AQA A Level Chemistry
AQA A Level Chemistry 3.3.10 Aromatic Chemistry Exam Questions
Free AQA A Level Chemistry exam questions for 3.3.10 Aromatic Chemistry, with a mark scheme for every pack. Covers benzene structure and stability, delocalisation evidence, electrophilic substitution, nitration and Friedel-Crafts acylation.
3.3.10 Aromatic Chemistry exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.3.10 Aromatic Chemistry
Aromatic chemistry on the AQA A Level course focuses on benzene and its distinctive stability. You need to describe the delocalised model of benzene, use thermochemical evidence from enthalpies of hydrogenation to show its extra stability, and explain why benzene undergoes electrophilic substitution rather than addition. You study two mechanisms in detail: nitration with a mixture of concentrated nitric and sulfuric acids, and Friedel-Crafts acylation with an acyl chloride and aluminium chloride. Nitration is important for making explosives and as a first step to aromatic amines in 3.3.11. Examiners test the generation of the electrophile, accurate mechanisms and calculations of stabilisation energy.
What you need to know
- Structure of benzene Benzene is a planar hexagon of six carbon atoms with all C-C bonds the same length, between a single and a double bond. Each carbon contributes one p electron to a ring of delocalised π electrons above and below the plane.
- Thermochemical evidence Cyclohexene has ΔH of hydrogenation of about -120 kJ mol-1, so a Kekulé structure with three C=C bonds would be expected to give about -360 kJ mol-1. The actual value for benzene is about -208 kJ mol-1, so benzene is about 152 kJ mol-1 more stable.
- Electrophilic substitution The delocalised ring attracts electrophiles, but addition would destroy the stable delocalisation. Substitution of a hydrogen keeps the ring intact.
- Nitration Concentrated HNO3 and concentrated H2SO4 at about 50 °C. HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4-. The NO2+ attacks the ring and H+ is lost to give nitrobenzene. Higher temperatures give multiple substitution.
- Friedel-Crafts acylation An acyl chloride with an AlCl3 catalyst forms an acylium ion: CH3COCl + AlCl3 → CH3CO+ + AlCl4-. The product is an aromatic ketone such as phenylethanone, and H+ + AlCl4- → AlCl3 + HCl regenerates the catalyst.
Common mistakes in 3.3.10 Aromatic Chemistry
- Saying benzene undergoes addition like alkenes. It undergoes substitution, which keeps the delocalised system.
- Drawing the intermediate with a full circle or with the horseshoe covering all six carbons. The partial ring should span five carbons, with the positive charge inside, leaving the carbon bonded to both H and the electrophile outside it.
- Starting the first curly arrow from a single carbon atom instead of from the delocalised ring.
- Leaving out the equation for forming the electrophile, which is usually worth a mark.
- Calculating stabilisation energy with the wrong sign or forgetting to multiply the cyclohexene value by three.
Try three 3.3.10 Aromatic Chemistry exam-style questions
1. Use enthalpy of hydrogenation data to explain the stability of benzene. Cyclohexene -120 kJ mol-1; benzene -208 kJ mol-1. (3 marks)
Show the mark scheme answer
Expected value for a Kekulé structure = 3 x -120 = -360 kJ mol-1; benzene releases 152 kJ mol-1 less energy than expected; so benzene is more stable because of its delocalised π electrons.
2. Write an equation for the formation of the electrophile in the nitration of benzene, and outline the mechanism for the reaction. (4 marks)
Show the mark scheme answer
HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4-; curly arrow from the delocalised ring to the N of NO2+; intermediate with a partial ring (horseshoe) and a positive charge; curly arrow from the C-H bond into the ring, releasing H+ to give nitrobenzene.
3. Benzene reacts with ethanoyl chloride in the presence of aluminium chloride. Write an equation to show how the catalyst forms the electrophile, and an equation to show how the catalyst is regenerated. (2 marks)
Show the mark scheme answer
CH3COCl + AlCl3 → CH3CO+ + AlCl4-; H+ + AlCl4- → AlCl3 + HCl.
3.3.10 Aromatic Chemistry FAQs
What does AQA A Level Chemistry 3.3.10 Aromatic Chemistry cover?
It covers the structure and stability of benzene, thermochemical evidence for delocalisation, electrophilic substitution, nitration and Friedel-Crafts acylation.
Which paper is 3.3.10 Aromatic Chemistry examined in?
It is examined in Paper 2, and Paper 3 can also include it.
What are the key details in an electrophilic substitution mechanism?
Show the electrophile forming, an arrow from the ring to the electrophile, the intermediate with a horseshoe over five carbons and a positive charge, and an arrow from the C-H bond back into the ring. Then show how the catalyst or acid is regenerated.
