Dibenzalacetone by Aldol Condensation
49
ALDOL SYNTHESIS of DIBENZALACETONE, AN ORGANIC ( SCREEN
Overview: The reaction of an aldehyde with a ketone employing sodium hydroxide as the base is
an example of a mixed aldol condensation reaction. You will do a double mixed-aldol
condensation reaction between acetone and benzaldehyde. Acetone has a-hydrogens (on both
sides) and thus can be deprotonated to give a nucleophilic enolate anion. The aldehyde carbonyl
is much more electrophilic than that of a ketone, and therefore reacts rapidly with the enolate. The
alkoxide produced is protonated by solvent, giving a b-hydroxyketone, which undergoes base-
catalyzed dehydration. The elimination process is particularly fast in this case because the alkene
is stabilized by conjugation to not only the carbonyl but also the benzene. In today’s experiment
you will use excess benzaldehyde, such that the aldol condensation can occur on both sides of the
ketone.
Mechanism for Aldol Condensation
H3CCCH3
O
1
Acetone
molar mass: 58 g/mol
density: 0.79g/mL
limiting reagent
+ 2 CH
O
Benzaldehyde
molar mass: 106 g/mol
density: 1.04 g/mL
an excess will be used
NaOH, H2O,
CH3CH2OH
CC
C
O
C
H
C
H
H H
original acetone
carbons
Dibenzalacetone
molar mass: 234 g/mol
melting point: somewhere
between 80 and 120ºC
1
H3CCC
H2
O
H
OH
H3CCCH2
O
C
H
O
CH
C
O
H3CC
H2
O
enolate
CH
C
O
H3CC
HO
H2O
alkoxide hydroxy-
ketone
OH
H H
CH
C
O
H3CC
H
HO
hydroxyenolate
H
C
C
O
H3CC
H
Benzalacetone
"enone"
H
C
C
O
C
H
H
CC
H
Dibenzalacetone
Step 1 Step 2 Step 3
Repeat
Steps 1-5
Again
Summary:
Step 1: Deprotonation (makes nucleophilic enolate)
Step 2: Attack by nucleophile on electrophile
Step 3: Protonate to give neutral hydroxy-ketone
Step 4: Deprotonate again (makes enolate)
Step 5: Eliminate hydroxide to generate alkene pi bond
Step 5
Step 4
