Halogenoalkanes are used as intermediates in the synthesis of other organic compounds. A chemist reacts three different halogenoalkanes — 1-fluorobutane, 1-chlorobutane, and 1-iodobutane — with aqueous sodium hydroxide solution under the same conditions. The rate at which each halogenoalkane undergoes nucleophilic substitution differs. Explain why the rate of reaction differs across the three halogenoalkanes, and state which one reacts the fastest.

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).

Halogenoalkanes undergo nucleophilic substitution reactions in which the halogen atom is replaced by another group. The strength of the carbon–halogen bond determines how readily this substitution occurs. Bond enthalpies: C–F = 485 kJ/mol, C–Cl = 339 kJ/mol, C–I = 218 kJ/mol.

Model answer (5 marks)

1‑iodobutane reacts the fastest because the C–I bond has the lowest bond enthalpy (218 kJ mol⁻¹) and is the weakest bond. Less energy is required to break the C–I bond, so the nucleophilic substitution proceeds more readily and at a greater rate. 1‑fluorobutane reacts the slowest because the C–F bond has the highest bond enthalpy (485 kJ mol⁻¹) and is the strongest bond. The rate therefore increases in the order 1‑fluorobutane < 1‑chlorobutane < 1‑iodobutane, following the decreasing bond enthalpy (C–F > C–Cl > C–I).

Examiner tips

  • Use the bond enthalpy trend to justify the rate order; mention the weakest bond first.
  • State the fastest reacting halogenoalkane explicitly.
  • Explain that lower bond enthalpy means less energy to break the bond, leading to a higher rate.

Common mistakes

  • Confusing bond enthalpy with bond length; stating that longer bonds are weaker without citing enthalpy.
  • Failing to identify 1‑iodobutane as the fastest reacting compound.
  • Using vague terms like ‘more reactive’ without linking to bond enthalpy.

Mark scheme (5 marks)

  1. 1-iodobutane reacts the fastest
  2. The C–I bond has the lowest bond enthalpy / is the weakest bond
  3. Less energy is required to break the C–I bond, so the substitution reaction occurs more readily / at a greater rate
  4. 1-fluorobutane reacts the slowest because the C–F bond has the highest bond enthalpy / is the strongest bond
  5. The rate increases going from fluorine to chlorine to iodine because bond enthalpy decreases in that order (C–F > C–Cl > C–I)

Key terms in this question

halogenoalkane · nucleophilic substitution

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