Topic 30: Hydrocarbons

Question 1

Definitions I:
(a) Arene
(b) Friedel-Crafts reaction
(c) Electrophile

Answer 1

Definitions I:
(a) Aromatic hydrocarbon with benzene ring, e.g., C₆H₆.
(b) Alkylation/acylation of arenes using AlCl₃, e.g., CH₃Cl on benzene.
(c) Electron-pair acceptor attacking aromatic ring, e.g., NO₂⁺.

Question 2

Predict the product of C₆H₅CH₃ with KMnO₄ (hot, alkaline) then H⁺. [2 Marks]

Answer 2

C₆H₅CH₃ + 4[O] → C₆H₅COOH + H₂O (1 mark)
Benzoic acid after acidification (1 mark)

Question 3

Write equations for:
(a) Benzene + Cl₂ (AlCl₃) [1 Mark]
(b) Methylbenzene + HNO₃/H₂SO₄ [1 Mark]

Answer 3

(a) C₆H₆ + Cl₂ → C₆H₅Cl + HCl (AlCl₃) (1 mark)
(b) C₆H₅CH₃ + HNO₃ → C₆H₄(CH₃)NO₂ + H₂O (H₂SO₄, 25-60°C) (1 mark)

Question 4

In a lab, benzene reacts with CH₃COCl (AlCl₃, heat). Write the equation. [2 Marks]

Answer 4

C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (AlCl₃, heat) (1 mark)
Acetophenone formed (1 mark)

Question 5

Show the mechanism for nitration of benzene with HNO₃/H₂SO₄. [3 Marks]

Answer 5

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O (1 mark)
NO₂⁺ attacks ring, carbocation intermediate (1 mark)
H⁺ lost, nitrobenzene forms (1 mark)

Question 6

Why does C₆H₅CH₃ halogenate the side-chain with Cl₂ (UV) but the ring with AlCl₃? [2 Marks]

Answer 6

UV: free radical substitution on CH₃ (C₆H₅CH₂Cl) (1 mark)
AlCl₃: electrophilic substitution on ring (C₆H₄(CH₃)Cl) (1 mark)

Question 7

Predict the substitution position on C₆H₅OH with Br₂ (AlBr₃) and explain. [2 Marks]

Answer 7

C₆H₅OH → C₆H₄(OH)Br, ortho/para (2 or 4) (1 mark)
-OH electron-donating, activates ring (1 mark)

Question 8

In a factory, benzene is hydrogenated to cyclohexane. Write the equation and conditions. [2 Marks]

Answer 8

C₆H₆ + 3H₂ → C₆H₁₂ (1 mark)
H₂, Pt/Ni, heat (1 mark)

Question 9

Describe the role of AlCl₃ in Friedel-Crafts alkylation of benzene with CH₃Cl. [2 Marks]

Answer 9

AlCl₃ + CH₃Cl → CH₃⁺ + AlCl₄⁻, generates electrophile (1 mark)
Catalyses substitution on benzene (1 mark)

Question 10

Predict the product of C₆H₅NO₂ with Cl₂ (AlCl₃) and explain the directing effect. [2 Marks]

Answer 10

C₆H₅NO₂ → C₆H₄(NO₂)Cl, meta (3-position) (1 mark)
-NO₂ electron-withdrawing, directs meta (1 mark)

Question 11

Explain why benzene prefers electrophilic substitution over addition with Br₂. [2 Marks]

Answer 11

Delocalised π system stabilises ring (1 mark)
Substitution retains aromaticity, addition doesn’t (1 mark)

Question 12

Compare the reactivity of benzene vs. methylbenzene in nitration and explain the mechanism. [3 Marks]

Answer 12

Methylbenzene more reactive, -CH₃ donates electrons (1 mark)
Stabilises carbocation intermediate (1 mark)
Same NO₂⁺ mechanism, faster rate (1 mark)