Question

Explain aldol condensation reaction in detail.

Answer 1

1. Aldehydes containing at least one α–hydrogen atom undergo a reaction in presence of dilute alkali (dilute NaOH, KOH, or Na₂CO₃) as the catalyst to form β-hydroxy aldehydes (aldol). This reaction is known as the aldol reaction.
2. Formation of aldol is an addition reaction.
3. Aldol formed from aldehyde having α-hydrogens undergoes subsequent elimination of water molecule on warming. This gives rise to α,β-unsaturated aldehyde.
4. The overall reaction is called aldol condensation. It is a nucleophilic addition-elimination reaction.
5. The reaction can be given as,
   \[\begin{array}{cc}
   \ce{\underset{\text{Aldehyde}}{2R - CH2 - CHO} ->[aq.NaOH] R - CH2 - CH - CHR - CHO}\\
   \phantom{.......................}|\\
   \phantom{..........................}\ce{\underset{\text{Aldol}}{OH}}
   \end{array}\]
   \[\begin{array}{cc}
   \ce{R - CH2 - CH - CHR - CHO ->[-H2O][warm] \underset{\text{α,β-Unsaturated aldehyde}}{R - CH = CR - CHO}}\\
   |\phantom{.......................................}\\
   \ce{\underset{Aldol}{OH}}\phantom{.....................................}
   \end{array}\]
   e.g.
   
6. Ketones containing at least two α-hydrogen also undergo aldol condensation reaction and give an α,β-unsaturated ketone.
   e.g.