Q.16 The CORRECT option(s) of Y for the following reaction is/are
Question overview
The reagent on the left is 2,4‑dinitrophenylhydrazine (2,4‑DNPH), which reacts with aldehydes or ketones to form 2,4‑dinitrophenylhydrazones via nucleophilic addition–elimination.
The question asks which structure of Y, among the four options, will predominantly behave as a carbonyl compound under dilute H₂SO₄ in ethanol at 60 °C so that the major product is the 2,4‑dinitrophenylhydrazone.
- Free aldehyde/ketone carbonyls react rapidly with 2,4‑DNPH.
- Esters and amides have strongly deactivated carbonyls and usually do not form hydrazones in these mild, protic conditions.
- If the carbonyl is tied up in a derivative that cannot easily hydrolyse back to the aldehyde/ketone, hydrazone formation is negligible.
Mechanism of 2,4‑dinitrophenylhydrazone formation
- The carbonyl oxygen is protonated by dilute H₂SO₄, increasing the electrophilicity of the carbonyl carbon.
- The terminal NH₂ of 2,4‑DNPH attacks this activated carbonyl carbon to give a carbinolamine intermediate.
- Proton transfer occurs, followed by loss of water, giving the C=N‑NH‑ (hydrazone) linkage attached to the aromatic 2,4‑dinitrophenyl ring.
Because 2,4‑DNPH itself is strongly deactivated by the nitro groups, it requires a reasonably reactive carbonyl (aldehyde or ketone) rather than the resonance‑stabilized carbonyls of esters and amides.
Option (A): isobutyraldehyde (correct)
Option (A) shows a simple branched aliphatic aldehyde, (CH3)2CH–CHO.
It contains a free aldehyde group; aldehydes are the most reactive carbonyls toward nucleophilic addition of 2,4‑DNPH.
Under dilute H₂SO₄ in ethanol at 60 °C, it smoothly forms the corresponding 2,4‑dinitrophenylhydrazone as the major product.
Therefore, option (A) satisfies the requirement that Y gives a 2,4‑dinitrophenylhydrazone as major product.
Option (B): ester (incorrect)
Option (B) is an ester, roughly of the form (CH3)2CH–COO–Et.
In an ester, the carbonyl carbon is strongly deactivated by resonance with the –OR group; the lone pair on oxygen delocalizes into the carbonyl, reducing its electrophilicity.
2,4‑DNPH is a relatively weak nucleophile in this acidic, protic medium, and esters do not normally undergo hydrazone formation under these mild conditions; they instead require much harsher nucleophilic acyl‑substitution conditions if they are to react at all.
Thus, the ester in (B) does not form a 2,4‑dinitrophenylhydrazone in appreciable yield, and 2,4‑DNPH is not a standard qualitative test for esters. The major reaction under the stated conditions is simply no reaction, so (B) is incorrect as Y.
Option (C): ketone derivative not accessible as free ketone (incorrect)
Option (C) is a substituted ketone‑derived carboxylic ester (acyl derivative), not a simple free ketone.
The carbonyl in (C) is again part of an acyl group whose reactivity is governed by nucleophilic acyl substitution rather than nucleophilic addition.
Just like esters, such acyl carbonyls are far less reactive toward 2,4‑DNPH and do not readily form hydrazones in dilute acid/ethanol conditions.
Because the effective carbonyl electrophilicity is too low and hydrazone formation is not favored, (C) cannot give 2,4‑dinitrophenylhydrazone as major product and is therefore not a valid Y.
Option (D): amide (incorrect)
Option (D) is an amide of the type (CH3)2CH–CONH–Et.
Amide carbonyls are even less reactive than esters due to strong resonance donation from the nitrogen lone pair into the carbonyl, making the carbonyl carbon very poor as an electrophile for nucleophilic addition.
Amides do not form hydrazones with 2,4‑DNPH under such mild conditions; they require severe hydrolysis (strong acid or base, higher temperatures) to give back the parent acid or amine, not an aldehyde or ketone.
Hence, option (D) cannot serve as Y to yield 2,4‑dinitrophenylhydrazone as the major product.
Final answer
Correct Y: Option (A) – the aldehyde.
Incorrect Y: Options (B), (C) and (D), because the carbonyls are part of ester/amide acyl systems that do not undergo hydrazone formation with 2,4‑DNPH under the given conditions.
