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Neeta was experimenti… (Class 12 Chemistry SQP 2024-25, Q26)

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ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2024-25: question 26

From the official CBSE sample question paper for 2024-25, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerCompetency-basedAlcohols, Phenols and Ethers

Neeta was experimenting in the lab to study the chemical reactivity of alcohols. She carried out a dehydration reaction of propanol at 140C to 180C. Different products were obtained at these two temperatures.
a. Identify the major product formed at 140C and the substitution mechanism followed in this case. (1+½)
b. Identify the major product formed at 180C and the substitution mechanism followed in this case. (1+½)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Lower temperature with excess alcohol favours intermolecular dehydration.

Hint 2

Which product forms when two alcohol molecules lose water, and which forms when one molecule does?

Hint 3

At higher temperature, which product results from loss of H and OH from neighbouring carbons?

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) At 140 °C with conc. (excess alcohol), intermolecular dehydration gives an ether. The major product is dipropyl ether (1-propoxypropane):

    1 mark

  2. Mechanism: **bimolecular nucleophilic substitution ()**. A second propanol molecule attacks the protonated alcohol (), displacing water.

    ½ mark

  3. (b) At about 180 °C, intramolecular dehydration gives the alkene. The major product is propene:

    1 mark

  4. Mechanism: protonation of –OH and loss of water (unimolecular step), then loss of . This is an elimination (E1-type, unimolecular) pathway rather than a substitution.

    ½ mark

Answer: (a) Dipropyl ether, S_N2; (b) Propene, unimolecular elimination (E1-type)

Where the marks are

  • Dipropyl ether at 140 °C (1 mark)
  • S_N2 (bimolecular nucleophilic substitution) (½ mark)
  • Propene at 180 °C (1 mark)
  • Unimolecular mechanism (E1-type elimination) (½ 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 paperSample paper 2024-25: 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.