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Draw the structures of… (Class 12 Chemistry 2024, Q20a)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2024: question 20(a)

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section B, question 20(a), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20(a) · 2 marks · Very short answerHaloalkanes and Haloarenes

Draw the structures of major monohalo products in each of the following reactions :
(i) styrene () + HBr
(ii) cyclohexene +

Figure in the paper: (i) benzene ring with CH=CH2 substituent plus HBr; (ii) cyclohexene ring plus Br2 under UV light. See it in the official paper.

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

Hints

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

Hint 1

In (i) decide which carbocation is more stable after H⁺ adds.

Hint 2

In (ii) UV light means free radicals; which hydrogen gives the most stable radical?

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. (i) Electrophilic addition follows Markovnikov's rule; H⁺ adds to the terminal CH₂ giving the more stable benzylic carbocation, so Br attaches to the carbon bearing the ring: (1-bromo-1-phenylethane).

    1 mark

  2. (ii) In UV light Br₂ gives free-radical substitution at the allylic position (the ring CH₂ next to the double bond): cyclohexene + Br₂ → 3-bromocyclohexene (six-membered ring with C=C between C1 and C2 and Br on C3) + HBr.

    1 mark

Answer: (i) ; (ii) 3-bromocyclohexene

Where the marks are

  • Markovnikov product 1-bromo-1-phenylethane (1 mark)
  • Allylic substitution product 3-bromocyclohexene (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 56/1/1: 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.