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(i) Arrange the following… (Class 12 Chemistry 2026, Q28a)

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ChemistryCBSE Class 12 · Board exam 2026, set 56/5/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2026: question 28(a)

Set in the CBSE Class 12 board exam of 2026, paper 56/5/1, section C, question 28(a), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28(a) · 3 marks · Short answerAldehydes, Ketones and Carboxylic Acids

(i) Arrange the following compounds in increasing order of their acidic strengths :
, ,
(ii) Why is more reactive than acetone towards reaction with HCN ?
(iii) Complete the equation :

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/5/1.

Hints

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

Hint 1

Electron-withdrawing groups stabilise the carboxylate ion, and the effect falls off with distance; electron-donating groups do the opposite.

Hint 2

Compare the number of alkyl groups around the C=O carbon. For (iii), the –NH₂ of hydrazine condenses with C=O, losing water.

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) The –Br group has a –I effect that increases acidity, and it is stronger the closer Br is to –COOH. The –CH₃ group has a +I effect that decreases acidity. Increasing order of acidic strength:

    1 mark

  2. (ii) Acetone has two methyl groups, so its carbonyl carbon is more crowded (steric hindrance) and less electrophilic (+I effect of two CH₃ groups). In ethanal, which has only one CH₃, the nucleophile attacks the carbonyl carbon more easily. Hence is more reactive.

    1 mark

  3. (iii) Ethanal forms its hydrazone:

    1 mark

Answer: (i) CH₃CH(CH₃)COOH < CH₃CH(Br)CH₂COOH < CH₃CH₂CH(Br)COOH; (ii) less steric hindrance and greater electrophilicity in CH₃CHO; (iii) CH₃CH=N–NH₂ + H₂O

Where the marks are

  • Correct increasing order of acidity (1 mark)
  • Steric and electronic reason (1 mark)
  • Ethanal hydrazone with H₂O (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 56/5/1: 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.