Detect a carbonyl with 2,4-DNPH
Use 2,4-dinitrophenylhydrazine to test for a carbonyl group in an aldehyde or ketone.
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 does a positive 2,4-DNPH test indicate?
What you will learn
- Recall the reagent name
- Describe the precipitate observation
- Limit the conclusion to carbonyl presence
2,4-DNPH gives a precipitate with carbonyls
2,4-dinitrophenylhydrazine reagent, often called 2,4-DNPH, reacts with aldehydes and ketones. A yellow or orange precipitate indicates a carbonyl compound is present.
The test confirms an aldehyde-or-ketone carbonyl, but it does not distinguish aldehydes from ketones.
Use a second test for identification
A positive 2,4-DNPH result narrows an unknown to a carbonyl-containing compound within this syllabus context.
Use Fehling's or Tollens' reagent, or ease of oxidation, to distinguish an aldehyde from a ketone. Do not infer a specific molecule from the precipitate alone.
Pause and explain
Can 2,4-DNPH alone distinguish an aldehyde from a ketone?
Worked example
An unknown forms an orange precipitate with 2,4-DNPH. What can you conclude, and what remains unresolved?
Show the worked solution
- The precipitate is a positive carbonyl test.
- Aldehydes and ketones both react.
- A second test is needed to distinguish them.
Answer The unknown contains an aldehyde or ketone carbonyl; 2,4-DNPH alone does not tell which one.
Try a topic test
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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.
State the reagent and positive observation for an unknown carbonyl compound.
Give me a hint
Name the full reagent and describe the solid.
Compare my reasoning
- Use 2,4-dinitrophenylhydrazine.
- A positive result gives a yellow or orange precipitate.
- Infer carbonyl presence, not a unique identity.
2,4-DNPH gives a yellow/orange precipitate, supporting an aldehyde or ketone carbonyl.
Look for these in your work
- Reagent named
- Observation stated
- Inference limited
An orange precipitate forms with 2,4-DNPH, then Tollens' reagent forms silver. What is the likely class?
Give me a hint
Combine the carbonyl test with the aldehyde/ketone distinction.
Compare my reasoning
- 2,4-DNPH supports a carbonyl.
- Tollens' silver mirror supports an aldehyde.
- Both results are consistent with an aldehyde.
An aldehyde is likely: 2,4-DNPH detects carbonyl and Tollens' positive result distinguishes an aldehyde.
Look for these in your work
- Tests combined
- Second result interpreted
- Conclusion stated
Why is '2,4-DNPH turned orange, therefore it is propanone' too strong?
Give me a hint
Separate functional-group evidence from exact molecular identification.
Compare my reasoning
- The colour indicates a carbonyl compound.
- Many aldehydes and ketones give the positive result.
- Additional tests or evidence are required to identify a specific molecule.
2,4-DNPH identifies a carbonyl class, not a unique molecule; more evidence is needed.
Look for these in your work
- Test scope stated
- Non-uniqueness explained
- Evidence need stated
Common mistakes
- Claiming a positive result proves the unknown is an aldehyde
- Calling the precipitate silver
- Using 2,4-DNPH as a reducing reagent
What can you explain now?
What does a positive 2,4-DNPH test indicate?
Compare with the explanation
A carbonyl compound is present
Aldehydes and ketones form a yellow or orange precipitate.
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Make it stick
What does a 2,4-DNPH precipitate show, and what does it not show?
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