A-Level Chemistry: Aromatic Chemistry and Amines
Aromatic chemistry is Year 2 organic at its most conceptual: a ring that refuses addition, an electrophile that must be manufactured, and a basicity order that runs on delocalisation. This quiz covers benzene, amines, amino acids and polymers as the specification groups them.
The benzene questions start with the evidence: why the enthalpy of hydrogenation falls short of three-double-bond expectations, the thermodynamic signature of delocalisation, tested both as evidence selection and as explanation. The mechanistic core follows: why substitution preserves what addition would destroy (the delocalised π-system), nitration with the nitronium electrophile generated by the nitric/sulfuric acid pair, and Friedel-Crafts acylation with aluminium chloride tested twice — as the required reagent set and, more deeply, through its Lewis-acid role in creating the acylium electrophile.
The amine questions centre on the basicity order every paper asks: alkyl groups pushing electron density onto nitrogen versus the aryl ring delocalising the lone pair away — primary aliphatic amines above ammonia above phenylamine, tested through the reasons. Preparation appears through nitrile reduction to primary amines.
Amino acids are covered as bifunctional molecules: amphoteric behaviour and the zwitterion. The polymer questions close the loop begun in Organic I: condensation polymerisation losing a small molecule per link, the amide linkage formed from diacid and diamine, and the contrast with addition polymerisation.
The blind-solver verification pass returned complete agreement across all ten questions. Explanations consistently argue from electron density and delocalisation — the register in which aromatic chemistry is actually marked.
- Use hydrogenation enthalpy as evidence for benzene's delocalised structure
- Explain why benzene substitutes rather than adds
- Describe nitration and Friedel-Crafts acylation including electrophile generation
- Rank amine basicity through electron-density reasoning
- Contrast condensation and addition polymerisation and identify amide links
Aligned to the Year 2 organic content of the DfE A level science subject content as delivered in current specifications: benzene's structure and electrophilic substitution, amines and their basicity, amino acids, and condensation polymers.
Sample question
Which experimental evidence most strongly supports the delocalised model of benzene over the Kekulé structure?
- The infrared spectrum shows a distinct C=C stretching frequency at 1650 cm-1.
- The reaction of benzene with bromine water causes rapid decolorization.
- The enthalpy of hydrogenation of benzene is significantly less exothermic than three times that of cyclohexene.
- Benzene reacts with hydrogen in the presence of a nickel catalyst to form cyclohexane.
See the answer
The enthalpy of hydrogenation of benzene is significantly less exothermic than three times that of cyclohexene.
The Kekulé structure predicts three isolated double bonds, which would result in an enthalpy of hydrogenation three times that of cyclohexene. The actual value is much lower, indicating the extra stability provided by delocalisation.