ALDOL condensation
A property of carobnyl
compounds having hydrogen at
α-carbon
ALDOL condensation
• Aldol means aldehyde and alcohol groups on the same
molecule
• May occur between two aldehydes (aldols) or ketones
(ketols) in the presence of a catalytic base
• Reaction is only possible between two components
having α-hydrogen
ALDOL condensation
• Formation of aldols or ketols reactions are
reversible
• Mechanism: involves enolate formation
and nucleophilic attack at a carbonyl
carbon
• Aldols and ketols loose water in the
presence of an acid forming α-ßunsaturated aldehydes or ketones and ßhydroxy aldehydes or ketones
Aldol condensation
• Aldehydes containing α-hydrogen undergo self
addition in the presence of a base to form aldols
• For example
2 molecules of acetaldehyde condense to form
aldol haviving the name 3-hydroxy butanal
Mechanism
Step-1 One carbonyl partner with an alpha hydrogen atom is
converted by base into its enolate ion
O
C
R
H
OH
C
O
OH2
N u c le o p h ilic
Donor
C
:
:
O
C:
C
R
Step 2 This enolate ion
acts as a nucleophilic and
adds to the electrophilic
carbonyl group of the
acceptor partner
O
..
C
C
R
E le c tro p h ilic
Acceptor
O
..
C
:
OH2
O
C
R
C
C
OH
+
-O H
New C-C Bond
Step 3 addition of water resulting a neutral
condensation product
Mechanism of the Aldol Reaction
HO :
Base removes an
acidic alpha hydrogen
from one aldehyde
molecule, yielding
a resonancestabilized enolate ion.
O
H
C
C
H
H
H
:O: H
C
CH3
O
.. C
C
H H
+ H 2O
H
The enolate ion attacks
a second aldehyde
molecule in a
nucleophilic addition
reaction to give a
tetrahedral alkoxide ion
intermediate
H2O
Protonation of the alkoxide ion intermediate yields
neutral aldol product and regenerates the base
catalyst
Aldols loss water in the presence of
an acid forming α-ß-unsaturated
aldehydes
Heat/acid
Ketol condensation
• Like aldehydes, ketones having alpha
hydrogen condense to form ketols
For example
2 molecules of acetone condense to produce
4-hydroxy, 4-methyl, 2-pentanone
4-hydroxy, 4-methyl, 2-pentanone
4 methyl-3 pentene-2 one
Mixed aldol condensation
• If two different aldehydes condense, the
process is called mixed aldol condensation.
• For example
• Benzaldehyde and acetaldehyde condense
to form cinnamaldehyde