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 reactions involved in the formation of compound B, C and D, respectively are:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Oxidation, Aldol reaction , Dehydration
The passage describes three changes that start from compound A, butanal:
A to B, oxidation: the CHO group gains an oxygen atom and becomes COOH, giving butanoic acid.
\( \displaystyle \ce{CH3CH2CH2CHO} \xrightarrow{[\ce{O}]} \ce{CH3CH2CH2COOH} \)
A to C, aldol reaction: two molecules of the aldehyde combine in dilute alkali to give a β-hydroxy aldehyde.
\( \displaystyle \ce{2CH3CH2CH2CHO} \xrightarrow{\text{dil. NaOH}} \ce{CH3CH2CH2-CH(OH)-CH(C2H5)-CHO} \)
C to D, dehydration: the aldol loses a molecule of water to give the α,β-unsaturated aldehyde.
\( \displaystyle \ce{CH3CH2CH2-CH(OH)-CH(C2H5)-CHO} \xrightarrow[-\ce{H2O}]{\text{Heat}} \ce{CH3CH2CH2-CH=C(C2H5)-CHO} \)
So the reactions are oxidation, aldol reaction and dehydration.
Why the other options are incorrect
Option (1) Oxidation, Aldol reaction , Reduction — Incorrect. D is formed by loss of water, not by addition of hydrogen.
Option (3) Elimination, Aldol reaction , Dehydration — Incorrect. B, a carboxylic acid, is formed by oxidation of the aldehyde, not by elimination.
Option (4) Oxidation, Aldol reaction , Dehydrohalogenation — Incorrect. Dehydrohalogenation removes HX from an alkyl halide, but C contains no halogen; it loses \( \ce{H2O} \).