OxfordAQA International A Level Chemistry 3.3.10 Aromatic Chemistry Exam Questions

About OxfordAQA International A Level Chemistry 3.3.10 Aromatic Chemistry

Aromatic chemistry is an A2 organic topic examined in Unit 4. You need to describe benzene as a planar molecule with delocalised electrons and intermediate bond lengths, and use enthalpies of hydrogenation to explain its extra stability. You also need the mechanisms of electrophilic substitution for nitration, sulfonation and Friedel-Crafts acylation and alkylation using an aluminium chloride catalyst. The OxfordAQA specification also includes free-radical chlorination of benzene, which adds chlorine to the ring, and of methylbenzene, which substitutes into the side chain, and the different reactivity of chlorine attached to a ring or a side chain. It links to amines, dyes and polymers.

What you need to know

• Structure of benzene Benzene is a planar ring of six carbons with a ring of delocalised pi electrons above and below. All the C-C bonds are the same length, between that of a C-C and a C=C bond.
• Stability The enthalpy of hydrogenation of benzene is much less exothermic than three times that of cyclohexene, so benzene is more stable than the Kekulé structure suggests. This is evidence for delocalisation.
• Electrophilic substitution Benzene undergoes substitution rather than addition to keep the stable delocalised ring. The electrophile attacks, forming an intermediate, and loss of H+ restores the ring.
• Nitration and sulfonation Nitration uses concentrated nitric and sulfuric acids to form NO2+, which substitutes onto the ring. Sulfonation introduces an SO3H group.
• Friedel-Crafts reactions Acylation uses an acyl chloride with an AlCl3 catalyst to form an acylium ion, RCO+, giving a phenyl ketone. Alkylation uses a halogenoalkane with AlCl3. AlCl3 is regenerated at the end.
• Free-radical chlorination In UV light chlorine adds to the benzene ring by free-radical addition. With methylbenzene, chlorine substitutes into the methyl side chain. Chlorine on a side chain is more reactive than chlorine attached directly to the ring.

Common mistakes in 3.3.10 Aromatic Chemistry

• Drawing the delocalised ring as a complete circle in the intermediate. In the intermediate the circle is broken into a horseshoe that does not cover the carbon being attacked.
• Saying benzene does not react with bromine because it has no double bonds. It is less reactive because the delocalised electrons have lower electron density than a localised C=C.
• Forgetting to show regeneration of the catalyst, for example H+ + AlCl4- → AlCl3 + HCl.
• Saying benzene undergoes electrophilic addition. It undergoes electrophilic substitution to keep its stability.

Try three 3.3.10 Aromatic Chemistry exam-style questions