Topic 15: Halogen Compounds

Question 1

Definitions I:
(a) Halogenoalkane
(b) Nucleophilic substitution
(c) Elimination reaction

Answer 1

Definitions I:
(a) Organic compound with a halogen atom bonded to an alkyl group.
(b) Reaction where a nucleophile replaces a leaving group, e.g., OH⁻ for Cl⁻.
(c) Reaction forming an alkene by removing HX, e.g., from halogenoalkanes.

Question 2

Definitions II:
(a) Primary halogenoalkane
(b) SN1 mechanism
(c) SN2 mechanism

Answer 2

Definitions II:
(a) Halogenoalkane with one alkyl group on C-X carbon, e.g., CH₃CH₂Cl.
(b) Substitution via carbocation intermediate, one-step rate-determining, e.g., tertiary.
(c) Substitution via concerted transition state, two-step process, e.g., primary.

Question 3

Write equations for producing CH₃CH₂Cl:
(a) From ethane [1 Mark]
(b) From ethanol with PCl₅ [1 Mark]

Answer 3

(a) CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl (UV light) (1 mark)
(b) CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl (1 mark)

Question 4

Classify and describe the substitution reaction of:
(a) CH₃CH₂CH₂Cl with NaOH(aq) [1 Mark]
(b) (CH₃)₃CCl with KCN [1 Mark]

Answer 4

(a) Primary, CH₃CH₂CH₂Cl + NaOH → CH₃CH₂CH₂OH + NaCl (heat) (1 mark)
(b) Tertiary, (CH₃)₃CCl + KCN → (CH₃)₃CCN + KCl (ethanol, heat) (1 mark)

Question 5

In a lab, 1-bromobutane reacts with AgNO₃(aq) in ethanol. What’s observed, and why? [2 Marks]

Answer 5

White AgBr ppt forms slowly (1 mark)
C-Br bond breaks, Br⁻ reacts with Ag⁺ (1 mark)

Question 6

Show the SN2 mechanism for CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻ with curly arrows. [3 Marks]

Answer 6

OH⁻ attacks C, arrow from OH⁻ lone pair to C (1 mark)
C-Br bond breaks, arrow from bond to Br (1 mark)
Concerted, forms CH₃CH₂OH + Br⁻ (1 mark)

Question 7

Why does C-I in CH₃CH₂I react faster with AgNO₃(aq) than C-Cl in CH₃CH₂Cl? [2 Marks]

Answer 7

C-I bond weaker (149 kJ/mol) than C-Cl (238 kJ/mol) (1 mark)
Faster heterolysis, quicker AgI ppt (1 mark)

Question 8

Write the elimination equation for CH₃CH₂CH₂Br with ethanolic NaOH, heat. [2 Marks]

Answer 8

CH₃CH₂CH₂Br + NaOH → CH₃CH=CH₂ + NaBr + H₂O (ethanol, heat) (1 mark)
Elimination forms propene (1 mark)

Question 9

Predict the mechanism for (CH₃)₂CHCl with NH₃ in ethanol under pressure. [2 Marks]

Answer 9

Secondary, mixture of SN1/SN2 possible (1 mark)
Likely SN2, NH₃ → CH₃CH(NH₂)CH₃ (1 mark)

Question 10

In industry, how is CH₂=CH₂ converted to CH₃CH₂Cl? Include conditions. [2 Marks]

Answer 10

CH₂=CH₂ + HCl → CH₃CH₂Cl (1 mark)
Room temp, electrophilic addition (1 mark)

Question 11

Compare SN1 and SN2 for (CH₃)₃CBr vs. CH₃CH₂Br with NaOH(aq), heat. [3 Marks]

Answer 11

(CH₃)₃CBr: SN1, tertiary, stable carbocation (1 mark)
CH₃CH₂Br: SN2, primary, direct attack (1 mark)
Inductive effects stabilise tertiary C⁺ (1 mark)

Question 12

Classify CH₃CHClCH₃ and predict its substitution product with KCN. [2 Marks]

Answer 12

Secondary, CH₃CHClCH₃ (1 mark)
CH₃CHClCH₃ + KCN → CH₃CH(CN)CH₃ + KCl (1 mark)