Question

Explain Aldol condensation of ethanal.

Answer 1

Aldehydes containing at least one α –hydrogen atom undergo a reaction in the presence of dilute alkali (dilute NaOH, KOH or Na₂CO₃) as a catalyst to form β-hydroxy aldehydes (aldol). This reaction is known as the aldol reaction.

1) Ethanal contains an α hydrogen atom so it undergoes the aldol condensation reaction.

2) Reaction:

1. \[\ce{\underset{Ethanal}{2CH3 - CHO} ->[dil.NaOH] \underset{3-Hydroxybutanal}{CH3 - CHOH - CH2 - CHO}}\]
2. \[\ce{\underset{3-Hydroxybutanal}{CH3 - CHOH - CH2 - CHO} ->[H^+] \underset{But-2-enal}{CH3 - CH = CH - CHO} + H2O}\]

3) When ethanal is treated with dilute base NaOH, KOH or sodium carbonate, two molecules of ethanal add together to form 3-Hydroxybutanal.

4) When 3-Hydroxybutanal is heated in the presence of an acid, a molecule of water is lost But-2-enal is formed.