Methanol (CH₃OH) and 1-chloropropane (CH₃CH₂CH₂Cl) are examples of an alcohol and a halogenoalkane respectively. Explain the similarities and differences between the structures of alcohols and halogenoalkanes, and describe what happens when a halogenoalkane undergoes hydrolysis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alcohols contain a hydroxyl group (–OH) bonded to a carbon chain. Halogenoalkanes are formed when the hydroxyl group of an alcohol is replaced by a halogen atom.
Model answer (5 marks)
Both alcohols and halogenoalkanes have a hydrocarbon backbone.
Alcohols contain a hydroxyl group (–OH) attached to a carbon atom of the chain.
Halogenoalkanes have a halogen atom (F, Cl, Br or I) bonded to a carbon in place of the –OH group.
In hydrolysis of a halogenoalkane the C–halogen bond is broken and the halogen is replaced by a hydroxyl group, giving an alcohol.
Water (or aqueous alkali/hydroxide ions) is the reagent that supplies the –OH group and drives the substitution.
Alcohols contain a hydroxyl group (–OH) attached to a carbon atom of the chain.
Halogenoalkanes have a halogen atom (F, Cl, Br or I) bonded to a carbon in place of the –OH group.
In hydrolysis of a halogenoalkane the C–halogen bond is broken and the halogen is replaced by a hydroxyl group, giving an alcohol.
Water (or aqueous alkali/hydroxide ions) is the reagent that supplies the –OH group and drives the substitution.
Examiner tips
- Use the exact terms – ‘hydrocarbon backbone’, ‘hydroxyl group’, ‘halogen atom’. Include the reaction step: C–X + H₂O → C–OH + HX. Mention the role of water or alkali as the reagent.
- common_mistakes
- :
- Confusing the hydroxyl group with the halogen as the functional group. Forgetting to state that the halogen is replaced by –OH during hydrolysis. Using vague language such as ‘oxygen replaces halogen’ instead of specifying C–X → C–OH.
Mark scheme (5 marks)
- Both alcohols and halogenoalkanes have a carbon chain / hydrocarbon backbone
- Alcohols contain a hydroxyl group (–OH) attached to the carbon chain
- Halogenoalkanes contain a halogen atom (F, Cl, Br or I) in place of the hydroxyl group
- During hydrolysis of a halogenoalkane, the halogen atom is replaced by a hydroxyl group (–OH)
- Water (or aqueous alkali / hydroxide ions) acts as the reagent in hydrolysis, breaking the C–halogen bond
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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