Aromatic chemistryAQA A-Level Chemistry: Topic test

1. 1
   Benzene, C₆H₆, is a planar molecule in which each carbon atom is bonded to one hydrogen atom and to two other carbon atoms in a hexagonal ring.

   (a)

   Which statement describes the p electrons in a benzene molecule?

   [1 mark]

   • AThey are delocalised in rings of electron density above and below the plane of the carbon atoms
   • BThey form three localised C=C double bonds
   • CThey are lone pairs on the carbon atoms
   • DThey form σ bonds between the carbon atoms and the hydrogen atoms

   (b)

   How do the carbon-carbon bond lengths in benzene compare with those in other hydrocarbons?

   [1 mark]

   • AThree are short and three are long, alternating around the ring
   • BAll are equal to the C=C bond length in alkenes
   • CAll are equal to the C-C bond length in alkanes
   • DAll are equal, and between the C-C and C=C bond lengths

   (c)

   Explain why the carbon atoms in a benzene ring are arranged with bond angles of 120°.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The enthalpy change of hydrogenation of cyclohexa-1,4-diene to cyclohexane is −240 kJ mol⁻¹, and cyclohexa-1,4-diene contains two isolated C=C double bonds. The enthalpy change of hydrogenation of benzene to cyclohexane is −208 kJ mol⁻¹.

   (a)

   What value would be expected for the enthalpy change of hydrogenation of the hypothetical molecule cyclohexa-1,3,5-triene, which has three localised C=C double bonds, each behaving like those in cyclohexa-1,4-diene?

   [1 mark]

   • A−208 kJ mol⁻¹
   • B−240 kJ mol⁻¹
   • C−360 kJ mol⁻¹
   • D−480 kJ mol⁻¹

   (b)

   Which conclusion is supported by the difference between the predicted and measured values?

   [1 mark]

   • ABenzene is less stable than the triene, by 152 kJ mol⁻¹
   • BBenzene is more stable than the triene, by 152 kJ mol⁻¹
   • CBenzene is more stable than the triene, by 28 kJ mol⁻¹
   • DBenzene and the triene have the same stability

   (c)

   Explain, in terms of its electrons, why benzene is more stable than the hypothetical cyclohexa-1,3,5-triene.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Nitration of benzene is an important step in the manufacture of explosives and of compounds that are later converted into amines. The electrophile is the nitronium ion, NO₂⁺, which is generated by mixing concentrated nitric acid with concentrated sulfuric acid.

   (a)

   Write an equation for the formation of the nitronium ion in the nitrating mixture and explain the role of sulfuric acid.

   [3 marks]

   (b)

   Outline the mechanism for the reaction of benzene with the nitronium ion to form nitrobenzene.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Benzene reacts with butanoyl chloride, CH₃CH₂CH₂COCl, in the presence of anhydrous aluminium chloride to form 1-phenylbutan-1-one, C₆H₅COCH₂CH₂CH₃. Cyclohexene, which has a localised C=C double bond, reacts with hydrogen bromide at room temperature, but benzene does not.

   (a)

   Describe the mechanism for the reaction of benzene with butanoyl chloride in the presence of aluminium chloride.

   [6 marks]

   (b)

   Explain why cyclohexene reacts with hydrogen bromide at room temperature while benzene does not, and why benzene reacts with strong electrophiles by substitution rather than addition.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Benzene reacts with 2-methylpropanoyl chloride, (CH₃)₂CHCOCl, in the presence of anhydrous aluminium chloride to form a ketone. The first step of the mechanism is the formation of the electrophile.

   (a)

   What is the role of aluminium chloride in the formation of the electrophile?

   [1 mark]

   • AIt acts as a nucleophile that attacks benzene
   • BIt accepts a chloride ion from the acyl chloride
   • CIt oxidises the acyl chloride
   • DIt removes a hydrogen ion from benzene

   (b)

   Which species is the electrophile?

   [1 mark]

   • A(CH₃)₂CHCOCl
   • B(CH₃)₂CHCOO⁻
   • C(CH₃)₂CHCO⁺
   • DAlCl₄⁻

   (c)

   Write an equation for the formation of the electrophile and an equation showing how the catalyst is regenerated in the final step of the mechanism.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   In a Friedel-Crafts acylation, 7.80 g of benzene (M_r = 78.0) is reacted with an excess of propanoyl chloride in the presence of anhydrous aluminium chloride. The product is 1-phenylpropan-1-one, C₆H₅COCH₂CH₃ (M_r = 134.0). The mass of product obtained is 9.38 g.

   (a)

   What is the organic product of this reaction?

   [1 mark]

   • APropylbenzene
   • BPhenylamine
   • CChlorobenzene
   • D1-Phenylpropan-1-one

   (b)

   Which type of reaction takes place between benzene and the electrophile?

   [1 mark]

   • AElectrophilic substitution
   • BNucleophilic substitution
   • CElectrophilic addition
   • DNucleophilic addition-elimination

   (c)

   Calculate the percentage yield of 1-phenylpropan-1-one.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   The nitration of benzene can be represented by C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O. The relative molecular masses are C₆H₆ = 78.0, HNO₃ = 63.0 and C₆H₅NO₂ = 123.0.

   (a)

   Explain why the nitronium ion is attracted to a benzene ring, but a nucleophile such as the hydroxide ion is not.

   [3 marks]

   (b)

   Calculate the atom economy for the formation of nitrobenzene by this reaction, to three significant figures.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Benzene, C₆H₆, is a planar molecule. When it reacts with the nitronium ion, NO₂⁺, the product is nitrobenzene, C₆H₅NO₂, a substitution product, and not an addition product.

   (a)

   Describe the structure and bonding of a benzene molecule and explain why it is more stable than a molecule with three localised double bonds.

   [6 marks]

   (b)

   Explain, with reference to the mechanism, why the reaction of benzene with the nitronium ion gives a substitution product rather than an addition product.

   [6 marks]

   Total for question 8: 12 marks

End of questions