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(i) (I) Which of the… (Class 12 Chemistry 2026, Q33a)

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ChemistryCBSE Class 12 · Board exam 2026, set 56/5/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2026: question 33(a)

Set in the CBSE Class 12 board exam of 2026, paper 56/5/1, section E, question 33(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(a) · 5 marks · Long answerHaloalkanes and Haloarenes

(i) (I) Which of the following is more reactive towards reaction : 2-Bromo-2-methylbutane or 1-Bromopentane (1)
(II) What type of halide is present in the following compound : (1)
(III) Why is chloroform stored in dark coloured bottles ? (1)
(ii) Define the following terms : (1+1)
(I) Ambident Nucleophiles
(II) Racemic mixture

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/5/1.

Hints

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

Hint 1

S_N1 goes through a carbocation, so compare the carbocations each halide would form.

Hint 2

Look at the hybridisation of the carbon carrying Cl. For chloroform, think about what light and air do to it.

Hint 3

For the definitions, think of CN⁻ (which atom can attack?) and of a 50:50 mixture of mirror-image isomers.

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)(I) 2-Bromo-2-methylbutane is more reactive towards . It is a tertiary halide, so it ionises to a stable tertiary carbocation . That carbocation is stabilised by +I effect and hyperconjugation. 1-Bromopentane is primary and would form an unstable 1° carbocation.

    1 mark

  2. (i)(II) The Cl atom is bonded directly to an carbon of the C=C double bond (). It is therefore a vinylic halide.

    1 mark

  3. (i)(III) In the presence of light, air slowly oxidises chloroform to the highly poisonous gas phosgene: . Storing it in dark coloured, completely filled bottles keeps out light and air.

    1 mark

  4. (ii)(I) Ambident nucleophiles: nucleophiles that have two nucleophilic centres and can attack through either of them. Example: cyanide ion attacks through C (giving cyanides) or N (giving isocyanides). Nitrite ion attacks through O or N.

    1 mark

  5. (ii)(II) Racemic mixture: a mixture containing equal amounts of the dextrorotatory (+) and laevorotatory (–) enantiomers. It shows zero optical rotation because the rotation by one enantiomer is cancelled by the other (external compensation).

    1 mark

Answer: (i)(I) 2-Bromo-2-methylbutane; (II) vinylic halide; (III) to prevent its oxidation by air in light to poisonous phosgene (COCl₂); (ii)(I) nucleophiles with two nucleophilic sites, e.g. CN⁻, NO₂⁻; (II) equimolar mixture of d- and l-enantiomers, optically inactive.

Where the marks are

  • Chooses 2-bromo-2-methylbutane (tertiary, stable carbocation) (1 mark)
  • Identifies the compound as a vinylic halide (1 mark)
  • States oxidation of CHCl₃ in light to poisonous phosgene (equation optional) (1 mark)
  • Defines ambident nucleophile as one with two nucleophilic centres (1 mark)
  • Defines racemic mixture as equal amounts of dextro and laevo enantiomers (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 paper2026 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.