Carbonyl Compounds
2,4-dinitrophenylhydrazine (2,4-DNP) which is also known as Brady’s reagent is used to identify the presence of carbonyl group in aliphatic as well as aromatic compounds. A solution of 2,4-DNP in sulphuric acid is added to the alcoholic solution of an organic compound (A) with molecular formula C4H8O. An orange-red precipitate is obtained. On warming Compound A with freshly prepared ammoniacal silver nitrate solution, a bright silver mirror is produced due to the formation of silver metal. Compound A is easily oxidised to carboxylic acid B with molecular formula C4H8O2 on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. A on catalytic hydrogenation give a straight chain alcohol. Two moles of compound A in the presence of dilute alkali form a β-hydroxy aldehyde C which readily loses water to give α,β-unsaturated carbonyl compound D.
Which of the compound is formed by the oxidation of compound A?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Butanoic acid
Aldehydes are easily oxidised to carboxylic acids with the same number of carbon atoms, because the carbonyl carbon carries a hydrogen atom. Even mild oxidising agents such as Tollens’ reagent can do this; ketones are oxidised only under vigorous conditions, with breaking of C–C bonds.
From the passage, compound A (\( \ce{C4H8O} \)) gives an orange-red precipitate with 2,4-DNP (a carbonyl compound), a silver mirror with Tollens’ reagent (an aldehyde), and a straight-chain alcohol on hydrogenation. So A is butanal, and its oxidation gives compound B:
\( \displaystyle \ce{CH3CH2CH2CHO} \xrightarrow{[\ce{O}]} \ce{CH3CH2CH2COOH} \)
So the compound formed is butanoic acid, \( \ce{C4H8O2} \), as the passage states.
Why the other options are incorrect
Option (1) 2-hydroxybenzoic acid — Incorrect. Salicylic acid is an aromatic acid, \( \ce{C7H6O3} \), and cannot come from a four-carbon aldehyde.
Option (3) Propanoic acid — Incorrect. It has only three carbon atoms, while the oxidation of an aldehyde keeps all its carbon atoms.
Option (4) Butanol — Incorrect. Butan-1-ol is the product of reduction (catalytic hydrogenation) of butanal, not of oxidation.
NCERT (2026-27 edition):
Oxidation
