Question

Explain Aldol condensation with a chemical reaction.

Hint:

Two carbonyl molecules that have an \(\alpha\)-hydrogen combine in dilute base to give a \(\beta\)-hydroxy carbonyl (aldol), which on heating loses water. Try acetaldehyde.

Answer 1

Step 1: Requirement for the reaction. Only carbonyl compounds having an \(\alpha\)-hydrogen can undergo aldol condensation; formaldehyde and benzaldehyde cannot because they have no \(\alpha\)-hydrogen.
Step 2: Role of the alkali. A dilute alkali pulls off one \(\alpha\)-hydrogen and generates a resonance-stabilised enolate (carbanion). One molecule then behaves as the nucleophile and the other as the electrophile.
Step 3: Addition step. The enolate carbon adds to the electrophilic carbonyl carbon of the second molecule, and after protonation a \(\beta\)-hydroxy carbonyl compound (aldol) is obtained.
Step 4: Illustration with acetaldehyde.
\[2\,CH_3CHO \xrightarrow{dil.\ NaOH} CH_3CH(OH)CH_2CHO\ (3\text{-hydroxybutanal})\]
Step 5: Dehydration. Heating removes water from the \(\beta\)-carbon to give a stable conjugated product.
\[CH_3CH(OH)CH_2CHO \xrightarrow{\Delta} CH_3CH=CHCHO + H_2O\]
The compound obtained is but-2-enal. Ketones such as acetone behave similarly, giving ketols and then unsaturated ketones.