The aldehydes and ketones undergo many nucleophilic reactions. Aldehydes are generally more reactive than Ketones in nucleophilic addition reactions due to steric and electronic reasons. The aldehydes and ketones undergo a number of reactions due to acidic nature of α-Hydrogen also. Aldol and cross aldol reactions occur in carbonyl compounds which contain α-Hydrogen. Aldehydes and ketones having at least one α-Hydrogen undergo reaction in dil. alkali to form β-hydroxy aldehyde or β-hydroxy ketone. This is aldol reaction. Aldol or ketol lose water to form α,β unsaturated carbonyl compounds which is called aldol condensation. Carbonyl compounds react with ammonia derivatives such as NH2OH, NH2NHCONH2, 2,4 DNP to give products useful in their identification.
Out of following given compounds, identify which carbonyl compound gives aldol reaction?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → ethanal, CH3CHO
An aldol reaction needs an α-hydrogen, a hydrogen on the carbon next to the carbonyl group. A base removes it to form a carbanion (enolate), which then adds to a second carbonyl molecule:
\( \displaystyle \ce{2CH3CHO} \xrightarrow{\text{dil. NaOH}} \ce{CH3CH(OH)CH2CHO} \)
Ethanal has three α-hydrogens on its \( \ce{CH3} \) group, so it gives the aldol reaction.
Why the other options are incorrect
Option (1) methanal — Incorrect. It has no α-carbon at all, so no α-hydrogen; it gives the Cannizzaro reaction instead.
Option (2) benzaldehyde — Incorrect. The α-carbon is part of the benzene ring and has no hydrogen; it also gives the Cannizzaro reaction.
Option (4) 2,2-dichloropropanal — Incorrect. Its α-carbon carries two Cl atoms and a \( \ce{CH3} \) group, so it has no α-hydrogen.
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.”