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Answer the following… (Class 12 Chemistry SQP 2025-26, Q31B)

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ChemistryCBSE Class 12 · Sample paper, session 2025-26 · 5 marks · Long answer

CBSE Class 12 Chemistry sample paper 2025-26: question 31(B)

From the official CBSE sample question paper for 2025-26, 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

Answer the following questions:
I. Write the structure of expected product of Wolf-Kishner reduction of 2 –methylbutanal.
II. How would the presence of group effect the acidic strength of benzoic acid
III. Prepare acetic acid from ethanamine.
IV. Convert Aniline to benzoic acid.
V. Give a chemical test to distinguish between: propanal and ethanal.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Wolff–Kishner reduction turns C=O into CH₂.

Hint 2

Electron-withdrawing groups stabilise the carboxylate anion.

Hint 3

Only one of the two aldehydes has a CH₃–C=O group, so which test detects it?

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. (2-methylbutane); the >C=O becomes >CH.

    1 mark

  2. II. –SOH is electron-withdrawing. It disperses the negative charge of the benzoate ion and stabilises the conjugate base, which makes proton release easier. The acidic strength of benzoic acid increases.

    1 mark

  3. III. ; then

    1 mark

  4. IV.

    1 mark

  5. V. Iodoform test: warm each compound with I and NaOH. Ethanal (has a CHCO– group) gives a yellow precipitate of iodoform, CHI: CHCHO + 3I + 4NaOH → CHI↓ + HCOONa + 3NaI + 3HO. Propanal gives no yellow precipitate.

    1 mark

Answer: I. 2-methylbutane, CH₃CH₂CH(CH₃)CH₃. II. Acidity increases. III. Ethanamine → HNO₂ → ethanol → acidified KMnO₄ → acetic acid. IV. Diazotisation → CuCN → benzonitrile → hydrolysis → benzoic acid. V. Iodoform test: ethanal gives a yellow ppt, propanal does not.

Where the marks are

  • Correct Wolff–Kishner product, 2-methylbutane (1 mark)
  • Electron-withdrawing –SO₃H increases acidity (1 mark)
  • Correct ethanamine → acetic acid steps (1 mark)
  • Correct aniline → benzoic acid via diazonium and nitrile (1 mark)
  • Iodoform test with both observations (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 paperSample paper 2025-26: 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.