Explain aldol condensation reaction with respect to ethanal.

Answer: Aldehydes having an α-hydrogen, on treatment with dilute alkali, undergo self-condensation to give a β-hydroxy aldehyde (aldol), which on heating loses water to give an α,β-unsaturated aldehyde.

2 CH₃CHO \(\xrightarrow{\text{dil. NaOH}}\) CH₃–CH(OH)–CH₂–CHO (3-hydroxybutanal, aldol)

CH₃–CH(OH)–CH₂–CHO \(\xrightarrow{\Delta}\) CH₃–CH=CH–CHO (but-2-enal, crotonaldehyde) + H₂O

Mechanism in brief: OH^– removes an α-H from one ethanal to give the carbanion ^–CH₂CHO, which adds to the carbonyl carbon of a second ethanal molecule; the alkoxide formed takes a proton from water to give the aldol.