Benzaldehyde is an aromatic carbonyl compound used in food flavouring. It contains a benzene ring attached to an aldehyde functional group. Explain how you could distinguish between benzaldehyde and benzene using chemical tests, and describe what you would observe in each case.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Benzaldehyde is an aromatic compound with the molecular formula C₇H₆O. It has a benzene ring bonded directly to a –CHO (aldehyde) group. Benzene itself contains only carbon and hydrogen and has no carbonyl functional group.
Model answer (5 marks)
1. Use Fehling’s solution (or Tollens’ reagent) to test for an aldehyde.
2. Add benzaldehyde to Fehling’s solution – the aldehyde is oxidised to benzoic acid and Cu⁺ is reduced to Cu₂O, giving a bright brick‑red/orange precipitate.
3. Add benzene to Fehling’s solution – no oxidation occurs, so the solution remains colourless and no precipitate forms.
4. The positive result for benzaldehyde shows the presence of the –CHO group; the negative result for benzene confirms the absence of any oxidisable functional group.
2. Add benzaldehyde to Fehling’s solution – the aldehyde is oxidised to benzoic acid and Cu⁺ is reduced to Cu₂O, giving a bright brick‑red/orange precipitate.
3. Add benzene to Fehling’s solution – no oxidation occurs, so the solution remains colourless and no precipitate forms.
4. The positive result for benzaldehyde shows the presence of the –CHO group; the negative result for benzene confirms the absence of any oxidisable functional group.
Examiner tips
- Mention the specific reagent (Fehling’s or Tollens’) and the expected visual change for each compound.
- Show the chemical change (oxidation of –CHO to –COOH and reduction of Cu²⁺ to Cu₂O).
- Explain why benzene does not react – lack of a functional group that can be oxidised.
Common mistakes
- Confusing the colour of the precipitate (red vs orange).
- Saying benzene gives a colour change – it does not.
- Not specifying that the aldehyde is oxidised to a carboxylic acid.
Mark scheme (5 marks)
- Fehling's solution (or Tollens' reagent) can be used to test for the aldehyde group
- Benzaldehyde gives a positive result with Fehling's solution — a brick-red/orange precipitate forms (or silver mirror with Tollens')
- Benzene gives no reaction / no colour change with Fehling's solution (or Tollens' reagent) because it has no aldehyde functional group
- The aldehyde group (–CHO) in benzaldehyde is oxidised during the test
- Benzene does not react because it lacks a functional group that can be oxidised under these conditions / it is relatively unreactive
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 →
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