Chloroethane can be converted into ethanol by reacting it with aqueous sodium hydroxide. This is an example of a substitution reaction involving a halogenoalkane. Explain why halogenoalkanes are reactive and describe what happens during this substitution reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Chloroethane has the molecular formula C₂H₅Cl. It reacts with aqueous sodium hydroxide solution to produce ethanol and sodium chloride.
Model answer (5 marks)
The C–Cl bond in chloroethane is polar because chlorine is more electronegative than carbon, giving the C–Cl bond a partial negative charge on Cl (δ−) and a partial positive charge on C (δ+). The hydroxide ion is a strong nucleophile and is attracted to the δ+ carbon. It attacks the carbon, forming a new C–O bond and displacing the chlorine atom. The chlorine leaves as a chloride ion (Cl⁻), and ethanol (C₂H₅OH) is produced.
Examiner tips
- Use the term ‘polar C–Cl bond’ and explain the charge distribution; mention ‘nucleophile’ and ‘electrophilic carbon’; describe the attack and leaving group; state the products clearly.
- Keep the answer concise – one or two sentences per point – and use correct chemical symbols and charges.
Common mistakes
- Writing ‘chlorine is electronegative’ without linking to the polar bond; confusing the nucleophile with the electrophile; omitting the chloride ion as the leaving group or the formation of ethanol.
Mark scheme (5 marks)
- The carbon–halogen bond is polar / the C–Cl bond is polar
- Chlorine is more electronegative than carbon, so it carries a partial negative charge (δ−) and carbon carries a partial positive charge (δ+)
- The hydroxide ion (OH⁻) acts as a nucleophile and is attracted to the δ+ carbon
- The hydroxide ion replaces / substitutes the chlorine atom
- The chlorine leaves as a chloride ion (Cl⁻) and ethanol is formed
Key terms in this question
halogenoalkane · substitution reaction
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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