Skip to content
MarkSchemeMarkScheme
Sign up — no card

9701 · 31.1

Halogen compounds — practice questions

Practice and worked examples for 9701 Halogen compounds. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

1-bromopropane is reacted separately with (a) warm aqueous sodium hydroxide and (b) hot ethanolic potassium hydroxide. For each reaction, state the type of reaction and draw the structure of the main organic product.

Show solution outline

Part (a): with warm aqueous sodium hydroxide

  • Reaction Type: Nucleophilic Substitution.
  • Explanation: The aqueous conditions favour substitution. The OH−OH^- ion acts as a nucleophile, attacking the δ+\delta+ carbon and displacing the Br−Br^- ion.
  • Organic Product: Propan-1-ol. Structure: CH3CH2CH2OHCH_3CH_2CH_2OH.

Part (b): with hot ethanolic potassium hydroxide

  • Reaction Type: Elimination.
  • Explanation: The hot, ethanolic conditions favour elimination. The OH−OH^- ion acts as a base, removing a proton from the carbon adjacent to the C-Br bond.
  • Organic Product: Propene. Structure: CH3CH=CH2CH_3CH=CH_2.

Worked example 2

Samples of 1-chlorobutane, 1-bromobutane, and 1-iodobutane are placed in separate test tubes. An equal volume of aqueous silver nitrate solution is added to each, and the tubes are placed in a water bath at 50°C. Describe and explain the expected observations.

Show solution outline

Observations:

  • 1-iodobutane: A yellow precipitate (of AgI) will form almost immediately.
  • 1-bromobutane: A cream precipitate (of AgBr) will form after a short time.
  • 1-chlorobutane: A white precipitate (of AgCl) will form very slowly, or perhaps not be visible during the experiment.

Explanation: The reaction is nucleophilic substitution, where water acts as the nucleophile, followed by precipitation of the resulting halide ion with Ag+(aq)Ag^+(aq). The rate of reaction depends on the strength of the carbon-halogen bond, which must be broken.

R−X+H2O→R−OH+H++X−R-X + H_2O \rightarrow R-OH + H^+ + X^- Ag+(aq)+X−(aq)→AgX(s)Ag^+(aq) + X^-(aq) \rightarrow AgX(s)

The bond enthalpies are in the order: C-Cl > C-Br > C-I. The C-I bond is the weakest and requires the least energy to break, so the reaction is fastest for 1-iodobutane. The C-Cl bond is the strongest, so the reaction is slowest for 1-chlorobutane. The rate of formation of the silver halide precipitate directly reflects the rate of hydrolysis of the C-X bond.

Mark a 9701 question on this topicFull lessonOpen in course studio