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Give reasons for the… (Class 12 Chemistry 2025, Q27)

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ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2025: question 27

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerAldehydes, Ketones and Carboxylic Acids

Give reasons for the following :

  1. (a)

    Benzoic acid does not undergo Friedel-Crafts reaction.

    1 mark
  2. (b)

    HCHO is more reactive than towards addition of HCN.

    1 mark
  3. (c)

    Vinyl group directly attached with carboxylic acid should decrease the acidity of corresponding carboxylic acid due to resonance, but on the contrary it increases the acidity.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/7/1.

Hints

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

Hint 1

What does a Lewis acid do when it meets a group with lone pairs?

Hint 2

Compare the electron density on the carbonyl carbon in the two aldehydes.

Hint 3

Think about how s-character affects electronegativity.

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. (a) The –COOH group is electron-withdrawing, so it deactivates the benzene ring towards electrophilic substitution. Also, the Lewis acid catalyst bonds to the carboxyl group (forms a salt or complex). The catalyst is then unavailable, so the Friedel-Crafts reaction does not occur.

    1 mark

  2. (b) In CH₃CHO, the methyl group's +I effect lowers the positive charge on the carbonyl carbon, and it also adds steric hindrance. HCHO has no alkyl group, so its carbonyl carbon is more electrophilic and less hindered. HCHO is therefore more reactive towards nucleophilic addition of .

    1 mark

  3. (c) In , the carboxyl carbon is attached to an carbon. An carbon has more s-character, so it is more electronegative than an carbon. It withdraws electrons and stabilises the carboxylate ion. This effect outweighs resonance, so acidity increases.

    1 mark

Answer: (a) –COOH deactivates the ring and AlCl₃ binds to it; (b) HCHO carbonyl carbon is more electrophilic (no +I or steric effect); (c) the electronegative sp² carbon withdraws electrons and stabilises the carboxylate.

Where the marks are

  • Deactivating –COOH / AlCl₃ bonds to carboxyl group (1 mark)
  • +I or steric effect of CH₃ makes CH₃CHO less reactive (1 mark)
  • Greater electronegativity of the sp² carbon (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 paper2025 board paper, set 56/7/1: 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.