Topic 31: Halogen Compounds

Question 1

Definitions I:
(a) Halogenoarene
(b) Halogenoalkane
(c) Electrophilic substitution

Answer 1

Definitions I:
(a) Aromatic compound with halogen on benzene ring, e.g., C₆H₅Cl.
(b) Aliphatic compound with halogen on carbon chain, e.g., CH₃CH₂Cl.
(c) Electrophile replaces H on aromatic ring, e.g., Cl⁺ on benzene.

Question 2

In a lab, C₆H₆ reacts with Br₂ (AlBr₃). Write the equation and name the product. [2 Marks]

Answer 2

C₆H₆ + Br₂ → C₆H₅Br + HBr (AlBr₃) (1 mark)
Bromobenzene (1 mark)

Question 3

Write equations for:
(a) Benzene → chlorobenzene [1 Mark]
(b) Methylbenzene → 4-chloromethylbenzene [1 Mark]

Answer 3

(a) C₆H₆ + Cl₂ → C₆H₅Cl + HCl (AlCl₃) (1 mark)
(b) C₆H₅CH₃ + Cl₂ → C₆H₄(CH₃)Cl (AlCl₃) (1 mark)

Question 4

Why does chloroethane react with NaOH(aq) but chlorobenzene does not? [2 Marks]

Answer 4

Chloroethane: C-Cl breaks, nucleophilic substitution (1 mark)
Chlorobenzene: C-Cl delocalised, resists breaking (1 mark)

Question 5

Predict the products of methylbenzene with Cl₂ (AlCl₃) and explain the positions. [2 Marks]

Answer 5

C₆H₄(CH₃)Cl: 2- and 4- positions (1 mark)
-CH₃ directs ortho/para, electron-donating (1 mark)

Question 6

Compare the reactivity of CH₃CH₂Cl and C₆H₅Cl with KCN (ethanol, heat). [3 Marks]

Answer 6

CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl (fast, SN2) (1 mark)
C₆H₅Cl: no reaction, stable C-Cl (1 mark)
Halogenoalkane reactive, halogenoarene inert (1 mark)

Question 7

Describe the role of AlCl₃ in converting C₆H₆ to C₆H₅Cl. [2 Marks]

Answer 7

AlCl₃ + Cl₂ → Cl⁺ + AlCl₄⁻, generates electrophile (1 mark)
Catalyses substitution on ring (1 mark)

Question 8

In a factory, why is AlBr₃ used to brominate benzene instead of UV light? [2 Marks]

Answer 8

AlBr₃: electrophilic substitution on ring (1 mark)
UV: free radical substitution, not desired (1 mark)

Question 9

Explain why chlorobenzene resists nucleophilic substitution unlike chloroethane. [2 Marks]

Answer 9

C₆H₅Cl: Cl lone pairs in π system, stable (1 mark)
CH₃CH₂Cl: C-Cl σ bond, breaks easily (1 mark)

Question 10

Predict the product of C₆H₅CH₃ with Br₂ (AlBr₃) and name the major isomer. [2 Marks]

Answer 10

C₆H₅CH₃ + Br₂ → C₆H₄(CH₃)Br (AlBr₃) (1 mark)
4-Bromomethylbenzene (major, para) (1 mark)

Question 11

Why is the C-Cl bond stronger in C₆H₅Cl than in CH₃CH₂Cl? [2 Marks]

Answer 11

C₆H₅Cl: sp² C, π overlap strengthens bond (1 mark)
CH₃CH₂Cl: sp³ C, weaker σ bond (1 mark)

Question 12

Analyse the substitution of C₆H₆ with Cl₂ (AlCl₃) vs. CH₃CH₃ with Cl₂ (UV). [3 Marks]

Answer 12

C₆H₆ + Cl₂ → C₆H₅Cl (AlCl₃), electrophilic (1 mark)
CH₃CH₃ + Cl₂ → CH₃CH₂Cl (UV), free radical (1 mark)
Different mechanisms, stability drives arene (1 mark)