Halogenoalkanes can be hydrolysed when heated with aqueous sodium hydroxide solution. A student investigates the hydrolysis of 1-fluorobutane, 1-bromobutane and 1-iodobutane under the same conditions. Explain why the rate of hydrolysis differs between these three compounds, and predict which compound hydrolyses fastest.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Halogenoalkanes undergo hydrolysis when warmed with aqueous sodium hydroxide solution. The carbon–halogen bond must break for the reaction to proceed. Bond enthalpies for C–F, C–Cl, C–Br and C–I bonds decrease in that order down Group 7.
Model answer (5 marks)
The rate of hydrolysis depends on the strength of the C–X bond.
1. C–F bonds have the highest bond enthalpy, C–Cl next, C–Br then C–I.
2. As the bond enthalpy decreases down Group 7, the C–X bond becomes weaker.
3. A weaker C–X bond requires less energy to break, so the nucleophilic substitution proceeds more readily.
4. Consequently, 1‑iodobutane hydrolyses fastest.
5. 1‑fluorobutane hydrolyses slowest because its C–F bond is the strongest.
1. C–F bonds have the highest bond enthalpy, C–Cl next, C–Br then C–I.
2. As the bond enthalpy decreases down Group 7, the C–X bond becomes weaker.
3. A weaker C–X bond requires less energy to break, so the nucleophilic substitution proceeds more readily.
4. Consequently, 1‑iodobutane hydrolyses fastest.
5. 1‑fluorobutane hydrolyses slowest because its C–F bond is the strongest.
Examiner tips
- Use the command word ‘explain’ – describe the relationship between bond strength and rate, then predict the order.
- Show the bond enthalpy trend (F > Cl > Br > I) to justify the prediction.
- Mention that a weaker bond means easier SN2 attack by OH⁻.
- Keep the answer concise – 5 points, no extra wording.
Common mistakes
- Confusing bond enthalpy with bond length; students say ‘longer bond’ instead of ‘weaker bond’.
- Failing to state the order of hydrolysis (I > Br > Cl > F).
- Using vague terms like ‘more reactive’ without linking to bond strength.
Mark scheme (5 marks)
- The rate of hydrolysis depends on the strength (bond enthalpy) of the carbon–halogen bond
- Going down Group 7 from fluorine to iodine, the carbon–halogen bond becomes weaker / bond enthalpy decreases
- A weaker bond requires less energy to break, so the carbon–halogen bond breaks more easily
- Therefore 1-iodobutane hydrolyses fastest because it has the weakest C–halogen bond
- 1-fluorobutane hydrolyses slowest because it has the strongest C–halogen bond / C–F bond has the highest bond enthalpy
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Halogenoalkanes questions
- Halogenoalkanes can undergo hydrolysis reactions with aqueous silver nitrate sol…
- Halogenoalkanes are a family of organic compounds. Explain why chloroalkanes, br…
- Halogenoalkanes are used as intermediates in the synthesis of other organic comp…
- Halogenoalkanes are organic compounds in which one or more hydrogen atoms in an …