A industrial chemist is producing a batch of 1-bromopropane from propan-1-ol. Explain how the chemist could confirm that the product is a haloalkane and not an alcohol, and describe what is observed at each stage of the test.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propan-1-ol is reacted with a reagent to produce 1-bromopropane. Both compounds are colourless liquids at room temperature. The chemist needs to confirm the identity of the final product.
Model answer (5 marks)
Add a few drops of (acidified) silver nitrate solution to a small sample of the product dissolved in water.
If the product is 1‑bromopropane, a cream‑to‑pale yellow precipitate of AgBr will form.
If the product were propan‑1‑ol, no precipitate would appear because no halide ion is present.
The appearance of the AgBr precipitate therefore proves that the product contains a bromide ion and is a haloalkane, not an alcohol.
This confirms the identity of 1‑bromopropane.
If the product is 1‑bromopropane, a cream‑to‑pale yellow precipitate of AgBr will form.
If the product were propan‑1‑ol, no precipitate would appear because no halide ion is present.
The appearance of the AgBr precipitate therefore proves that the product contains a bromide ion and is a haloalkane, not an alcohol.
This confirms the identity of 1‑bromopropane.
Examiner tips
- Use the command word ‘explain’ to describe the test and the observation; include the colour of the precipitate.
- Show the reasoning that a precipitate indicates a halide, linking to the product’s identity.
Common mistakes
- Writing that the precipitate is white instead of cream‑to‑pale yellow.
- Forgetting to mention that no precipitate forms with the alcohol.
Mark scheme (5 marks)
- Add (acidified) silver nitrate solution to a sample of the product dissolved in water / add AgNO₃ solution
- A precipitate forms with the haloalkane (1-bromopropane) because haloalkanes react with silver nitrate to produce a silver halide precipitate
- The precipitate is cream / pale yellow in colour (confirming bromide ions / a bromoalkane)
- Propan-1-ol (the alcohol) would not produce a precipitate with silver nitrate because it does not contain a halide ion
- This shows the product contains a halide / is a haloalkane and is not an alcohol, confirming the identity of 1-bromopropane
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Alcohols, haloalkanes and analysis questions
- A student is given three unlabelled bottles, each containing a different colourl…
- A forensic scientist is analysing an unknown organic liquid recovered from a cri…
- Propan-1-ol and 1-chloropropane are two organic compounds. Explain how a chemist…
- Ethanol is an alcohol used in alcoholic drinks and as a fuel. When ethanol burns…