Distinguish aldehydes with Tollens' reagent
Warm a carbonyl sample with Tollens' reagent: aldehydes form a silver mirror while ketones do not.
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 Tollens' test look like?
What you will learn
- State the silver-mirror observation
- Interpret positive and negative results
- Combine Tollens' with 2,4-DNPH evidence
Aldehydes reduce silver(I)
On warming with Tollens' reagent, an aldehyde reduces silver(I) ions to metallic silver, which coats the clean glass as a silver mirror or dark silver deposit.
The aldehyde is oxidised to a carboxylate under alkaline test conditions. Ketones do not give the positive silver-mirror result in this syllabus test.
A negative result needs context
If 2,4-DNPH first confirms a carbonyl, a negative Tollens' result is consistent with a ketone. Without a positive carbonyl test, no silver mirror alone does not prove that the sample is a ketone.
Keep Fehling's and Tollens' observations distinct: brick-red Cu₂O versus deposited silver. Both exploit the easier oxidation of aldehydes relative to ketones.
Pause and explain
Which carbonyl class gives a positive Tollens' test in this syllabus?
Worked example
A carbonyl unknown gives a positive 2,4-DNPH test and no silver mirror on warming with Tollens' reagent. What class is supported?
Show the worked solution
- 2,4-DNPH supports an aldehyde or ketone.
- Aldehydes normally give a silver mirror.
- A negative result with confirmed carbonyl supports a ketone.
Answer A ketone is supported, because the carbonyl is present but Tollens' reagent is not reduced to silver.
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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.
Describe the result when propanal is warmed with Tollens' reagent.
Give me a hint
Name the metal deposited and its appearance.
Compare my reasoning
- Propanal is an aldehyde.
- It reduces silver(I) ions.
- Metallic silver coats the glass.
A silver mirror or silver deposit forms; propanal is oxidised to propanoate in alkaline solution.
Look for these in your work
- Aldehyde recognized
- Observation correct
- Oxidation linked
A sample gives a silver mirror but has not been tested with 2,4-DNPH. What can you infer, and what remains unknown?
Give me a hint
A positive Tollens' test indicates a reducing aldehyde under these conditions.
Compare my reasoning
- Aldehyde behavior is supported.
- The test does not identify the exact aldehyde molecule.
- Further evidence is needed for molecular identity.
The result supports an aldehyde, but it does not reveal the exact molecule or all other groups present.
Look for these in your work
- Class inference made
- Identity limitation stated
- Evidence need stated
A student observes a silver mirror in a tube containing AgNO₃ and calls it a Tollens' test. What should be checked?
Give me a hint
The reagent composition matters for identifying the named test.
Compare my reasoning
- Tollens' reagent is not simply aqueous silver nitrate.
- Use the specified ammoniacal silver(I) reagent.
- Distinguish it from silver nitrate tests for halide ions.
Confirm the reagent is Tollens' reagent; ethanolic/aqueous AgNO₃ is used for halide tests and is not the same test.
Look for these in your work
- Reagent identity checked
- Test purpose distinguished
- Observation not overinterpreted
Common mistakes
- Calling silver metal a silver halide
- Using the no-mirror result without first confirming a carbonyl
- Reporting a blue solution as the positive result
What can you explain now?
What does a positive Tollens' test look like?
Compare with the explanation
A silver mirror or silver deposit
Silver(I) is reduced to metallic silver by an aldehyde.
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Make it stick
What does Tollens' reagent detect in an unknown carbonyl compound, and what does it not identify?
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.