Experiment 15
Preparation of Dibenzalacetone
In this experiment we will prepare dibenzalacetone from acetone and two
equivalents of benzaldehyde under basic conditions. This is an example of a crossed-
aldol (or mixed-aldol) reaction. The overall reaction is shown in Figure 15.1 and a
detailed mechanism is given in Figure 15.2
C
O
H
CH3
C
O
CH3
NaOH CC
O
C
C C
H
H
H
H
2 H2O
Figure 15.1 Dibenalacetone Formation
+
+
dibenzalacetone
benzaldehyde acetone
Acetone enolizes in the strongly basic conditions. Note that benzaldehye cannot
enolize and so it must act as the electrophile. The nucleophilic alpha carbon then attacks
the carbonyl of benzaldehyde. After proton transfer there is loss of water to give the α,β-
unsaturated carbonyl that is stabilized by conjugation with the phenyl substituent. Notice
how the π-electrons of the phenyl ring are delocalized all the way onto the carbonyl and
onto the other carbonyl in the final dibenzylacetone product.
Figure 15.2 Mechanism for Dibenzalacetone Formation
C
O
H
CH3
C
O
CH2
H-OH
CH3
C
O
CH2
CH2
CH
O-
C
O
CH3
H O
H
CH
CH
OH
C
O
CH3
+-OH
H-OH
CH
HC
C
O
CH2
H
-OH
CH
HC
C
O
CH2
C
O
H
as before
dibenzylacetone
- H2O
Physical Constants
Compound
Mol. Wt (g/mol)
Density
(g/mL)
b.p. (°C)
m.p. (°C)
Acetone
58.08
0.791
56
-94
Benzaldehyde
106.12
1.044
178-179
-26
Dibenzalacetone
234.28
-
-
110-111 °C
