Arenes
Cambridge A-Level Chemistry (9701) · Unit 30: Hydrocarbons · 8 flashcards
Arenes is topic 30.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 30 — Hydrocarbons . In one line: Hot alkaline KMnO₄ followed by dilute acid. This oxidizes the alkyl group to a carboxyl group, forming a benzoic acid derivative.
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 8 flashcards — 4 definitions and 4 key concepts — covering the precise wording mark schemes reward. Use the 4 definition cards 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 reagents and conditions for the complete oxidation of a side-chain on an arene
Hot alkaline KMnO₄ followed by dilute acid. This oxidizes the alkyl group to a carboxyl group, forming a benzoic acid derivative.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- describe the chemistry of arenes as exemplified by the following reactions of benzene and methylbenzene: (a) substitution reactions with Cl ₂ and with Br₂ in the presence of a catalyst, Al Cl ₃ or Al Br₃, to form halogenoarenes (aryl halides) (b) nitration with a mixture of concentrated HNO₃ and concentrated H₂SO₄ at a temperature between 25 °C and 60 °C (c) Friedel–Crafts alkylation by CH₃Cl and Al Cl ₃ and heat (d) Friedel–Crafts acylation by CH₃COCl and Al Cl ₃ and heat (e) complete oxidation of the side-chain using hot alkaline KMnO₄ and then dilute acid to give a benzoic acid (f) hydrogenation of the benzene ring using H₂ and Pt/Ni catalyst and heat to form a cyclohexane ring
- describe the mechanism of electrophilic substitution in arenes: (a) as exemplified by the formation of nitrobenzene and bromobenzene (b) with regards to the effect of delocalisation (aromatic stabilisation) of electrons in arenes to explain the predomination of substitution over addition
- predict whether halogenation will occur in the side-chain or in the aromatic ring in arenes depending on reaction conditions
- describe that in the electrophilic substitution of arenes, different substituents direct to different ring positions (limited to the directing effects of –NH₂, –OH, –R, –NO₂, –COOH and –COR)
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Arenes
- › Explain activation by detailing how the lone pair on the oxygen overlaps with the pi-ring system, increasing electron density.
- › Check if elimination products form a side chain or ring C=C bond, as this determines the optical isomerism of subsequent reaction products.
- › Treat the cyclohexene ring as non-planar and ensure straight-chain alkenes with the formula C6H10 have two C=C double bonds.
- › Identify every C=C double bond in the molecule and determine if cleavage results in a ketone, aldehyde, or carboxylic acid.
- › Only count hybridised orbitals belonging to carbon atoms; internal carbons in alkenes are sp2 hybridised, while terminal methyl carbons are sp3.
What type of reaction is characteristic of arenes, and why?
Arenes undergo electrophilic substitution reactions due to the delocalized π-electron system (aromatic stabilization). Addition reactions would disrupt the stable delocalized system, requiring more energy.
Describe the mechanism for the nitration of benzene.
1. Formation of electrophile: HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O. 2. Electrophilic attack: NO₂⁺ attacks benzene ring, forming a carbocation intermediate. 3. Deprotonation: HSO₄⁻ removes a proton, regenerating the benzene ring and forming nitrobenzene.
What are the reagents and conditions for the complete oxidation of a side-chain on an arene?
Hot alkaline KMnO₄ followed by dilute acid. This oxidizes the alkyl group to a carboxyl group, forming a benzoic acid derivative.
What catalyst is required for the halogenation of benzene, and what is its role?
A Lewis acid catalyst such as AlCl₃ or AlBr₃ is required. It polarizes the halogen molecule (
Describe the Friedel-Crafts alkylation reaction.
It involves reacting an arene with an alkyl halide (
What product is formed when benzene is hydrogenated using H₂ and a Pt/Ni catalyst with heat?
Cyclohexane (C₆H₁₂). The benzene ring is saturated with hydrogen atoms, converting the aromatic ring into a saturated cyclic alkane.
Explain how substituents on a benzene ring can affect the position of further electrophilic substitution.
Substituents direct incoming groups to specific positions. Electron-donating groups (
Under what conditions will halogenation occur in the side-chain of an arene, rather than on the aromatic ring?
Halogenation of the side-chain occurs under UV light or high temperatures without a Lewis acid catalyst. This promotes free radical substitution in the alkyl side-chain.
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