C3: Aromatic Chemistry and Reaction Mechanisms

Year 1 / ASYear 2 / A-Level All Boards (AQA, Edexcel, OCR, WJEC, CCEA) AQA

Benzene delocalisation stability, electrophilic aromatic substitution, nitration, Friedel-Crafts acylation, amines.

Exam Board:

📋 Key Definitions and Core Concepts

Electrophilic Aromatic Substitution: Reaction where electrophile replaces H on benzene ring, preserving the stable delocalised π-system.

Delocalisation Energy: 152 kJ/mol extra stability of benzene over theoretical cyclohexa-1,3,5-triene.

🔍 Key Principles & Specification Requirements

Evidence against Kekule model: hydrogenation enthalpy (-208 kJ/mol vs -360 kJ/mol), equal C-C bond lengths (0.139 nm), lack of addition reactions.
Nitration electrophile NO₂⁺ generated via: HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + 2HSO₄⁻ + H₃O⁺.
Friedel-Crafts acylation uses RCOCl with AlCl₃ catalyst to form acylium ion R-C⁺=O.

💡 Worked Example Question

Exam-Style Question

Question:

Outline the mechanism for electrophilic nitration of benzene to nitrobenzene.

Model Solution & Mark Scheme:

1. Generation: HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺.
2. Attack: Benzene π-electrons attack NO₂⁺ forming arenium intermediate carbocation (horseshoe delocalised over 5 carbons).
3. Deprotonation: HSO₄⁻ removes H⁺ from sp³ carbon, restoring aromatic ring: Arenium + HSO₄⁻ → Nitrobenzene + H₂SO₄.

❓ Practice Questions & Mark Schemes

Q1: Why is phenol more reactive than benzene?

Show Model Answer

Answer: Lone pair on oxygen partially delocalises into benzene π-system, increasing electron density and activating ring toward electrophiles.

Q2: Order the basicity of phenylamine, ammonia, ethylamine.

Show Model Answer

Answer: Ethylamine > Ammonia > Phenylamine. Ethyl group is electron-donating (+I); phenylamine delocalises N lone pair into ring.

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🎬 Video Resources

Aromatic Chemistry and Reaction Mechanisms - Overview -- YouTube Search
Aromatic Chemistry and Reaction Mechanisms - Exam Questions -- YouTube Search

📄 Past Papers & Exam Resources

AQA A-Level - Past Papers -- AQA
Edexcel A-Level - Past Papers -- Edexcel
OCR A-Level - Past Papers -- OCR

🔗 Further Reading & Resources

Physics & Maths Tutor -- Free notes, worksheets, and past paper questions by topic
Dr Frost Maths -- Free resources, slides, and interactive questions
BBC Bitesize -- BBC Bitesize revision resources

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