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Complete the… (Class 12 Chemistry SQP 2024-25, Q20)

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ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 2 marks · Very short answer

CBSE Class 12 Chemistry sample paper 2024-25: question 20

From the official CBSE sample question paper for 2024-25, section B, question 20, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20 · 2 marks · Very short answerHaloalkanes and Haloarenes

Complete the following reactions by writing the major and minor product in each case (any 2)
a. (1)
b. (1)
c. + alc KOH (1)

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

CN⁻ is ambident: which end attacks when the cyanide is ionic KCN?

Hint 2

For HBr addition, decide which carbocation is more stable.

Hint 3

For elimination with alcoholic KOH, the more substituted alkene is favoured.

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) (major, propanenitrile) (minor, ethyl isocyanide).
    KCN is ionic, so attacks mainly through carbon, forming the stronger C–C bond.

    1 mark

  2. (b) (major, 2-bromobutane) (minor, 1-bromobutane).
    By Markovnikov's rule, Br goes to the carbon that gives the more stable 2° carbocation.

    1 mark

  3. (c) (Alternative to (a) or (b); 1 mark if attempted instead.)
    (major, 2-methylbut-2-ene) (minor, 3-methylbut-1-ene).
    By Saytzeff's rule, the more substituted alkene is the major product.

Answer: (a) CH₃CH₂CN (major), CH₃CH₂NC (minor); (b) CH₃CH₂CHBrCH₃ (major), CH₃CH₂CH₂CH₂Br (minor); (c) (CH₃)₂C=CHCH₃ (major), (CH₃)₂CHCH=CH₂ (minor)

Where the marks are

  • Each correct major product (½ mark each, any two reactions)
  • Each correct minor product (½ mark each, any two reactions)

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.