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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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RELATED QUESTIONS
Write the products formed when CH3CHO reacts with the following reagents: CH3CHO in the presence of dilute NaOH
Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction.
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- Cyclohexanone
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Why are carboxylic acids more acidic than alcohols or phenols although all of them have hydrogen atom attached to a oxygen atom \[\ce{(-O-H)}\]?
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Presence of Alpha hydrogen in aldehydes and ketones is essential for aldol condensation.
Which of the following gives aldol con~ensation reaction?
Identify A and B from the following reaction:
\[\begin{array}{cc}
\ce{CH3}\phantom{.................}\\
|\phantom{....................}\\
\phantom{}\ce{2CH3 - C = O ->[Ba(OH)2] A ->[Δ] B + H2O}
\end{array}\]
Explain Aldol condensation of ethanal.
Which of the following does not give aldol condensation reaction?
Why is the α-hydrogens of aldehydes and ketones are acidic in nature?
