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Various isomeric… (Class 12 Chemistry SQP 2024-25, Q27)

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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 27

From the official CBSE sample question paper for 2024-25, 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

Various isomeric haloalkanes with the general formula undergo hydrolysis reaction. Among them, compound “A” is the most reactive through mechanism. Identify “A” citing the reason for your choice. Write the mechanism for the reaction.

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

The rate-determining step of S_N1 forms a carbocation.

Hint 2

Among the C₄H₉Cl isomers, which gives a tertiary carbocation?

Hint 3

Write the mechanism in two steps: ionisation (slow), then nucleophilic attack (fast).

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. The isomers of are 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane. **A = 2-chloro-2-methylpropane, ** (tertiary butyl chloride).

    ½ mark

  2. Reason: goes through a carbocation. A gives a tertiary carbocation, , which is the most stable because of the +I effect and hyperconjugation of three methyl groups. So A reacts fastest of all the isomers.

    ½ mark

  3. Step 1 (slow, rate-determining): ionisation of the C–Cl bond

    1 mark

  4. Step 2 (fast): attack of the nucleophile on the planar carbocation
    (2-methylpropan-2-ol)
    Rate = k[(CH₃)₃CCl], i.e. first order.

    1 mark

Answer: A = (CH₃)₃CCl (2-chloro-2-methylpropane); forms the most stable 3° carbocation; S_N1: slow ionisation then fast OH⁻ attack giving (CH₃)₃COH

Where the marks are

  • Identifies A as 2-chloro-2-methylpropane (½ mark)
  • Reason: most stable tertiary carbocation (½ mark)
  • Slow ionisation step forming carbocation (1 mark)
  • Fast nucleophilic attack giving tert-butyl alcohol (1 mark)

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 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.