Learn in your language
Skip to content
A Level · Cambridge (CIE) · 9701

Detect a carbonyl with 2,4-DNPH

Use 2,4-dinitrophenylhydrazine to test for a carbonyl group in an aldehyde or ketone.

Section 17 · Carbonyl compounds

Carbonyl compounds · Cambridge 9701 AS Chemistry · 17.1

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
Each test answers a different question2,4-DNPHYellow/orange precipitateDetects: aldehyde or ketoneFehling’sBrick-red Cu₂O precipitateDetects: aldehyde positiveTollens’Silver mirrorDetects: aldehyde positiveAlkaline iodineYellow CHI₃ precipitateDetects: CH₃CO– groupA negative subgroup test does not identify every structure
Carbonyl identification uses tests that answer distinct questions. 2,4-DNPH gives a yellow/orange precipitate with aldehydes and ketones. Fehling's solution gives a brick-red copper(I) oxide precipitate with aldehydes; Tollens' reagent gives a silver mirror. Alkaline iodine gives yellow CHI₃ with a CH₃CO– group. Negative results need cautious interpretation.

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?

Put the idea to work

Worked example

An unknown forms an orange precipitate with 2,4-DNPH. What can you conclude, and what remains unresolved?

Show the worked solution
  1. The precipitate is a positive carbonyl test.
  2. Aldehydes and ketones both react.
  3. 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

Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

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.

1 · Guided

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
  1. Use 2,4-dinitrophenylhydrazine.
  2. A positive result gives a yellow or orange precipitate.
  3. 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
2 · Independent

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
  1. 2,4-DNPH supports a carbonyl.
  2. Tollens' silver mirror supports an aldehyde.
  3. 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
3 · Transfer

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
  1. The colour indicates a carbonyl compound.
  2. Many aldehydes and ketones give the positive result.
  3. 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
Recall without your notes

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.

After trying it yourself, choose your next review. This is your self-assessment.

Your review choice appears on Today. Sign in to sync it across devices.

Continue to next lesson Back to Today

Make it stick

What does a 2,4-DNPH precipitate show, and what does it not show?

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.

Keep learning Chemistry

Check the official syllabus 2028–2030
Course scope and assessment

Four hundred and forty-two original lessons map the published learning outcomes across AS Chemistry Topics 1–22 and A Level Topics 23–37, with guided, independent and transfer practice and explained answers. Every numbered subject-content topic and listed outcome tag now has a mapped lesson sequence. Topic 32 maps all 12 hydroxy-compound outcome tags in 11 lessons; Topic 33 maps all 15 carboxylic-acid and derivative outcome tags in 16 lessons; Topic 34 maps all 22 nitrogen-compound outcome tags in 20 lessons; Topic 35 maps all 12 polymerisation outcome tags in 16 lessons; Topic 36 maps all four organic-synthesis outcome tags in 14 lessons; Topic 37 maps all 22 chromatography and NMR outcome tags in 18 lessons. Cambridge's 2028–2030 Version 1 syllabus states that there are no significant changes which affect teaching from the preceding edition; the routes are aligned to that edition. This is 37 of the 37 numbered subject-content topics. Full cumulative exam assessment, Paper 3 laboratory competence, Paper 5 investigation practice and human subject-teacher review remain outstanding. Written practice is self-checked and does not certify exam readiness.

StudyForA · Learn with understanding

Original teaching content and links to official sources. Check your exam year and qualification before using a resource.