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JEE Main · 2021 · 20 Jul 2021 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

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Which among the above compound/s does / do not form silver mirror when treated with Tollen's reagent?
Structure I
I
Structure II
II
Structure III
III
Structure IV
IV
  1. (A)Only (IV)
  2. (B)(III) and (IV) only
  3. (C)Only (II)✓ Correct
  4. (D)(I), (III) and (IV) only

Answer: (C)

Solution

Tollens' reagent oxidises a free aldehyde group and deposits silver only from a compound able to supply one. (I) is 2-methylpropanal and already carries the group. (III) is the enol of cyclohexanecarbaldehyde and (IV) is the cyclic hemiacetal of 5-hydroxypentanal; each stands in equilibrium with its open chain aldehyde, which is consumed as it is oxidised, so the equilibrium is pulled across and both give the mirror. (II) is the enol of propanone, and its tautomer is a ketone, whose carbonyl carbon bears no hydrogen and so resists the mild oxidant. The one compound that gives no silver mirror is the enol of propanone, (II).

JEE Main · 2021 · 20 Jul 2021 · Shift 1 · Single correct option

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