Carbonyl tests: aldehydes and ketones
Key idea: Use 2,4-DNPH to identify an aldehyde or ketone, then an appropriate oxidation test to distinguish them. Tollens' detects aldehydes; a negative Fehling's result can still fit an aromatic aldehyde.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Use the precipitate to find which functional groups fit.
- Compare Tollens’ and Fehling’s reactions, including aromatic aldehydes.
- Explain how the results select an ordinary ketone.
- Choose the class, then use the reliable selective tests.
- Correct the aldehyde claim and explain the extra test.
- Choose what DNPH shows and useful extra evidence.
- Keep the aromatic aldehyde despite its negative Fehling’s result.
- Choose extra evidence that separates these two compounds.
- Explain why ethanoic acid is outside the DNPH test class.
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ContinueA carbonyl group is a carbon–oxygen double bond, but a positive carbonyl test does not identify every molecule containing one. The supplied 2,4-DNPH test detects aldehydes and ketones; carboxylic acids, esters and amides also contain carbonyl bonds but do not give that usual positive result.
Use a broad class test to check the possibilities, then a selective test to distinguish them. Interpret supplied observations here; 9476 does not require students to carry out the 2,4-DNPH test in Paper 4.
Check the aldehyde-or-ketone class
An orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNPH, also called Brady’s reagent) is the observation given in the supplied notes. The precipitate is a solid reaction product; it does not mean that the original unknown is orange. Depending on the compound, the precipitate can be yellow to orange, but use the observation actually supplied.
Both aldehydes and ketones react. Repeating the same positive test adds no evidence that distinguishes them.
Use the appropriate oxidation test
| Test under specified conditions | Positive observation | Inference within ordinary aldehyde/ketone possibilities |
|---|---|---|
| Tollens’ reagent on warming | Silver mirror | Aldehyde supported |
| Fehling’s solution on warming | Red-brown copper(I) oxide precipitate | Aliphatic aldehyde supported |
An aldehyde is oxidised while silver(I) in Tollens’ reagent is reduced to silver metal. In Fehling’s solution, copper(II) is reduced to copper(I) oxide, Cu₂O(s). Ordinary ketones do not give these positive results.
Fehling’s is narrower than Tollens’. An aromatic aldehyde such as benzaldehyde can give a negative Fehling’s result while remaining an aldehyde. Do not turn “no red-brown precipitate” into “ketone” without checking the possibilities and other evidence.
A negative result is useful only when the correct conditions and a working reagent make the reaction observable. Where supplied, a positive comparison sample helps check that the reagent worked. In laboratory work, follow the supervised method: Tollens’ reagent is prepared fresh and must not be stored or allowed to dry; residues need the specified disposal procedure.
Guided practice 1
Tell a ketone and an aldehyde apart
Problem
Supplied observations show an orange precipitate with 2,4-DNPH, but negative Tollens’ and Fehling’s results. The possibilities are ordinary aldehydes or ketones, and the supplied comparison samples confirm that both reagents worked under the specified conditions. Identify the carbonyl type and justify the evidence chain.
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Hints
Hint 1
broad test
Use 2,4-DNPH to check the carbonyl class only.
Hint 2
selective tests
Ask which carbonyl type would give a silver mirror or red-brown precipitate.
Show solution step by step
Check the carbonyl
Method
State that the orange 2,4-DNPH precipitate supports an aldehyde or ketone.
Reason
The reagent is a broad carbonyl test and does not distinguish the two classes.
Working
Answer
Possibilities: aldehyde or ketone.Exclude the aldehyde
Method
Use both negative selective results to support a ketone.
Reason
An aldehyde would reduce Tollens’ reagent and, for an aliphatic aldehyde, Fehling’s solution under the stated tests.
Working
Answer
Inference: ketone.
Quick check
The negative Tollens’ result distinguishes these possibilities only if the specified conditions were used and the reagent was working. Fehling’s is not a universal aldehyde test: aromatic aldehydes such as benzaldehyde do not give its usual positive result.
