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Explain why : (i) Carboxyl… (Class 12 Chemistry 2023, Q29a)

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ChemistryCBSE Class 12 · Board exam 2023, set 56/1/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2023: question 29(a)

Set in the CBSE Class 12 board exam of 2023, paper 56/1/1, section C, question 29(a), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q29(a) · 3 marks · Short answerAldehydes, Ketones and Carboxylic Acids

Explain why :
(i) Carboxyl group in benzoic acid is meta directing.
(ii) Sodium bisulphite is used for the purification of aldehydes and ketones.
(iii) Carboxylic acids do not give characteristic reactions of carbonyl group.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/1/1.

Hints

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

Hint 1

Consider whether –COOH donates or withdraws electrons from the ring.

Hint 2

Bisulphite addition is a reversible reaction that gives a crystalline solid.

Hint 3

Draw the resonance structures of the carboxyl group and look at the charge on 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) The –COOH group is electron-withdrawing (–R and –I effects). It reduces electron density mainly at the ortho and para positions, so the meta positions are left relatively richer in electrons. Therefore electrophilic substitution occurs at the meta position, and the group also deactivates the ring.

    1 mark

  2. (ii) Aldehydes and ketones form crystalline bisulphite addition compounds with , which can be separated from impurities. These adducts are decomposed by dilute acid or alkali to regenerate the pure aldehyde or ketone.

    1 mark

  3. (iii) In –COOH, the lone pair on the –OH oxygen is in resonance with the C=O group. This makes the carboxyl carbon much less electrophilic, so it does not undergo the typical nucleophilic addition reactions of aldehydes and ketones.

    1 mark

Answer: (i) –COOH is electron-withdrawing (–R), deactivating ortho/para; (ii) reversible formation of crystalline bisulphite adducts; (iii) resonance lowers electrophilicity of the carboxyl carbon.

Where the marks are

  • (i) Electron-withdrawing / deactivating (–R) nature of –COOH (1 mark)
  • (ii) Formation of addition compound that regenerates the pure carbonyl on hydrolysis (1 mark)
  • (iii) Resonance makes the carboxyl carbon less electrophilic (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 paper2023 board paper, set 56/1/1: 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.