Question
Explain aldol condensation reaction in detail.
✓
Answer
- Aldehydes containing at least one α–hydrogen atom undergo a reaction in presence of dilute alkali (dilute $NaOH, KOH$, or $Na_2CO_3$) as the catalyst to form $\beta$-hydroxy aldehydes (aldol). This reaction is known as the aldol reaction.
- Formation of aldol is an addition reaction.
- Aldol formed from aldehyde having $\alpha $-hydrogens undergoes subsequent elimination of water molecule on warming. This gives rise to α,β-unsaturated aldehyde.
- The overall reaction is called aldol condensation. It is a nucleophilic addition-elimination reaction.
- The reaction can be given as,
$\begin{array}{cc}
\ce{\underset{\text{Aldehyde}}{2R - CH2 - CHO} ->[aq.NaOH] R - CH2 - CH - CHR - CHO}\\
\phantom{......................}|\\
\phantom{........................}\ce{\underset{\text{Aldol}}{OH}}
\end{array}$
$\begin{array}{cc}
\ce{R - CH2 - CH - CHR - CHO ->[-H2O][warm] \underset{\text{α,β-Unsaturated aldehyde}}{R - CH = CR - CHO}}\\
|\phantom{...................................}\\
\ce{OH}\phantom{...................................}
\end{array}$
e.g.
- Ketones containing at least two α-hydrogen also undergo aldol condensation reaction and give an $\alpha ,\beta$-unsaturated ketone.
e.g.
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