Halogen compounds
Cambridge A-Level Chemistry (9701) · Unit 31: Hydroxy compounds (A Level) · 6 flashcards
Halogen compounds is topic 31.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 31 — Hydroxy compounds (A Level) . In one line: Methylbenzene reacts with chlorine (Cl₂) and AlCl₃ to form 2-chloromethylbenzene and 4-chloromethylbenzene. The methyl group directs the chlorine to the ortho (position 2) and para (position 4) positions.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 6 flashcards — 1 definition and 5 key concepts — covering the precise wording mark schemes reward. Use the definition card to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The possible products when methylbenzene reacts with chlorine in the presence of an AlCl₃ catalyst
Methylbenzene reacts with chlorine (Cl₂) and AlCl₃ to form 2-chloromethylbenzene and 4-chloromethylbenzene. The methyl group directs the chlorine to the ortho (position 2) and para (position 4) positions.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Recall how halogenoarenes are made: an arene is substituted with Cl₂ or Br₂ using an AlCl₃ or AlBr₃ catalyst, for example benzene giving chlorobenzene, and methylbenzene giving 2-chloromethylbenzene and 4-chloromethylbenzene.
- Explain why halogenoalkanes and halogenoarenes differ in reactivity, with chloroethane compared with chlorobenzene as the example.
Cambridge syllabus keywords to use in your answers
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Describe the reaction conditions and reagents required to produce chlorobenzene from benzene.
Benzene reacts with chlorine (Cl₂) in the presence of a catalyst such as aluminium chloride (AlCl₃). This electrophilic substitution reaction substitutes a hydrogen atom on the benzene ring with a chlorine atom, forming chlorobenzene and HCl.
What are the possible products when methylbenzene reacts with chlorine in the presence of an AlCl₃ catalyst?
Methylbenzene reacts with chlorine (Cl₂) and AlCl₃ to form 2-chloromethylbenzene and 4-chloromethylbenzene. The methyl group directs the chlorine to the ortho (position 2) and para (position 4) positions.
Explain why halogenoarenes are significantly less reactive than halogenoalkanes in nucleophilic substitution reactions.
The carbon-halogen bond in halogenoarenes is stronger due to the delocalization of the lone pair of electrons on the halogen into the π-system of the benzene ring, giving it partial double bond character. This increased bond strength makes it harder to break compared to the C-X bond in halogenoalkanes.
Describe one factor that affects the rate of nucleophilic substitution in halogenoalkanes, but is not relevant in halogenoarenes.
Steric hindrance around the carbon atom bonded to the halogen. Bulky alkyl groups in halogenoalkanes can hinder the approach of the nucleophile, slowing down the reaction. This is less relevant in halogenoarenes as the benzene ring provides a different type of steric environment.
How does the stability of the carbocation intermediate affect the reactivity of halogenoalkanes vs. halogenoarenes?
Tertiary (and some secondary) halogenoalkanes can ionise to give relatively stable carbocations in SN1 reactions. In a halogenoarene the C–X bond is strengthened by overlap of a halogen lone pair with the ring π system, and the aryl cation that would form is very unstable: its positive charge sits in an sp² orbital in the plane of the ring and cannot be delocalised. So it does not form under normal conditions.
Outline the mechanism for the reaction between benzene and chlorine in the presence of AlCl₃, forming chlorobenzene.
The reaction proceeds via electrophilic substitution. AlCl₃ acts as a Lewis acid catalyst, polarizing the Cl-Cl bond. The benzene ring acts as a nucleophile, attacking the partially positive chlorine. This forms a carbocation intermediate, which then loses a proton to regenerate the aromatic system, forming chlorobenzene and HCl.
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