A student is given an unknown organic compound and is told it is either propan-1-ol or propanal. Describe how the student could use chemical tests to identify which compound is present. Include the reagents used, the observations for each test, and what each result tells the student.

Eduqas A-Level Chemistry — 4.8 Organic synthesis and analysis · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

1. Add a few drops of acidified potassium dichromate(VI) to a small sample of the unknown and warm gently.
Observation: orange solution turns green.
Interpretation: both propan‑1‑ol and propanal are oxidised, so this test alone is not definitive.

2. Treat the unknown with Tollens’ reagent.
Observation with propanal: a silver mirror forms on the inner wall of the test tube.
Observation with propan‑1‑ol: no silver mirror appears.
Interpretation: a positive Tollens’ test indicates an aldehyde.

3. Perform Fehling’s test.
Observation with propanal: a brick‑red precipitate of Cu₂O appears.
Observation with propan‑1‑ol: the blue solution remains unchanged.
Interpretation: a positive Fehling’s test confirms an aldehyde.

4. To confirm the alcohol, add a few drops of sodium metal to the unknown.
Observation with propan‑1‑ol: vigorous bubbling of hydrogen gas.
Observation with propanal: no bubbling.
Interpretation: hydrogen evolution shows the presence of an –OH group.

5. Alternatively, esterify the unknown with ethanoic acid and an acid catalyst.
Observation with propan‑1‑ol: a clear ester (ethyl propanoate) forms.
Observation with propanal: no ester is formed.
Interpretation: ester formation confirms the alcohol.

Conclusion: Positive Tollens’/Fehling’s and negative sodium‑metal test identify the compound as propanal; positive sodium‑metal or esterification confirms propan‑1‑ol.

Examiner tips

  • Use the order of tests to avoid false positives – start with a non‑specific oxidising test, then a specific aldehyde test, and finish with a confirmatory alcohol test.
  • Write the observation exactly (e.g. ‘silver mirror’, ‘brick‑red precipitate’, ‘no change’).
  • Explain what each observation tells you – link the result to the functional group.
  • Mention an additional confirmatory test to show you can distinguish the two compounds conclusively.

Common mistakes

  • Confusing the colour change of potassium dichromate with the aldehyde test; it is not definitive for aldehydes.
  • Failing to note that the sodium‑metal test is negative for aldehydes and positive for alcohols.
  • Omitting the observation of the Fehling’s test (brick‑red precipitate) or mis‑labeling it as a colour change only.

Mark scheme (5 marks)

  1. Add acidified potassium dichromate(VI) (and warm) / use Tollens' reagent / use Fehling's solution as a test reagent
  2. State the observation with the aldehyde (propanal): e.g. acidified potassium dichromate turns from orange to green / Tollens' gives a silver mirror / Fehling's gives a brick-red precipitate
  3. State the observation with the alcohol (propan-1-ol): acidified potassium dichromate still turns from orange to green (oxidised to an acid/aldehyde) OR Tollens' reagent gives no silver mirror / Fehling's solution stays blue
  4. Correct interpretation: observation shows the compound is propanal (an aldehyde) because it is oxidised by Tollens'/Fehling's whereas propan-1-ol is not / aldehyde gives positive result with Tollens' or Fehling's that alcohol does not
  5. Suggest a confirmatory or additional test to distinguish them — e.g. sodium metal / sodium carbonate: propan-1-ol produces bubbles of hydrogen (alcohols react with sodium); OR use of ester formation with ethanoic acid and acid catalyst confirming the presence of an –OH group

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