Check your understanding 1 Check broad and selective carbonyl results
The ordinary aldehyde/ketone sample gives an orange DNPH precipitate and negative Tollens’ and Fehling’s tests with working reagents. Check the broad class and the final carbonyl type.
Show answer
Aldehyde or ketone; Ketone. With the aldehyde/ketone class checked and working reagents confirmed, the negative Tollens’ result supports a ketone within these possibilities.
Spot the mistake 2
Correct “2,4-DNPH identifies an aldehyde”
Learner inference
A learner sees an orange 2,4-DNPH precipitate and concludes that the unknown is an aldehyde. Correct the inference and name the minimum additional evidence needed.
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Show solution step by step
Limit the first inference
Method
State that the orange precipitate supports a carbonyl group in an aldehyde or ketone.
Reason
Both classes react positively with 2,4-DNPH.Working
Answer
One broad test leaves two possibilities.Add selective evidence
Method
Use a supplied Tollens’ silver-mirror result or the appropriate Fehling’s red-brown precipitate result.
Reason
A positive Tollens’ result supports an aldehyde within the stated carbonyl possibilities. A negative Fehling’s result alone leaves an aromatic aldehyde possible; interpret a negative test only under reliable specified conditions.
Working
Answer
Broad class test → selective discrimination.
Common mistake
A positive class test does not become specific because its observation is vivid; check which alternative classes share the result.
Check your understanding 2 Check what the DNPH result tells you
An orange DNPH precipitate is used to claim an aldehyde. Check what that result shows and which extra evidence can separate ordinary aldehydes and ketones.
Show answer
Aldehyde or ketone; Tollens’ or Fehling’s result. A positive Tollens’ or appropriate Fehling’s result supports an aldehyde.
A negative Fehling’s result need not mean ketone
Worked example 3
Keep the aromatic aldehyde possibility
Problem
The only possibilities are benzaldehyde and propanone. The supplied observations are an orange 2,4-DNPH precipitate, a silver mirror with Tollens’ reagent, and no red-brown precipitate with Fehling’s solution. Identify the compound and explain why the results do not conflict.
Show full solution
Use the broad result
Method
Retain both aldehyde and ketone possibilities after 2,4-DNPH.
Reason
Both possibilities contain the reactive carbonyl class.
Working
Answer
Both benzaldehyde and propanone fit the first result.
Use the selective positive result
Method
Identify benzaldehyde from the silver mirror.Reason
Benzaldehyde is an aldehyde; propanone is an ordinary ketone and does not reduce Tollens’ reagent under this test.
Working
Answer
The negative Fehling’s result is compatible with this aromatic aldehyde.
Check your understanding 3 Choose evidence that separates the possibilities
An unknown is either benzaldehyde or propanone. It gives an orange precipitate with 2,4-DNPH and a negative Fehling’s result. Can you choose one possibility? What additional supplied observation would distinguish them?
Show answer
Both remain possible. Both give the broad 2,4-DNPH result, and benzaldehyde need not give a positive Fehling’s result. A silver mirror with Tollens’ reagent supports benzaldehyde; a reliable negative Tollens’ result under the specified conditions supports propanone within these two possibilities. The set of possibilities and reagent conditions are part of the inference.
Check your understanding 4 Compare a carbonyl bond with the test class
A learner says, “Ethanoic acid contains a carbonyl bond, so it must give a positive 2,4-DNPH test.” What has the learner confused?
Show answer
A structural carbonyl bond is broader than the aldehyde/ketone class detected by this test. Ethanoic acid has a carboxylic acid group and does not give the usual positive 2,4-DNPH result. Match the functional-group class and specified reaction, rather than the bond alone.
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Syllabus and review details
- GCE A-Level H2 Chemistry 2027 · 9476-2027
9476 (2027), complete syllabus
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