheme.
Mark scheme (5 marks)
- Chloroethane is classified as a halogenoalkane because it contains a halogen atom (chlorine) bonded to a carbon chain / an alkyl group
- Halogenoalkanes do not contain an –OH (hydroxyl) group, whereas alcohols do
- Halogenoalkanes do not undergo combustion to produce carbon dioxide and water in the same way as alcohols / alcohols burn completely in excess oxygen to give CO₂ and H₂O whereas halogenoalkanes do not combust cleanly
- Halogenoalkanes undergo hydrolysis / substitution reactions with water or hydroxide ions to form alcohols, whereas alcohols do not undergo this reaction
- Halogenoalkanes do not react with sodium / do not produce hydrogen gas when added to sodium, whereas alcohols react with sodium to release hydrogen gas
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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