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For the pair phenol and… (Class 12 Chemistry 2023, Q23)

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ChemistryCBSE Class 12 · Board exam 2023, set 56/5/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2023: question 23

Set in the CBSE Class 12 board exam of 2023, paper 56/5/1, section B, question 23, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 2 marks · Very short answerAlcohols, Phenols and Ethers

For the pair phenol and cyclohexanol, answer the following :
(a) Why is phenol more acidic than cyclohexanol ?
(b) Give one chemical test to distinguish between the two.

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

Compare the stability of the anions formed after each loses .

Hint 2

For the test, think of a reagent that gives a characteristic colour only with phenols.

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) On losing , phenol forms phenoxide ion whose negative charge is delocalised over the benzene ring by resonance, so it is stabilised. Cyclohexoxide ion has no resonance and its charge is localised on O (further destabilised by the +I effect of the alkyl ring). Hence phenol ionises more readily and is more acidic.

    1 mark

  2. (b) **Neutral test:** phenol gives a violet colour with neutral ferric chloride; cyclohexanol gives no such colour. (Alternatively, bromine water gives a white precipitate of 2,4,6-tribromophenol with phenol but not with cyclohexanol.)

    1 mark

Answer: (a) Phenoxide ion is resonance-stabilised; (b) neutral FeCl₃ — violet colour with phenol only

Where the marks are

  • Resonance stabilisation of phenoxide ion compared with alkoxide (1 mark)
  • Correct distinguishing test with observation (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.