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Distinguishing Tests for Aldehydes, Ketones and Methyl Ketones

Distinguishing tests use characteristic observations to identify aldehydes, ketones, or specific carbonyl compounds such as methyl ketones.

In one line: Tollens tells aldehyde; iodoform identifies CH3CO pattern.

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Understand it

  • Aldehydes are readily oxidised, so they reduce Tollens' reagent to metallic silver and Fehling's solution to red cuprous oxide in suitable cases.
  • Ketones generally do not give these tests, except special cases in other reactions.
  • The iodoform test identifies methyl ketones and compounds oxidisable to methyl ketones.
  • Schiff's reagent gives a characteristic colour change with aldehydes.

Example

Ethanal gives Tollens' test and iodoform test. Propanone gives iodoform test but not Tollens' test.

Rule to remember

Tollens': aldehyde → carboxylate with silver mirror.

Fehling's: aliphatic aldehyde → red Cu2O precipitate.

Iodoform: CH3CO- group or suitable precursor → yellow CHI3 precipitate.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1To distinguish benzaldehyde and acetophenone, Tollens' reagent gives a silver mirror with benzaldehyde, while acetophenone does not.
  2. 2Iodoform test may be positive for acetophenone because it is a methyl ketone.

Method you can reuse

Identify whether the compound is aldehyde, ketone, methyl ketone, or aromatic aldehyde; choose the test with different outcomes; state reagent, observation, and reason.

Visual tip

Observation table is more useful than a diagram: compound type, reagent, positive observation, and limitation.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Which test can distinguish ethanal from propanone: Tollens' test or iodoform test?

Think first, then show answer
Tollens' test can distinguish them because ethanal gives a silver mirror, while propanone does not. Both can give iodoform test.

Common wrong answer

Using Fehling's solution to confirm benzaldehyde as if it always behaves like aliphatic aldehydes; aromatic aldehydes generally do not give Fehling's test.

Don't mix this up

Assuming every ketone gives iodoform test; only methyl ketones, or compounds that form methyl ketones under test conditions, give it.

Exam tip

Write both the reagent and the observation. Test-based answers are incomplete if the visible observation is missing.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked through compound pairs, observation-based short answers, reason questions, and conversion-test combinations.
  1. 1

    Enough for 1 mark

    Distinguishing tests use characteristic observations to identify aldehydes, ketones, or specific carbonyl compounds such as methyl ketones.

  2. 2

    Add for 2 marks

    Example: Ethanal gives Tollens' test and iodoform test. Propanone gives iodoform test but not Tollens' test.

  3. 3

    Add for 3 marks

    Aldehydes are readily oxidised, so they reduce Tollens' reagent to metallic silver and Fehling's solution to red cuprous oxide in suitable cases.

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