Chemistry · Aldehydes, ketones and carboxylic acids · chapter 8 of 10

Class 12 Chemistry Chapter 8: Aldehydes, Ketones and Carboxylic Acids case study question with answers

One case-based question of the kind the board asks: a passage, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks
A college laboratory has three starting materials on the shelf: pentanenitrile, CH₃(CH₂)₃CN; toluene, C₆H₅CH₃; and benzene. The instructor asks a group to make one carbonyl compound from each, choosing reagents that stop at the aldehyde or ketone and do not go on to an alcohol or acid. The group notes that a nitrile can be reduced to an imine and hydrolysed, that a methyl group on a ring must be protected from over-oxidation, and that an acyl group can be put directly onto a benzene ring with a Lewis acid catalyst.
Part 1 · 1 mark
The reagent that converts pentanenitrile into pentanal, followed by hydrolysis, is
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Answer: (a) DIBAL-H

DIBAL-H reduces a nitrile selectively to the imine, which hydrolyses to the aldehyde. Excess LiAlH₄ gives the amine; Pd–BaSO₄ is for acyl chlorides; a Grignard reagent gives a ketone.

Part 2 · 1 mark
Toluene is converted to benzaldehyde by chromic oxide in acetic anhydride. The intermediate that protects the aldehyde from further oxidation is
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Answer: (a) benzylidene diacetate, C₆H₅CH(OCOCH₃)₂

CrO₃ in acetic anhydride gives benzylidene diacetate at 273–283 K, which resists further oxidation and is hydrolysed by aqueous acid to benzaldehyde.

Part 3 · 2 marks
Write the equation for converting benzene into 1-phenylbutan-1-one, C₆H₅COCH₂CH₂CH₃, naming the reaction and the catalyst.
Answer and marking scheme

C₆H₆ + CH₃CH₂CH₂COCl → C₆H₅COCH₂CH₂CH₃ + HCl, with anhydrous AlCl₃: Friedel–Crafts acylation.

  • correct equation with butanoyl chloride (1)
  • Friedel–Crafts acylation, anhydrous AlCl₃ (1)
Or
Write the equation for converting benzene into benzaldehyde in one step, naming the reaction and the reagents.
Answer and marking scheme

C₆H₆ + CO + HCl → C₆H₅CHO, with anhydrous AlCl₃ or CuCl: the Gatterman–Koch reaction.

  • correct equation (1)
  • Gatterman–Koch reaction, CO and HCl over anhydrous AlCl₃/CuCl (1)
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