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(i) An organic compound… (Class 12 Chemistry 2026, Q31a)

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ChemistryCBSE Class 12 · Board exam 2026, set 56/1/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2026: question 31(a)

Set in the CBSE Class 12 board exam of 2026, paper 56/1/1, section E, question 31(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(a) · 5 marks · Long answerAldehydes, Ketones and Carboxylic Acids

(i) An organic compound (X) has the molecular formula . Draw structures for (X) if it :
(I) does not give Tollen's test but gives a positive iodoform test.
(II) does not give Tollen's test and iodoform test but undergoes Aldol condensation.
(III) undergoes Cannizzaro's reaction.
(ii) Show how each of the following compounds can be converted to benzoic acid :
(I) Acetophenone (II) Ethyl benzene

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Tollens' test identifies aldehydes, iodoform needs a CH₃CO– group, and aldol needs α-H.

Hint 2

Cannizzaro occurs only with aldehydes having no α-hydrogen.

Hint 3

Oxidation of any alkyl side chain on benzene gives –COOH.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i)(I) No Tollens' test means it is not an aldehyde; a positive iodoform test means it is a methyl ketone (). **X = ** (pentan-2-one); 3-methylbutan-2-one, , is also acceptable.

    1 mark

  2. (i)(II) It is a ketone that is not a methyl ketone but has α-hydrogens, so it can undergo aldol condensation. **X = ** (pentan-3-one).

    1 mark

  3. (i)(III) Cannizzaro reaction requires an aldehyde with no α-hydrogen. **X = ** (2,2-dimethylpropanal).

    1 mark

  4. (ii)(I) Acetophenone undergoes the haloform reaction:

    1 mark

  5. (ii)(II) The side chain of ethylbenzene is oxidised:

    1 mark

Answer: (i) (I) pentan-2-one; (II) pentan-3-one; (III) 2,2-dimethylpropanal. (ii) (I) NaOI then H₃O⁺; (II) alkaline KMnO₄, heat, then H₃O⁺.

Where the marks are

  • Methyl ketone structure, e.g. pentan-2-one (1 mark)
  • Pentan-3-one (1 mark)
  • 2,2-Dimethylpropanal (1 mark)
  • Acetophenone → benzoic acid via haloform/oxidation (1 mark)
  • Ethylbenzene → benzoic acid via alkaline KMnO₄ then acid (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Aldehydes, Ketones and Carboxylic Acids, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 56/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.