Halogenoalkanes are a family of organic compounds. Explain why chloroalkanes, bromoalkanes and iodoalkanes show different rates of hydrolysis when reacted with warm aqueous sodium hydroxide solution, and describe the trend in reactivity across this series.

Edexcel A-Level Chemistry (9CH0) — 6.4 Halogenoalkanes · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Chloroalkanes, bromoalkanes and iodoalkanes all undergo nucleophilic substitution reactions when warmed with aqueous sodium hydroxide solution. The rate at which each type of halogenoalkane reacts differs due to the nature of the carbon–halogen bond.

Model answer (5 marks)

The hydrolysis of a halogenoalkane requires cleavage of the C–X bond.
1. The C–X bond must be broken during the reaction.
2. Bond strength decreases from C–Cl to C–Br to C–I.
3. A weaker C–X bond is broken more easily, so the nucleophilic substitution is faster.
4. Consequently, iodoalkanes hydrolyse fastest, bromoalkanes intermediate, and chloroalkanes slowest.
5. The nucleophile (OH⁻) replaces the halogen, releasing the halide ion (Cl⁻, Br⁻ or I⁻) as a product.

Examiner tips

  • Use the word ‘break’ for bond cleavage, ‘weaker’ for bond strength, and state the trend explicitly (I > Br > Cl).
  • Mention the nucleophile and product to show full understanding of the mechanism.

Mark scheme (5 marks)

  1. The carbon–halogen bond must be broken during hydrolysis
  2. Bond strength decreases going from C–Cl to C–Br to C–I
  3. A weaker bond is broken more easily, so the reaction is faster
  4. Iodoalkanes hydrolyse fastest and chloroalkanes hydrolyse slowest
  5. The halide ion (Cl⁻, Br⁻ or I⁻) is released as a product of hydrolysis / the OH⁻ acts as the nucleophile replacing the halogen

Key terms in this question

halogenoalkane · hydrolysis

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