Use ease of oxidation to distinguish aldehydes and ketones
Combine carbonyl confirmation with oxidation and silver/copper tests to classify an unknown.
Section 17 · Carbonyl compounds
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
Why do aldehydes give positive Fehling's or Tollens' tests more readily than ketones?
What you will learn
- Explain the relative oxidation behavior
- Compare Fehling's and Tollens' tests
- Draw a bounded conclusion from results
Aldehydes oxidise more readily
Aldehydes are readily oxidised to carboxylic acids or carboxylates. Ketones resist oxidation under the mild syllabus test conditions.
Fehling's reagent gives brick-red Cu₂O with aldehydes; Tollens' reagent gives silver. Ketones do not give those positive results in the stated tests.
Use a carbonyl test before classifying
2,4-DNPH confirms a carbonyl group in an aldehyde or ketone. A positive Fehling's or Tollens' result then supports an aldehyde; a negative result after carbonyl confirmation supports a ketone.
The conclusion is only as strong as the test conditions and sample. Avoid using a negative aldehyde test to conclude 'ketone' if 2,4-DNPH did not establish a carbonyl first.
Pause and explain
What should a positive 2,4-DNPH result be followed by to distinguish aldehyde from ketone?
Worked example
An unknown gives an orange 2,4-DNPH precipitate, no Fehling's change and no Tollens' mirror. Interpret it.
Show the worked solution
- The precipitate confirms a carbonyl compound.
- Both aldehyde oxidation tests are negative.
- A ketone is supported under the stated test conditions.
Answer A ketone is supported: it gives a positive carbonyl test but resists the mild oxidation tests.
Try a topic test
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Test this topic →From guided practice to a new situation
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.
An unknown gives 2,4-DNPH positive and Fehling's positive. State the conclusion and evidence.
Give me a hint
Combine carbonyl presence with aldehyde reactivity.
Compare my reasoning
- 2,4-DNPH supports a carbonyl.
- Brick-red Cu₂O indicates Fehling's was reduced.
- This is consistent with an aldehyde.
An aldehyde is supported: carbonyl present and positive Fehling's oxidation result.
Look for these in your work
- Both tests interpreted
- Aldehyde supported
- Evidence connected
An unknown gives no 2,4-DNPH precipitate and no Tollens' mirror. Can you call it a ketone?
Give me a hint
Check whether the carbonyl group was confirmed first.
Compare my reasoning
- No 2,4-DNPH result gives no evidence for a carbonyl.
- The negative Tollens' result alone is not enough.
- A ketone conclusion is not justified from these results.
No. Confirm an aldehyde/ketone carbonyl with 2,4-DNPH before using negative Tollens' behavior to support a ketone.
Look for these in your work
- Missing premise recognized
- Negative evidence bounded
- Conclusion withheld
Explain the link between aldehyde oxidation and the silver mirror test.
Give me a hint
Identify which species is oxidised and which is reduced.
Compare my reasoning
- The aldehyde is oxidised to carboxylate.
- Silver(I) ions gain electrons and become Ag(s).
- Metallic silver deposits on the glass.
Aldehyde oxidation supplies electrons that reduce Ag⁺ to silver metal, forming the mirror.
Look for these in your work
- Oxidation identified
- Reduction identified
- Observation explained
Common mistakes
- Treating 2,4-DNPH as an aldehyde/ketone distinction
- Calling all negative results proof of ketone
- Confusing oxidation with reduction
What can you explain now?
Why do aldehydes give positive Fehling's or Tollens' tests more readily than ketones?
Compare with the explanation
Aldehydes oxidise more readily
They are oxidised to carboxylate while reducing Cu²⁺ or Ag⁺.
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Make it stick
How do 2,4-DNPH and an oxidation test work together to classify a carbonyl unknown?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.