Distinguish aldehydes with Fehling's reagent
Warm the sample with Fehling's reagent: aldehydes give brick-red Cu₂O while ketones do not react.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Distinguish aldehydes from ketones by structure and evidence; state reagent, condition and observation. Written practice is self-checked
Before you begin
- Use carbonyl structures, alcohol oxidation and reduction from Topics 13 and 16, nucleophiles and curly arrows, and the organic reaction patterns from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What positive result does an aldehyde give with warm Fehling's reagent?
What you will learn
- State the Fehling's observation
- Interpret aldehyde and ketone outcomes
- Link aldehyde oxidation to carboxylate formation
Aldehydes reduce copper(II)
On warming with Fehling's reagent, an aldehyde reduces blue Cu²⁺ ions to brick-red copper(I) oxide, Cu₂O. The aldehyde is oxidised to a carboxylate under the alkaline test conditions.
Ketones do not give this positive result under the stated test. The visible brick-red precipitate distinguishes a reactive aldehyde from a ketone in the syllabus test.
State colour, solid and inference
A positive result is a brick-red precipitate, not a silver mirror. A negative result with the carbonyl already confirmed by 2,4-DNPH is consistent with a ketone.
Warm Fehling's reagent as directed. Do not claim a negative result alone proves a ketone if the sample may contain no carbonyl at all.
Pause and explain
What happens to a ketone in the stated Fehling's test?
Worked example
An unknown gives an orange precipitate with 2,4-DNPH and brick-red solid with warmed Fehling's reagent. Interpret the evidence.
Show the worked solution
- 2,4-DNPH indicates a carbonyl.
- The brick-red Cu₂O indicates Fehling's was reduced.
- An aldehyde is supported.
Answer The unknown is consistent with an aldehyde: it contains a carbonyl and gives the positive Fehling's result.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the Fehling's test result expected for ethanal.
Give me a hint
Name the observation and relate it to oxidation/reduction.
Compare my reasoning
- Warm ethanal with Fehling's reagent.
- Blue Cu²⁺ is reduced.
- A brick-red Cu₂O precipitate forms.
A brick-red Cu₂O precipitate forms; ethanal is oxidised to ethanoate under the alkaline conditions.
Look for these in your work
- Warming stated
- Precipitate correct
- Redox roles linked
A 2,4-DNPH-positive sample remains blue with Fehling's reagent after warming. What class is supported?
Give me a hint
Use the first test to confirm carbonyl before interpreting the second.
Compare my reasoning
- 2,4-DNPH supports an aldehyde or ketone.
- No Fehling's change is consistent with a ketone.
- State the inference as supported, not a unique structure.
A ketone is supported: the carbonyl is present but no positive Fehling's reduction occurs.
Look for these in your work
- Evidence combined
- Ketone interpretation correct
- Inference bounded
A student reports a silver mirror after Fehling's test. Correct the reagent-observation pairing.
Give me a hint
Recall the metal ion in each reagent.
Compare my reasoning
- Fehling's contains copper(II).
- Its positive solid is brick-red Cu₂O.
- Silver mirror is observed with Tollens' reagent.
Fehling's gives brick-red Cu₂O; Tollens' reagent gives the silver mirror.
Look for these in your work
- Reagent identified
- Solid corrected
- Other test distinguished
Common mistakes
- Calling the Fehling's product silver
- Expecting ketones to give brick-red Cu₂O
- Ignoring warming conditions
What can you explain now?
What positive result does an aldehyde give with warm Fehling's reagent?
Compare with the explanation
Brick-red Cu₂O precipitate
Cu²⁺ is reduced to copper(I) oxide.
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Make it stick
How does a positive Fehling's result distinguish an aldehyde from a ketone?
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