2. In the presence of a strong base, two
different molecules of aldehyde or ketone
combine to form a β-hydroxyaldehyde or a
β-hydroxyketone.
This reaction is called as the aldol
condensation.
3. The enolate ion (carbanion) is the actual
nucleophilic reagent.
The reaction breaks down to the following:
The α-carbon of the donor attaches itself to
the carbonyl carbon of the acceptor.
4. CH3 C
H
O CH3 C
H
O
CH2
C
H
O
CH2
C
H
O
+ _ _
..
_
acceptor
donor
5. The carbonyl group plays two roles in this reaction:
1. It provides a site for nucleophilic attack by the
enolate ion.
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
Cannizarro reaction
6. 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
product.
7. C H
O
+
CH3 C H
O
NaOH
C CH2
OH
H
C H
O
- H2O
(spontaneously)
CH CH C H
O
CINNAMALDEHYDE
8. Step 1: Deprotonation and formation of nucleophile.
• Step 2: Attack by nucleophile: Formation of alkoxide
ion.
9. Step 3: Hydrolysis and formation of Aldol type product
Step 4: Intramolecular dehydration
10. The aldol condensation is very useful in the
preparation of large molecules from simple starting
materials.
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-
hydroxy hexanal
11. CH2 CH C H
O
OHO
P
OH
OO
CH2 C CH2 O P OH
OH
O O
O
CH2
CH C CH C CH2
O P OH
O
O
O
OH
OH
HOHO
P
OH
O O
+
Glyceraldehyde-3-phosphate
Dihydroxyacetone phosphate
Fructose-1,6-diphosphate
a
ALDOL
CONDENSATION
enzyme
