9701 · 30.1
Arenes flashcards
Revision flashcards for Cambridge 9701 Arenes (syllabus 30.1). Flip, recall, then mark a real past-paper question.
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What is an arene?
A hydrocarbon based on the benzene ring, C₆H₆, which is a planar, cyclic molecule with a delocalised system of pi electrons.
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Why does benzene undergo substitution rather than addition reactions?
The delocalised pi-system gives benzene extra stability (delocalisation energy). Substitution preserves this stable system, whereas addition would destroy it.
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What is the intermediate in the electrophilic substitution of benzene called?
The arenium ion (or a carbocation intermediate). It is unstable because the delocalised pi-system is temporarily broken.
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What are the reagents and conditions for the nitration of benzene?
Reagents: Concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄ catalyst). Conditions: Heat under reflux at 50-55°C.
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What is the role of concentrated sulfuric acid in the nitration of benzene?
It acts as a catalyst by protonating the nitric acid, which then loses a water molecule to generate the strong electrophile, the nitronium ion (NO₂⁺).
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What is the electrophile in the nitration of benzene?
The nitronium ion, NO₂⁺.
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What are the reagents and conditions for the bromination of benzene?
Reagents: Bromine (Br₂) and a halogen carrier catalyst (e.g., FeBr₃ or AlBr₃). Conditions: Room temperature, in the absence of UV light.
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Why is a halogen carrier needed for the halogenation of benzene?
Benzene's pi-system is not electron-rich enough to polarise a non-polar Br-Br molecule. The catalyst (e.g., FeBr₃) accepts a lone pair from a bromine atom, creating a powerful Br⁺ electrophile.
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What is Friedel-Crafts alkylation?
The substitution of a hydrogen atom on the benzene ring with an alkyl group, using a haloalkane and an aluminium chloride (AlCl₃) catalyst.
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What is a major limitation of Friedel-Crafts alkylation?
Polyalkylation. The initial alkylbenzene product is more reactive than benzene itself, so it can react further to give di- and tri-substituted products.
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What is the electrophile in the Friedel-Crafts acylation of benzene with ethanoyl chloride?
The acylium ion, CH₃CO⁺.