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.
   $$\begin{array}{c}\underset{(\mathrm{Aldehyde})}{2\mathrm{R}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CHO}}\stackrel{\mathrm{aq}\cdot \mathrm{NaOH}}{\stackrel{-\rightharpoonup }{\leftharpoondown -}}\mathrm{R}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}-\mathrm{CHR}-\mathrm{CHO}\\ \phantom{.....................}|\\ \phantom{.......................}\underset{(\mathrm{Aldol})}{\mathrm{OH}}\end{array}$$
3. 
   $$\begin{array}{c}\mathrm{R}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}-\mathrm{CHR}-\mathrm{CHO}\underset{\mathrm{warm}}{\stackrel{-\mathrm{H}{\phantom{A}}_2\mathrm{O}}{\stackrel{-\rightharpoonup }{\leftharpoondown -}}}\mathrm{R}-\mathrm{CH}=\mathrm{C}-\mathrm{CHO}\\ \phantom{....}|\phantom{..................................}|\\ \phantom{............}\underset{(\mathrm{aldol})}{\mathrm{OH}}\phantom{......................}\underset{(\mathrm{\alpha },\mathrm{\beta }-\mathrm{unsaturated}\mathrm{aldehyde})}{\mathrm{R}}\end{array}$$
4. 
   $$\text{begin{array}{cc} ce{H} | ce{H - C - C - H + H - overset{α}{C}H2 - C - H <=>[dil.NaOH][]H3overset{β}{C}H - CH - overset{α}{C}H - CHO <=>[Δ][-H2O]H3C - overset{β}{C}H = overset{α}{C}H - CHO} || || | | ce{underset{(Ethanal)}underset{(Ethanal)}{O}{H O}underset{(3- Hydroxybutanal)}{OH H}} end{array}}$$
5. The overall reaction is called aldol condensation. It is a nucleophilic addition - elimination reaction.