amazon.co.ukC3: Aromatic Chemistry and Reaction Mechanisms
Benzene delocalisation stability, electrophilic aromatic substitution, nitration, Friedel-Crafts acylation, amines.
📋 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.
🎬 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