CCEA A Level Chemistry 4.10 Aromatic Chemistry Exam Questions

About CCEA A Level Chemistry 4.10 Aromatic Chemistry

Section 4.10 completes Unit A2 1 with aromatic chemistry. You need to explain the structure of benzene in terms of a ring of delocalised pi electrons and use evidence such as bond lengths and enthalpy of hydrogenation to show it is more stable than a Kekule structure. You must explain why benzene resists addition with bromine, unlike alkenes, and draw the electrophilic substitution mechanisms for monobromination, mononitration, monoalkylation and monoacylation, including how each electrophile is formed. The required practical is the preparation of methyl 3-nitrobenzoate from methyl benzoate. This links to alkenes (2.4) and to phenylamine and azo dyes in A2 2.

What you need to know

• Structure Each carbon is bonded to two carbons and one hydrogen; the remaining p electrons overlap to form a ring of delocalised pi electrons above and below the plane. All C-C bonds are the same length, between C-C and C=C.
• Stability Hydrogenation of benzene releases about 208 kJ mol-1, much less than the 360 kJ mol-1 expected for three C=C bonds (3 x 120), so benzene is more stable by about 152 kJ mol-1 due to delocalisation.
• Resistance to addition Addition would destroy the stable delocalised system, so benzene undergoes substitution and does not decolourise bromine water.
• Electrophiles Nitration: concentrated HNO3 and H2SO4 give NO2+. Bromination: Br2 with an FeBr3 (or iron) halogen carrier gives Br+. Alkylation: a halogenoalkane with AlCl3. Acylation: an acyl chloride with AlCl3 gives RCO+.
• Mechanism The electrophile accepts a pair of electrons from the delocalised ring, forming an intermediate with a positive charge and a partly broken ring; then C-H breaks and the electrons restore the delocalised ring, releasing H+.
• Methyl 3-nitrobenzoate Add a cold mixture of concentrated nitric and sulfuric acids slowly to methyl benzoate in sulfuric acid, keeping the temperature low in an ice bath. Pour onto ice, filter under reduced pressure, recrystallise from ethanol and check purity by melting point.

Common mistakes in 4.10 Aromatic Chemistry

• Drawing the intermediate with a full circle. The ring in the intermediate is a horseshoe, open at the carbon bearing both H and the electrophile.
• Saying benzene reacts with bromine water. It needs liquid bromine and a halogen carrier catalyst.
• Forgetting to show regeneration of the catalyst. For example H+ + FeBr4- gives FeBr3 + HBr.
• Explaining stability with three double bonds. Benzene has no localised double bonds; it has a delocalised pi system.
• Letting the nitration mixture get hot. Higher temperatures lead to further nitration and lower yield of the mononitro product.

Try three 4.10 Aromatic Chemistry exam-style questions