ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2026-27: question 24

From the official CBSE sample question paper for 2026-27, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerAldehydes, Ketones and Carboxylic Acids

Identify each name reaction and complete the missing reactant/ product in the given reaction sequence. (3x1)

(I) $2 C H_{3} C H O ? C H_{3} C H (O H) C H_{2} C H O$

(II) Toluene $+_?_C S_{2}$ benzal compound $C_{6} H_{5} C H (O C r O H C l_{2})_{2}$ $H_{3} O^{+}$ benzaldehyde

(III) $> C = O N H_{2} N H_{2} - H_{2} O_?_K O H / ethylene glycol Heat > C H_{2} + N_{2}$

Figure in the paper: Part (II) shows toluene (CH3 on benzene ring), then an intermediate benzene ring bearing CH(OCrOHCl2)2, then benzaldehyde (benzene ring with CHO). See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

(I) Two aldehyde molecules join to give a β-hydroxy aldehyde.

Hint 2

(II) Which chromium reagent in CS₂ converts a methyl group on benzene to CHO?

Hint 3

(III) Hydrazine condenses with C=O, losing water. What product forms?

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.

(I) Aldol condensation (aldol addition). Missing reagent: dilute NaOH. Two molecules of ethanal give 3-hydroxybutanal.
1 mark
(II) Etard reaction. Missing reactant: **chromyl chloride, $C r O_{2} C l_{2}$ **. It forms the chromium complex, which is hydrolysed to benzaldehyde.
1 mark
(III) Wolff–Kishner reduction. Missing intermediate: the **hydrazone, $> C = N - N H_{2}$ **. Heating it with KOH in ethylene glycol gives $> C H_{2}$ and $N_{2}$ .
1 mark

Answer: (I) Aldol, dil. NaOH; (II) Etard, $C r O_{2} C l_{2}$ ; (III) Wolff–Kishner, $> C = N - N H_{2}$ (hydrazone)

Where the marks are

Aldol + dil NaOH (½ + ½ mark)
Etard + $C r O_{2} C l_{2}$ (½ + ½ mark)
Wolff–Kishner + hydrazone (½ + ½ mark)

Revise the chapter

Chapter: Aldehydes, Ketones and Carboxylic Acids (notes, formulas, where marks are lost and every past question)

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 paperSample paper 2026-27: every question, by section.

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CBSE Class 12 Chemistry sample paper 2026-27: question 24

Hints

Step-by-step solution

Revise the chapter

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