Chloroethane can undergo a substitution reaction with aqueous sodium hydroxide solution to form ethanol. Explain what happens during this reaction, including why chloroethane is reactive and what type of bond breaking occurs.

Eduqas A-Level Chemistry — 2.6 Halogenoalkanes · Explain · 5 marks · View as Markdown

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

Chloroethane (CH₃CH₂Cl) reacts with aqueous sodium hydroxide solution. The product is ethanol (CH₃CH₂OH).

Model answer (5 marks)

The C–Cl bond in chloroethane is polar because chlorine is more electronegative than carbon, giving the carbon a partial positive charge (δ⁺). This makes the carbon susceptible to nucleophilic attack. In aqueous NaOH the hydroxide ion (OH⁻) acts as a nucleophile and attacks the δ⁺ carbon. The C–Cl bond then breaks heterolytically, with both bonding electrons moving to chlorine. Chlorine leaves as a chloride ion (Cl⁻) and the OH group attaches to the carbon, giving ethanol.

Examiner tips

  • Use the word ‘nucleophile’ and ‘heterolytic cleavage’ – these are key terms the scheme rewards.
  • Show the polarity of the C–Cl bond and the δ⁺ on carbon to justify reactivity.

Common mistakes

  • Confusing SN1 with SN2 – the reaction is SN2 and should be stated as such.
  • Omitting the heterolytic nature of the bond breaking or the role of the hydroxide ion as nucleophile.

Mark scheme (5 marks)

  1. The C–Cl bond is polar (the chlorine atom is more electronegative than carbon)
  2. The carbon bonded to chlorine carries a partial positive charge (δ+), making it susceptible to attack
  3. The hydroxide ion (OH⁻) acts as a nucleophile
  4. The C–Cl bond breaks heterolytically (both electrons go to the chlorine)
  5. The chlorine leaves as a chloride ion (Cl⁻) and is replaced by the OH group, forming ethanol

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