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(i) Why is the C – O bond… (Class 12 Chemistry 2023, Q30a)

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ChemistryCBSE Class 12 · Board exam 2023, set 56/5/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2023: question 30(a)

Set in the CBSE Class 12 board exam of 2023, paper 56/5/1, section C, question 30(a), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q30(a) · 3 marks · Short answerAlcohols, Phenols and Ethers

(i) Why is the C – O bond length in phenols less than that in methanol ?
(ii) Arrange the following in order of increasing boiling point : Ethoxyethane, Butanal, Butanol, n-butane
(iii) How can phenol be prepared from anisole ? Give reaction.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/5/1.

Hints

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

Hint 1

Think about resonance of the oxygen lone pair with the ring and the hybridisation of the carbon.

Hint 2

Rank by intermolecular forces: none polar < weakly polar < polar C=O < H-bonding.

Hint 3

Ethers are cleaved by HI; decide which C–O bond breaks in anisole.

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) In phenol the lone pair on O is in conjugation with the benzene ring, giving the C–O bond partial double bond character by resonance; also the carbon is hybridised (more s-character) compared with carbon in methanol. Hence C–O bond in phenol (≈136 pm) is shorter than in methanol (≈142 pm).

    1 mark

  2. (ii) n-Butane < Ethoxyethane < Butanal < Butanol
    (only van der Waals forces < weak dipole forces < stronger dipole–dipole of C=O < intermolecular H-bonding in butanol)

    1 mark

  3. (iii) Anisole is cleaved by hot conc. HI (or HBr):

    The O–CH₃ bond breaks (not the O–aryl bond, which has partial double bond character), giving phenol.

    1 mark

Answer: (i) resonance/partial double bond and sp² carbon in phenol; (ii) n-butane < ethoxyethane < butanal < butanol; (iii) C₆H₅OCH₃ + HI → C₆H₅OH + CH₃I

Where the marks are

  • Partial double bond character / sp² vs sp³ (1 mark)
  • Correct increasing order of b.p. (1 mark)
  • Correct equation with HI/HX (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Alcohols, Phenols and Ethers, one at a time, marked against the answer key and marking points.Back to the full paper2023 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.