In the presence of a strong base, two
different molecules of aldehyde or ketone
combine to form a β-hydroxyaldehyde or a
This reaction is called as the aldol
The enolate ion (carbanion) is the actual
The reaction breaks down to the following:
The α-carbon of the donor attaches itself to
the carbonyl carbon of the acceptor.
O CH3 C
+ _ _
The carbonyl group plays two roles in this reaction:
1. It provides a site for nucleophilic attack by the
2. It makes the a-hydrogens sufficiently acidic
to enable the formation of reasonable
concentrations of enolate ion.
NOTE: There must be a-hydrogens in order to
undergo aldol condensation.
Without a-hydrogens, the only possible reaction is
If both carbonyl compounds have α-hydrogens, then
the donor and acceptor could be controlled.
If one carbonyl compound that does not have α-
hydrogens is chosen, it can only act as an acceptor.
Then, but adding the donor to a dilute solution of
acceptor, we can control the reaction to give only one
CH3 C H
CH CH C H
Step 1: Deprotonation and formation of nucleophile.
• Step 2: Attack by nucleophile: Formation of alkoxide
Step 3: Hydrolysis and formation of Aldol type product
Step 4: Intramolecular dehydration
The aldol condensation is very useful in the
preparation of large molecules from simple starting
In biosynthesis, where large molecules are products,
condensation reactions provide the only reasonable
route, since all of the starting materials are small
molecules containing only two or three carbons.
In synthesis of insect repellants such as 2-Ethyl-3-
CH2 CH C H
CH2 C CH2 O P OH
CH C CH C CH2
O P OH