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An organic compound… (Class 12 Chemistry SQP 2023-24, Q27)

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ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2023-24: question 27

From the official CBSE sample question paper for 2023-24, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerHaloalkanes and Haloarenes

An organic compound A with the molecular formula (+) undergoes hydrolysis to form (+) . Give the structure of A and write the mechanism of the reaction.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Optical activity in A means a chiral carbon; which C4H9Br isomer has one?

Hint 2

The product is racemic (±). Which substitution mechanism leads to racemisation?

Hint 3

SN1: a planar carbocation forms in the slow step, and OH⁻ attacks both faces equally to give a 50:50 mixture.

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 is optically active, so it must contain a chiral carbon. The only C4H9Br isomer with a chiral carbon is 2-bromobutane: A = CH3CH2CH(Br)CH3, (+)-2-bromobutane.

    1 mark

  2. The product is racemic (±)-butan-2-ol, so the reaction is SN1. Step 1 (slow, rate-determining): the C–Br bond ionises to give the planar sp2 secondary carbocation and bromide: CH3CH2CH(Br)CH3 → CH3CH2C⁺HCH3 + Br⁻. Rate = k[RBr].

    1 mark

  3. Step 2 (fast): the nucleophile (OH⁻/H2O) attacks the planar carbocation equally from both faces, CH3CH2C⁺HCH3 + OH⁻ → CH3CH2CH(OH)CH3. Attack from the front gives retention and attack from the back gives inversion, so a 50:50 mixture of (+) and (–) butan-2-ol forms, i.e. (±)-butan-2-ol (racemisation).

    1 mark

Answer: A = (+)-2-bromobutane, CH3CH2CH(Br)CH3. The SN1 mechanism has a slow step giving a planar carbocation and a fast step in which OH⁻ attacks from both faces, giving racemic (±)-butan-2-ol.

Where the marks are

  • Structure of A: 2-bromobutane with a chiral carbon (1 mark)
  • Slow step: formation of a planar carbocation (SN1) (1 mark)
  • Fast attack of OH⁻ from both faces giving racemic (±)-butan-2-ol (1 mark)
  • (3 marks)

Revise the chapter

Practise this chapterBoard-style questions from Haloalkanes and Haloarenes, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2023-24: 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.