Pentan-1-ol (C₅H₁₁OH) is an alcohol. When pentan-1-ol reacts with phosphorus pentachloride, 1-chloropentane (C₅H₁₁Cl) is produced. Explain why 1-chloropentane is classified as a halogenoalkane and describe three differences in the chemical properties of pentan-1-ol and 1-chloropentane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1-chloropentane is a halogenoalkane because it contains a chlorine atom bonded to a carbon chain, replacing the –OH group of pentan‑1‑ol.
Differences:
1. Functional group: pentan‑1‑ol has an –OH group; 1‑chloropentane has a C–Cl bond.
2. Oxidation: pentan‑1‑ol can be oxidised to a carboxylic acid or CO₂ and H₂O; 1‑chloropentane does not undergo this oxidation.
3. Reactivity with sodium: pentan‑1‑ol reacts with Na to give NaOCH₂(CH₂)₃CH₃ and H₂ gas; 1‑chloropentane does not react with Na in the same way.
4. Hydrolysis: 1‑chloropentane undergoes nucleophilic substitution with water or alkali to give pentan‑1‑ol; pentan‑1‑ol does not hydrolyse to an alcohol.
Differences:
1. Functional group: pentan‑1‑ol has an –OH group; 1‑chloropentane has a C–Cl bond.
2. Oxidation: pentan‑1‑ol can be oxidised to a carboxylic acid or CO₂ and H₂O; 1‑chloropentane does not undergo this oxidation.
3. Reactivity with sodium: pentan‑1‑ol reacts with Na to give NaOCH₂(CH₂)₃CH₃ and H₂ gas; 1‑chloropentane does not react with Na in the same way.
4. Hydrolysis: 1‑chloropentane undergoes nucleophilic substitution with water or alkali to give pentan‑1‑ol; pentan‑1‑ol does not hydrolyse to an alcohol.
Examiner tips
- Use the term ‘halogenoalkane’ and state the presence of a halogen bonded to carbon.
- Mention the –OH vs C–Cl functional groups first. List each property as a separate point to show clear comparison. Include the reaction type (oxidation, substitution, hydrolysis) to match the scheme.
Common mistakes
- Confusing the oxidation of the alcohol with the oxidation of the alkane; only the alcohol oxidises. Saying that both compounds react with sodium; only the alcohol does. Using vague terms like ‘more reactive’ without specifying the reaction type.
Mark scheme (5 marks)
- 1-chloropentane is a halogenoalkane because it contains a halogen atom (chlorine) bonded to a carbon chain / in place of the OH group
- Pentan-1-ol contains an –OH (hydroxyl) functional group; 1-chloropentane does not / contains a C–Cl bond instead
- Pentan-1-ol can be oxidised / burns to form carbon dioxide and water; 1-chloropentane does not undergo the same oxidation reaction
- Pentan-1-ol reacts with sodium to produce hydrogen gas; 1-chloropentane does not react with sodium in the same way
- 1-chloropentane undergoes hydrolysis / nucleophilic substitution with water or aqueous alkali to form pentan-1-ol / an alcohol; pentan-1-ol does not undergo this reaction
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
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