A carbonyl compound on treatment with dilute NaOH gives 2-ethyl-3-hydroxyhexanal. Identify the carbonyl compound.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Butanal
With dilute NaOH, an aldehyde having an α-hydrogen undergoes the aldol reaction: the α-carbon of one molecule bonds to the carbonyl carbon of a second molecule, giving a β-hydroxy aldehyde with twice the number of carbon atoms.
Product: 2-ethyl-3-hydroxyhexanal, \( \ce{CH3CH2CH2-CH(OH)-CH(C2H5)-CHO} \), has 8 carbon atoms and a –CHO group, so it comes from two molecules of a four-carbon aldehyde.
Check: butanal, \( \ce{CH3CH2CH2CHO} \), has two α-hydrogens on C-2; that carbon, carrying an ethyl group, becomes C-2 of the product, and the carbonyl carbon of the second molecule becomes the C-3 that carries –OH.
\( \displaystyle \ce{2CH3CH2CH2CHO} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{CH3CH2CH2-CH(OH)-CH(C2H5)-CHO} \)
Why the other options are incorrect
Option (1) Propanal — Incorrect. Two propanal molecules give a six-carbon aldol, 3-hydroxy-2-methylpentanal, with a methyl branch, not an ethyl branch.
Option (2) Propanone — Incorrect. A ketone gives a β-hydroxy ketone (ketol), here 4-hydroxy-4-methylpentan-2-one, not an aldehyde.
Option (4) Butanone — Incorrect. It is also a ketone, so it gives a ketol, not a hexanal.
NCERT (2026-27 edition):
4. Reactions due to α-hydrogen – “(i) 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.”