Question

Write the structure of the major product of the aldol condensation of benzaldehyde with acetone.

Answer 1

Step 1:

\[\begin{array}{cc}
\phantom{}\ce{O}\phantom{.....................}\ce{O}\phantom{...}\\
\phantom{}||\phantom{.....................}||\phantom{...}\\
\ce{\underset{(Acetone)}{H - CH2 - C - CH3} ->[OH^Θ] \underset{(Carbanion)}{\overset{Θ}{C}H2 - C - CH3} + H2O}
\end{array}\]

Step 2:

\[\begin{array}{cc}
\phantom{.}\ce{O}\phantom{................................}\ce{O^Θ}\phantom{...}\\
\phantom{}||\phantom{.................................}|\phantom{....}\\
\ce{\underset{(Benzaldehyde)}{C6H5 - C - H} + \overset{Θ}{C}H2COCH3 -> C6H5 - C - H + H2O}\\
\phantom{..............................}|\\
\phantom{.........................................}\ce{\underset{(Phenyl alkoxide ion)}{CH2COCH3}}
\end{array}\]

Step 3:

\[\begin{array}{cc}
\phantom{...}\ce{O^Θ}\phantom{..................}\ce{OH}\phantom{...............}\\
\phantom{}||\phantom{....................}|\phantom{...............}\\
\ce{C6H5 - C - H ->[H - OH] \underset{(major product)}{C6H5 - CH - CH2COCH3} + \overset{Θ}{OH}}\\
|\phantom{....................................}\\
\ce{CH2COCH3}\phantom{..........................}
\end{array}\]

Step 4:

\[\begin{array}{cc}
\ce{C6H5 - CH - CH2 - COCH3 ->[\Delta][-H2O] \underset{(Benzylidene acetone)}{C6H5 - CH = CH - COCH3}}\\
|\phantom{..............................................}\\
\ce{OH}\phantom{............................................}
\end{array}\]