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Question
What is meant by the following term? Give an example of the reaction in the following case.
Aldol
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Solution
Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as the aldol reaction.
\[\begin{array}{cc}
\ce{\underset{Ethanal}{2CH3-CHO}<=>[dil.NaOH] CH3 - CH - CH2 - CHO ->[\Delta][-H2O]\underset{But-2-enal}{CH3 - CH = CH - CHO}}\\
|\phantom{.....................}\\
\ce{\underset{(Aldol)}{\underset{3-Hydroxybutanal}{OH}}}\phantom{...................}\
\end{array}\]
\[\begin{array}{cc}
\phantom{..................}\ce{CH3}\phantom{.............................}\ce{CH3}\\
\phantom{..............}|\phantom{.................................}|\\
\ce{\underset{Propanone}{2CH3 - CO - CH3} <=>[Ba(OH)2] CH3 - C - CH2CO - CH3->[\Delta][-H2O]\underset{(Aldol condensation product)}{\underset{4-Methylpent-3-en-2-one}{CH3 - C = CH - CO - CH3}}}\\
|\phantom{...................}\\
\ce{\underset{4-Hydroxy-4-methylpentan-2-one}{\underset{(Ketol)}{OH}}}\phantom{................}\
\end{array}\]
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\[\begin{array}{cc}
\ce{CH3}\phantom{.................}\\
|\phantom{....................}\\
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\end{array}\]
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\[\begin{array}{cc}
\ce{2CH3 - CHO ->[dil.NaOH] CH3 - CH - CH2 - CHO}\\
\phantom{...............}|\\
\phantom{.................}\ce{OH}
\end{array}\]
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