Identify an unknown aldehyde or ketone from tests
Combine 2,4-DNPH, Fehling's, Tollens' and iodoform evidence without overclaiming molecular identity.
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
Which test first confirms an aldehyde-or-ketone carbonyl?
What you will learn
- Confirm a carbonyl first
- Use oxidation tests to classify aldehyde or ketone
- Use iodoform to identify a structural subgroup
Follow an evidence sequence
First use 2,4-DNPH: yellow/orange precipitate supports an aldehyde or ketone carbonyl. Then warm with Fehling's or Tollens' reagent to test ease of oxidation.
A positive Fehling's result gives brick-red Cu₂O; a positive Tollens' result gives a silver mirror. Either positive result supports an aldehyde. With a confirmed carbonyl, negative results in both tests support a ketone.
Use iodoform for a separate structural clue
Alkaline I₂(aq) gives yellow CHI₃ if the molecule contains CH₃CO–R. The accompanying carboxylate is RCO₂⁻.
The tests answer different questions: carbonyl presence, aldehyde versus ketone behavior, and presence of a methyl carbonyl group. They do not alone identify every substituent or exact molecule.
Pause and explain
What does a positive iodoform test add to carbonyl identification?
Worked example
An unknown gives orange 2,4-DNPH precipitate, no Fehling's change, no Tollens' mirror, and yellow CHI₃ with alkaline iodine. What can be concluded?
Show the worked solution
- 2,4-DNPH supports an aldehyde or ketone.
- Negative oxidation tests support a ketone.
- Yellow CHI₃ supports a CH₃CO–R group.
Answer The unknown is consistent with a methyl ketone: a carbonyl is present, oxidation tests are negative, and CH₃CO–R is detected. The exact molecule is not identified.
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.
A sample gives positive 2,4-DNPH and Tollens', but negative iodoform. State the supported class and one limitation.
Give me a hint
Combine carbonyl and oxidation evidence.
Compare my reasoning
- 2,4-DNPH supports a carbonyl.
- Tollens' positive supports an aldehyde.
- Negative iodoform means the diagnostic methyl carbonyl group was not detected, not that no carbonyl exists.
An aldehyde is supported; the negative iodoform result indicates no detected CH₃CO–R group, but does not identify the full structure.
Look for these in your work
- Class inferred
- Iodoform interpreted
- Limitation stated
A sample gives positive 2,4-DNPH and Fehling's, plus yellow CHI₃. What two structural conclusions are supported?
Give me a hint
Use the oxidation and subgroup tests independently.
Compare my reasoning
- Carbonyl is present.
- Positive Fehling's supports an aldehyde.
- Iodoform supports CH₃CO–R; ethanal is an example that can show both.
An aldehyde carbonyl is supported, and the CH₃CO–R group is present; exact identity is not fixed.
Look for these in your work
- Carbonyl confirmed
- Aldehyde inferred
- Group inference bounded
Why is it useful to perform 2,4-DNPH before interpreting a negative Fehling's test?
Give me a hint
A negative oxidation result only distinguishes known carbonyl candidates.
Compare my reasoning
- A sample without a carbonyl also gives no aldehyde oxidation test.
- 2,4-DNPH establishes an aldehyde/ketone carbonyl first.
- Then negative Fehling's and Tollens' support a ketone.
The initial carbonyl test prevents mistaking a non-carbonyl compound for a ketone based only on negative oxidation tests.
Look for these in your work
- Alternative explained
- Test order justified
- Conclusion bounded
Common mistakes
- Using iodoform as the aldehyde/ketone distinction
- Calling 2,4-DNPH a ketone-only test
- Naming a unique molecule from a few qualitative tests
What can you explain now?
Which test first confirms an aldehyde-or-ketone carbonyl?
Compare with the explanation
2,4-DNPH
A yellow/orange precipitate indicates a carbonyl compound.
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Make it stick
What question does each of 2,4-DNPH, Fehling's/Tollens' and alkaline iodine answer?
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.