Organic 2 Section 3 - Unit 27: Aromatic Chemistry

Question 1

Equations for the formation of the electrophile during nitration (2)

Answer 1

• H₂SO₄ + HNO₃ → H₂NO₃+ + HSO₄-

- H₂NO₃+2 → H₂O + NO₂+

Question 2

Reagents and conditions for nitration (3)

Answer 2

• Concentrated nitric acid
• Sulfuric acid
• Heat

Question 3

Equations for the formation of the electrophile during Friedel crafts acylation (1)

Answer 3

• R-COCl + AlCl₃ → R-CO+ + AlCl₄-

Question 4

Reagents and conditions for Friedel crafts acylation (3)

Answer 4

• Acyl chloride

- Anhydrous AlCl₃ catalyst

Question 5

State and explain the stability of benzene compared with the hypothetical cyclohexatriene (4 marks)

Answer 5

• Benzene is more stable than cyclohexatriene
• Expected ΔH hydrogenation of C₆H₆ is 3(–120)
• Actual ΔH hydrogenation of benzene is
  152 kJ mol-1 (less exothermic)
• Because of delocalisation / electrons spread out

Question 6

Give a reagent that could be used in a test-tube reaction to distinguish between benzene and cyclohexene and state the observations (3 marks)

Answer 6

• Bromine
• No colour change with benzene
• Decolourises with cyclohexene

Question 7

Give the formula of the reactive intermediate during Friedel crafts acylation (1 mark)

Answer 7

• CH₃CO-

Question 8

Suggest one reason why phenylamine cannot be prepared from bromobenzene via nucleophilic substitution. Outline a synthesis of phenylamine from benzene, giving only the reagents and conditions for each step (6 marks)

Answer 8

• Benzene ring repels nucleophiles

Step 1:

• Nitration to form nitrobenzene
• Concentrated HNO₃ + concentrated H₂SO₄
• 20-60 degrees

Step 2:

• Reduction to form phenylamine
• Sn/HCl