Question

What is aldol condensation? Explain it with suitable examples.

Answer 1

1. Aldehydes containing at least one α –hydrogen atom undergo a reaction in presence of dilute alkali ( dilute NaOH, KOH or Na2 CO3 ) as catalyst to form β-hydroxy aldehydes (aldol). This reaction is known as aldol reaction.
2. Formation of aldol is an addition reaction. Aldol formed from aldehyde having α-hydrogens undergoes subsequent elimination of water molecule on warming, giving rise to α, β - unsaturated aldehyde.
   \[\begin{array}{cc}
   \ce{\underset{(Aldehyde)}{2R - CH2 - CHO} <=>[aq.NaOH][]R - CH2 - CH - CHR - CHO}\\
   \phantom{.....................}|\\
   \phantom{.......................}\ce{\underset{(Aldol)}{OH}}
   \end{array}\]
3. 
   \[\begin{array}{cc}
   \ce{R - CH2 - CH - CHR - CHO<=>[-H2O][warm]R - CH = C - CHO}\\
   \phantom{....}|\ \phantom{..................................}|\\
   \phantom{............}\ce{\underset{(aldol)}{OH}\phantom{......................}\underset{(α, β - unsaturated aldehyde)}{R}}
   \end{array}\]
4. 
   \[\begin{array}{cc}
   \ce{H}\\
   |\\
   \ce{H - C - C - H + H - \overset{α}{C}H2 - C - H <=>[dil.NaOH][]H3\overset{β}{C}H - CH - \overset{α}{C}H - CHO <=>[Δ][-H2O]H3C - \overset{β}{C}H = \overset{α}{C}H - CHO}\\
   ||\ ||\ |\ |\\
   \ce{\underset{(Ethanal)}\underset{(Ethanal)}{O}{H O}\underset{(3- Hydroxybutanal)}{OH H}}
   \end{array}\]
5. The overall reaction is called aldol condensation. It is a nucleophilic addition - elimination reaction.