Problem on aldol reaction

Question: How many aldol reactions are occurred in the given transformation?

Solution:

Before start the question let us recall the aldol reaction.

The Aldol addition reaction involves the addition of α-carbon of an enolizable aldehyde or ketone (an enolizable aldehyde or ketone is a compound whose molecule has one or more alpha hydrogens) to the carbonyl group of another aldehyde or ketone and give a β-hydroxy carbonyl compound which is also known as an aldol. As the name suggest aldol (ald+ol) compound shows presence of both aldehyde and alcohol groups.  The reaction is catalyzed more commonly by a base or some times by an acid.

Enolizable aldehyde or ketone + carbonyl group of aldehyde or ketone = β hydroxyl-carbonyl compound

Now let’s go back to the question and perform aldol reaction on the given reactant:

Now after 3^rd aldol reaction the obtained product is neither an enolizable aldehyde nor enolizable ketone. Thus, no more aldol reaction here.

Now the obtained product when further reacts with NaOH, this leads to cannizzaro reaction. Since Cannizzaro Reaction is seen only in aldehydes that do not contain an alpha hydrogen atom. The obtained product in 3^rd aldol reaction is also an aldehyde and do not contain an alpha hydrogen atom.

This gives our final product. from her it is clear that in the whole transformation the aldol reaction is occurred 3 times.