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A-Level Chemistry May/June 2025 Q8(b)(iii): Explain the difference in reactivity of chlorobutane and chlorobenzene with warm aqueou…
A-Level Chemistry · Paper 9701/41 · May/June 2025 · Question 8(b)(iii) · [2 marks]
Explain the difference in reactivity of chlorobutane and chlorobenzene with warm aqueous AgNO3.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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Chlorobutane is a haloalkane and will undergo nucleophilic substitution with warm aqueous silver nitrate, forming a white precipitate of AgCl. Chlorobenzene is a haloarene and does not react under these conditions.
The difference in reactivity is due to the structure of chlorobenzene.
- In chlorobenzene, a lone pair of electrons on the chlorine atom is delocalised into the π system of the benzene ring.
- This delocalisation gives the C–Cl bond partial double bond character. This strengthens the C–Cl bond compared to the single C–Cl bond in chlorobutane.
Consequently, more energy is required to break the C–Cl bond in chlorobenzene, making it much less reactive towards nucleophilic substitution.
How the marks are awarded
- M1 — The first mark is awarded for correctly identifying the delocalisation of the chlorine atom's lone pair of electrons into the benzene ring's π system, as stated in the first point.
- M2 — The second mark is awarded for explaining the consequence of this delocalisation: the C-Cl bond gains partial double bond character, which strengthens it, as stated in the second point.
Common mistakes
- Stating that the C-Cl bond is 'stronger' without explaining that this is due to the delocalisation of the chlorine's lone pair into the ring.
- Incorrectly describing the delocalisation, for example, by stating that electrons from the ring move onto the chlorine atom.
- Only stating that the 'benzene ring is stable' without linking this concept to the strength of the specific C-Cl bond being broken.
- Confusing the reaction conditions and suggesting that chlorobenzene would undergo electrophilic substitution with AgNO3.
Examiner tip: Always connect the concept of electron delocalisation to its specific consequences on bond character, bond strength, and ultimately, chemical reactivity.
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