Halogenoalkanes can undergo hydrolysis reactions with aqueous silver nitrate solution. When 1-chloropropane, 1-bromopropane, and 1-iodopropane are each separately added to aqueous silver nitrate solution and warmed, different coloured precipitates form at different rates. Explain the differences in the rate of precipitate formation across these three halogenoalkanes, and name the colour of the precipitate formed by 1-iodopropane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Halogenoalkanes are organic molecules in which one or more hydrogen atoms have been replaced by a halogen atom. The carbon–halogen bond is polar and can be broken during hydrolysis. The strength of the carbon–halogen bond decreases going down Group 7: C–Cl > C–Br > C–I in terms of bond strength.
Model answer (5 marks)
The hydrolysis of a halogenoalkane requires breaking the C–X bond to release the halide ion.
1. The C–I bond is the weakest of the three (C–Cl > C–Br > C–I), so the C–I bond is most readily cleaved.
2. 1‑Iodopropane therefore reacts fastest, giving a rapid formation of the precipitate.
3. The C–Br bond is intermediate in strength; 1‑bromopropane reacts at a moderate rate.
4. The C–Cl bond is the strongest; 1‑chloropropane reacts slowest, giving the slowest precipitate.
5. The precipitate from 1‑iodopropane is yellow, as it is silver iodide (AgI).
1. The C–I bond is the weakest of the three (C–Cl > C–Br > C–I), so the C–I bond is most readily cleaved.
2. 1‑Iodopropane therefore reacts fastest, giving a rapid formation of the precipitate.
3. The C–Br bond is intermediate in strength; 1‑bromopropane reacts at a moderate rate.
4. The C–Cl bond is the strongest; 1‑chloropropane reacts slowest, giving the slowest precipitate.
5. The precipitate from 1‑iodopropane is yellow, as it is silver iodide (AgI).
Examiner tips
- Use the bond‑strength trend to explain rate differences; link weakest bond to fastest reaction.
- State the colour of the AgI precipitate explicitly.
- Show the order of reactivity (I > Br > Cl).
Common mistakes
- Confusing the order of bond strengths; writing C–Cl > C–Br > C–I.
- Failing to mention the colour of the AgI precipitate.
- Not linking bond strength to rate of hydrolysis.
Mark scheme (5 marks)
- The carbon–halogen bond must be broken during hydrolysis
- The C–I bond is the weakest (or C–Cl bond is the strongest) of the three
- 1-iodopropane reacts fastest because it has the weakest C–X bond, so it is most easily broken
- 1-chloropropane reacts slowest because the C–Cl bond is the strongest and requires the most energy to break
- The precipitate formed by 1-iodopropane is yellow (silver iodide)
Key terms in this question
halogenoalkane · hydrolysis · precipitate · silver nitrate
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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