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A. Out of formic… (Class 12 Chemistry SQP 2024-25, Q32b)

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ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 5 marks · Long answer

CBSE Class 12 Chemistry sample paper 2024-25: question 32(b)

From the official CBSE sample question paper for 2024-25, section E, question 32(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(b) · 5 marks · Long answerAldehydes, Ketones and Carboxylic Acids

a. Out of formic acid and acetic acid, which one will give the HVZ reaction? Give a suitable reason in support of your answer and write the chemical reaction involved. (2)
b. Alcohols are acidic but they are weaker acids than water. Arrange various isomers of butanol in the increasing order of their acidic nature. Give a reason for the same. (1)
c. An organic compound A which is a Grignard reagent is used to obtain 2-methylbutan-2-ol on reaction with a carbonyl compound ‘B’. Identify A’ and ‘B’. Write the equation for the reaction between A and B. (2)

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

HVZ halogenates a particular carbon of the acid. Which carbon, and does formic acid have it?

Hint 2

For acidity, think about how the electron-releasing effect of alkyl groups changes from 1° to 2° to 3° alcohols.

Hint 3

2-methylbutan-2-ol is tertiary. Which class of carbonyl compound gives a 3° alcohol with RMgX? Break off one CH₃ from the carbinol carbon.

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. Acetic acid gives the HVZ reaction. The HVZ reaction replaces an α-hydrogen of a carboxylic acid by halogen, using or with a little red phosphorus. Acetic acid () has three α-hydrogens. Formic acid () has no α-carbon, so it has no α-hydrogen and cannot react.

    1 mark

  2. Reaction:

    1 mark

  3. b. Increasing order of acidic strength of the butanol isomers:

    ½ mark

  4. Reason: the acidity of an alcohol comes from the polar O–H bond. Alkyl groups are electron-releasing (+I). More alkyl groups on, or close to, the carbinol carbon push more electron density onto oxygen. This makes the O–H bond less polar and destabilises the alkoxide ion. So acidity falls in the order .

    ½ mark

  5. c. The product is 2-methylbutan-2-ol, , a tertiary alcohol. A Grignard reagent gives a tertiary alcohol only with a ketone, so B is a ketone.

    Disconnect one methyl group from the carbinol carbon. This gives **A = (methylmagnesium bromide) and B = ** (butan-2-one).

    1 mark

  6. Equation:

    (Another valid pair is with propanone, which gives the same alcohol.)

    1 mark

Answer: a. Acetic acid (it has α-H; formic acid has none): CH₃COOH + Br₂/red P → BrCH₂COOH + HBr. b. 2-methylpropan-2-ol < butan-2-ol < 2-methylpropan-1-ol < butan-1-ol (+I effect of alkyl groups). c. A = CH₃MgBr, B = butan-2-one.

Where the marks are

  • Identifies acetic acid and states that HVZ needs an α-hydrogen, which formic acid lacks (1 mark)
  • Writes the HVZ equation giving bromoacetic acid (1 mark)
  • Correct increasing order of acidity of the butanol isomers (½ mark)
  • Reason: +I effect of alkyl groups lowers O–H polarity, so 1° > 2° > 3° (½ mark)
  • Identifies A = CH₃MgBr and B = butan-2-one (ketone, since the product is a 3° alcohol) (1 mark)
  • Correct equation for Grignard addition followed by hydrolysis (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Aldehydes, Ketones and Carboxylic Acids, 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.