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(I) Arrange the… (Class 12 Chemistry SQP 2026-27, Q31B)

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ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 5 marks · Long answer

CBSE Class 12 Chemistry sample paper 2026-27: question 31(B)

From the official CBSE sample question paper for 2026-27, section E, question 31(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

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

(I) Arrange the following according to the property indicated (3x1)
a) , HCHO, (decreasing order of the ease of oxidation)
b) , , , (increasing order of nucleophilic acyl substitution)
c) , , (increasing order of electrophilicity of carbonyl carbon)

(II) Write any two possible isomers of the compound having molecular formula . Identify which of these isomer/s will give a positive Tollen’s test and explain the reason. (2)

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

Electron-donating groups make the carbonyl carbon less electrophilic; electron-withdrawing groups make it more so.

Hint 2

For acyl substitution, compare how good a leaving group each of Cl, OCH₃, OH and NH₂ is.

Hint 3

Tollens' reagent is a mild oxidant. Which carbonyl compounds have an oxidisable C–H on the carbonyl carbon?

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)(a) HCHO > CH₃CHO > C₆H₅CHO. The +I effect of the methyl group and the resonance of the phenyl ring progressively reduce the ease of oxidation.

    1 mark

  2. (I)(b) CH₃CONH₂ < CH₃COOH < CH₃COOCH₃ < CH₃COCl. Reactivity rises as the leaving group gets better and as its resonance donation to C=O gets weaker. is the best leaving group; donates strongly by resonance and is the poorest leaving group.

    1 mark

  3. (I)(c) CH₃COCH₃ < CH₃CHO < CF₃CHO. Two electron-donating methyls lower the electrophilicity, one methyl lowers it less, and the strongly –I group raises it the most.

    1 mark

  4. (II) Two isomers of : propanal, , and propanone, .

    1 mark

  5. Propanal gives a positive Tollens' test (silver mirror). Its aldehydic C–H allows it to be oxidised to propanoate, reducing to metallic Ag. Propanone, a ketone, has no such hydrogen and is not oxidised by this mild reagent.

    1 mark

Answer: (I)(a) HCHO > CH₃CHO > C₆H₅CHO; (b) CH₃CONH₂ < CH₃COOH < CH₃COOCH₃ < CH₃COCl; (c) CH₃COCH₃ < CH₃CHO < CF₃CHO; (II) propanal and propanone; propanal gives Tollens' test.

Where the marks are

  • Correct order (a) (1 mark)
  • Correct order (b) (1 mark)
  • Correct order (c) (1 mark)
  • Two correct isomers (½ + ½ mark)
  • Propanal identified with reason (oxidation of aldehyde, Ag⁺ reduced to Ag) (½ + ½ 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 paperSample paper 2026-27: 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.