30.1 A2 Level

Arenes

8 flashcards to master this topic

Key Concept Flip

What type of reaction is characteristic of arenes, and why?

Answer Flip

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.

Key Concept Flip

Describe the mechanism for the nitration of benzene.

Answer Flip

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.

Definition Flip

What are the reagents and conditions for the complete oxidation of a side-chain on an arene?

Answer Flip

Hot alkaline KMnO₄ followed by dilute acid. This oxidizes the alkyl group to a carboxyl group, forming a benzoic acid derivative.

Definition Flip

What catalyst is required for the halogenation of benzene, and what is its role?

Answer Flip

A Lewis acid catalyst such as AlCl₃ or AlBr₃ is required. It polarizes the halogen molecule (

Example: Cl₂) to create a stronger electrophile (. Cl⁺), facilitating the electrophilic attack on the benzene ring.
Definition Flip

Describe the Friedel-Crafts alkylation reaction.

Answer Flip

It involves reacting an arene with an alkyl halide (

Example: CH₃Cl) in the presence of a Lewis acid catalyst (. AlCl₃) and heat. The alkyl group is added to the benzene ring through electrophilic substitution.
Definition Flip

What product is formed when benzene is hydrogenated using H₂ and a Pt/Ni catalyst with heat?

Answer Flip

Cyclohexane (C₆H₁₂). The benzene ring is saturated with hydrogen atoms, converting the aromatic ring into a saturated cyclic alkane.

Key Concept Flip

Explain how substituents on a benzene ring can affect the position of further electrophilic substitution.

Answer Flip

Substituents direct incoming groups to specific positions. Electron-donating groups (

Example: -NH₂, -OH, -R) are ortho/para-directing, while electron-withdrawing groups (. -NO₂, -COOH, -COR) are meta-directing.
Key Concept Flip

Under what conditions will halogenation occur in the side-chain of an arene, rather than on the aromatic ring?

Answer Flip

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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29.2 Characteristic organic reactions 31.1 Halogen compounds