Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl more efficiently than in the former.
4-Hydroxy-4-methylpentan-2-one and 4-methylpent-3-en-2-one are the products obtained from 'A' by aldol condensation. 'A' is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(CH_3COCH_3\)
A carbonyl compound with at least one α-hydrogen undergoes the aldol reaction with dilute alkali. A ketone gives a β-hydroxy ketone (ketol), which loses water on heating to give an α,β-unsaturated ketone.
Both products named in the question have six carbon atoms, a C=O at C-2 and two methyl groups on C-4, so they come from two molecules of a three-carbon ketone, acetone:
\( \displaystyle \ce{2CH3COCH3} \xrightarrow{\text{dil. }\ce{NaOH}} \underset{\text{4-hydroxy-4-methylpentan-2-one}}{\ce{(CH3)2C(OH)CH2COCH3}} \)
\( \displaystyle \ce{(CH3)2C(OH)CH2COCH3} \xrightarrow[-\ce{H2O}]{\Delta} \underset{\text{4-methylpent-3-en-2-one}}{\ce{(CH3)2C=CHCOCH3}} \)
So A is acetone.
Why the other options are incorrect
Option (2) \(CH_3CHO\) — Incorrect. Ethanal gives 3-hydroxybutanal and but-2-enal, four-carbon aldehydes, not six-carbon ketones.
Option (3) \(CH_3CH_2CH_2CH_2CHO\) — Incorrect. Pentanal would give a ten-carbon aldol (an aldehyde), not a pentan-2-one.
Option (4) \(CH_3CH_2CH_2CHO\) — Incorrect. Butanal gives 2-ethyl-3-hydroxyhexanal, an eight-carbon aldehyde.
NCERT (2026-27 edition):
Aldol Condensation – “Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction. The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products. The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 12-06 S3 12-06 S2 10-06 S1 | – |