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.
The compound A discussed in above passage is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Butan-1-al
The identity of an unknown carbonyl compound is built up from its molecular formula and the tests it gives:
Molecular formula C4H8O: one degree of unsaturation, which fits one C=O group.
Orange-red precipitate with 2,4-DNP: A contains a carbonyl group (aldehyde or ketone).
Silver mirror with ammoniacal silver nitrate (Tollens’ reagent): A is an aldehyde.
Straight-chain alcohol on catalytic hydrogenation: the carbon chain is unbranched.
The only unbranched aldehyde with the formula \( \ce{C4H8O} \) is \( \ce{CH3CH2CH2CHO} \). Its oxidation gives butanoic acid (\( \ce{C4H8O2} \)) and its aldol condensation fits the rest of the passage. So A is butanal. The carbon of the –CHO group is always C-1 of the chain, so no locant is written for it.
Why the other options are incorrect
Option (2) Butan-2-one — Incorrect. This ketone has the same formula and gives the 2,4-DNP test, but it does not give a silver mirror with Tollens’ reagent.
Option (3) 2-methoxypropanal — Incorrect. \( \ce{CH3CH(OCH3)CHO} \) has the formula \( \ce{C4H8O2} \), with one oxygen atom too many.
Option (4) 2-hydroxybutane — Incorrect. This is butan-2-ol (\( \ce{C4H10O} \)), an alcohol with no carbonyl group, so it does not react with 2,4-DNP.
Note: It seems that there is a mistake in the original NTA paper, which prints Butan-1-al in Option (1) when it should read butanal.