Exam Practice

Topic 3.3.10: Aromatic Chemistry

Test your understanding of benzene structure, electrophilic substitution mechanisms, and directing effects.

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📋 Structured Questions

Question 1: Structure of Benzene

6 marks

(a) Compare the carbon-carbon bond lengths in benzene with those in cyclohexene and cyclohexane. Explain what this reveals about the structure of benzene. [3]

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  • In cyclohexene, C=C bond is shorter (0.134 nm); in cyclohexane, C-C bond is longer (0.154 nm) [1]
  • In benzene, all six C-C bonds are equal in length and intermediate between single and double bonds (0.140 nm) [1]
  • This proves benzene has a delocalised ring system and does not contain alternating single and double bonds (disproving the cyclohexa-1,3,5-triene Kekule structure) [1]
Examiner tip: Quote exact or comparative lengths when possible. The key concept is that the bond length is intermediate, showing that the C-C bonds are identical due to delocalisation.

(b) Explain how the enthalpy of hydrogenation of benzene provides thermodynamic evidence for its stability compared to Kekule's proposed cyclohexa-1,3,5-triene. Show calculations. [3]

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  • Hydrogenation of one C=C double bond (cyclohexene) is -120 kJ/mol, so the theoretical Kekule cyclohexa-1,3,5-triene structure is expected to release 3 * -120 = -360 kJ/mol [1]
  • The actual enthalpy of hydrogenation of benzene is -208 kJ/mol [1]
  • Benzene is 152 kJ/mol more stable (lower in energy) than expected. This difference (delocalisation/resonance stability energy) is due to the delocalised pi-electrons [1]
Examiner tip: Show the calculation 3 * -120 = -360 and subtract the actual value (-360 - (-208) = -152 kJ/mol). Always state that the smaller-than-expected release of energy shows the molecule is more stable/lower in starting energy.

Question 2: Nitration of Benzene

6 marks

(a) Write the equation for the reaction of nitric acid and sulfuric acid to generate the electrophile for nitration. [1]

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HNO3 + 2H2SO4 -> NO2+ + H3O+ + 2HSO4- [1]

(Accept: HNO3 + H2SO4 -> NO2+ + H2O + HSO4-)

Examiner tip: The first equation shows the full acid-base behaviour where sulfuric acid protonates nitric acid, which then loses water. AQA accepts both variations, but the equation with 2H2SO4 is preferred as it reflects the correct stoichiometry in concentrated solutions.

(b) Describe the mechanism for the electrophilic substitution of benzene by this electrophile. Mention the shape and bonding of the intermediate. [4]

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  • Curly arrow goes from the benzene pi-ring to the nitrogen of the NO2+ electrophile [1]
  • The intermediate is drawn with a horseshoe shape open towards the sp3 carbon (which is bonded to both -H and -NO2) [1]
  • A positive charge is located inside the horseshoe loop of the intermediate ring [1]
  • Curly arrow goes from the C-H bond back into the pi-system to restore delocalisation, releasing H+ [1]
Examiner tip: Pay close attention to the drawing of the intermediate: the horseshoe must cover at least 4 carbons, and the open end must face the carbon containing both the H and NO2 groups. The positive charge must be inside the loop, not on a single carbon.

(c) State the temperature condition for this reaction and explain why it is controlled. [1]

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Maintain temperature below 55 degrees Celsius to prevent further substitution / nitration reactions [1]

Question 3: Friedel-Crafts Acylation

4 marks

(a) Write the equation to show the generation of the electrophile when propanoyl chloride reacts with aluminium chloride catalyst. [1]

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CH3CH2COCl + AlCl3 -> CH3CH2CO+ + AlCl4- [1]

(Must show the positive charge on the carbonyl carbon: CH3CH2C+O or similar)

(b) Explain the role of the AlCl3 catalyst in Friedel-Crafts acylation, and write an equation showing how the catalyst is regenerated at the end of the reaction. [3]

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  • AlCl3 acts as a halogen carrier / Lewis acid / electron pair acceptor [1]
  • It pulls the chlorine atom off the acyl chloride to form AlCl4- and create a highly reactive carbocation (acylium ion) electrophile [1]
  • Regeneration: AlCl4- + H+ -> AlCl3 + HCl [1]
Examiner tip: Remember that catalyst regeneration involves the hydrogen ion (H+) lost from the benzene ring during the substitution process reacting with the AlCl4- ion. This forms gaseous HCl as a byproduct.

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