A student tests two unknown organic compounds, Compound A and Compound B, using Fehling's solution. Compound A produces a brick-red precipitate when warmed with Fehling's solution. Compound B produces no colour change when warmed with Fehling's solution. Both compounds contain only carbon, hydrogen, and oxygen, and both contain a carbonyl group. Describe what these results tell the student about the identity of each compound, and explain why Fehling's solution gives different results with the two compounds.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Compound A is an aldehyde, because it gives a brick‑red precipitate when heated with Fehling’s solution, showing that it reduces Cu²⁺ to Cu⁺.
Compound B is a ketone, because it shows no colour change or precipitate, indicating it does not reduce Cu²⁺.
Aldehydes are reducing agents; the –CHO group is oxidised to –COOH, reducing Cu²⁺ to Cu⁺.
Ketones lack a hydrogen on the carbonyl carbon; they cannot be oxidised under these conditions and therefore do not reduce Cu²⁺.
The brick‑red precipitate is copper(I) oxide (Cu₂O), formed when Cu²⁺ ions are reduced to Cu⁺ by the aldehyde.
Compound B is a ketone, because it shows no colour change or precipitate, indicating it does not reduce Cu²⁺.
Aldehydes are reducing agents; the –CHO group is oxidised to –COOH, reducing Cu²⁺ to Cu⁺.
Ketones lack a hydrogen on the carbonyl carbon; they cannot be oxidised under these conditions and therefore do not reduce Cu²⁺.
The brick‑red precipitate is copper(I) oxide (Cu₂O), formed when Cu²⁺ ions are reduced to Cu⁺ by the aldehyde.
Examiner tips
- State the identity of each compound first. Explain the red precipitate as Cu₂O. Mention that aldehydes are reducing, ketones are not.
Common mistakes
- Confusing the red precipitate with copper metal. Saying both compounds are aldehydes. Forgetting that the red precipitate is Cu₂O, not Cu⁰.
Mark scheme (5 marks)
- Compound A is an aldehyde
- Compound B is a ketone
- Aldehydes can be oxidised (are reducing agents) / aldehydes reduce Fehling's solution
- Ketones cannot be oxidised (are not reducing agents) / ketones do not reduce Fehling's solution
- The brick-red precipitate is copper(I) oxide, formed when copper(II) ions in Fehling's solution are reduced
Key terms in this question
carbonyl group · Fehling's solution
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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