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Account for the following : (Class 12 Chemistry 2024, Q26)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/5/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2024: question 26

Set in the CBSE Class 12 board exam of 2024, paper 56/5/1, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerHaloalkanes and Haloarenes

Account for the following :

  1. (a)

    Haloalkanes react with AgCN to form isocyanide as main product.

    1 mark
  2. (b)

    Allyl chloride shows high reactivity towards reaction.

    1 mark
  3. (c)

    Haloarenes are extremely less reactive towards nucleophilic substitution reactions.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/5/1.

Hints

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

Hint 1

Compare how ionic KCN and covalent AgCN release the cyanide ion.

Hint 2

Think about resonance in the carbocation for (b) and in the C–X bond for (c).

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) AgCN is mainly covalent, so the carbon of cyanide is tied up in the Ag–C bond. Only the nitrogen lone pair is free to attack the alkyl carbon, so the alkyl isocyanide R–NC is the main product.

    1 mark

  2. (b) Allyl chloride ionises to the allyl carbocation . This ion is resonance-stabilised (), so it forms easily and the reaction is fast.

    1 mark

  3. (c) In haloarenes the halogen lone pair is in resonance with the ring, giving the C–X bond partial double-bond character. The carbon is also hybridised, so the bond is shorter and stronger. In addition, the phenyl cation is unstable and the electron-rich ring repels the nucleophile, so nucleophilic substitution is very difficult.

    1 mark

Answer: (a) Covalent AgCN – only N donates; (b) resonance-stabilised allyl carbocation; (c) partial double-bond character of C–X (resonance, sp² carbon)

Where the marks are

  • Covalent nature of AgCN, attack through N (1 mark)
  • Resonance stabilisation of allyl carbocation (1 mark)
  • Resonance/partial double bond/sp² carbon/unstable phenyl cation (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 paper2024 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.