Lab 8: Preparation of Dibenzylideneacetone
(Dibenzalacetone)
from K. L. Williamson, Macroscale and Microscale Organic
Experiments, 2nd Ed. 1994, Houghton Mifflin, Boston d. p522;
revised 10/10/98, adapted from the microscale synthesis posted
The reaction of an aldehyde with a ketone employing sodium
hydroxide as the base is an example of a mixed aldol
condensation reaction. This is also called the Claisen-Schmidt
reaction. Dibenzalacetone can be prepared by condensation of 1
mole of acetone with two moles of benzaldehyde. The intermediate
β-hydroxyketone, undergoes a base catalyzed dehydration step in
the presence of base, NaOH.
O
O
O
NaOH
H
2
+
CH3
CH3
dibenzalacetone
Synthesis of Dibenzalacetone
Obtain a sample of fresh benzaldehyde. Weigh 2.9 g
(27 mmol) of benzaldehyde directly into a 50-100 mL
round bottom flask. Add 15 mL 95% ethanol and 20 mL of
3M sodium hydroxide solution. Using a 1-mL pipet, add
0.9 mL (12 mmol) of acetone. Stopper the flask and
shake the mixture vigorously. Continue to shake the RBflask from time to time for the next 30 minutes. You
should see a pale yellow solution as time goes on. This
solution will start to become cloudy, and a yellow
precipitate will start to form. Shake the flask ever 23 minutes, for 30 min. If you don’t see crystals, then
open the flask and scratch the inside of the flask with
a glass rod or a spatula. If necessary, seed the flask
with some one else’s crystals. BE PATIENT!! Also try
cooling the flask in an ice bath to aid the formation
of crystals.
Isolation and Purification
Filter the solid from the flask using a Buchner
funnel. Wash the crystals 3 X with 5ml each of cold
water.
Squeeze the product between sheets of filter paper
to dry it, and then recrystallize the crude product
using a minimum amount of 70:30 mixture of
ethanol:water in a 50 mL Erlenmeyer flask. Remove the
boiling solution from the hot plate, and place it aside
to cool to room temperature. If the product separates
as an oil, get a seed crystal (from a neighbor), keep
heating the solution to dissolve the oil, and then add
the seed crystal. If it still continues to oil out, add
a little ethanol and heat and cool again. Keep the
flask containing your product on ice till you see
crystals form. You may need to scratch your flask with
a spatula to initiate crystallization.
Collect the product on a Buchner funnel. Wash the
crystals once with about 3 mL of ice-cold 70% ethanol.
Dry the product under vacuum by attaching the tube to
an aspirator for ~20 minutes.
Determine the weight of the dibenzalacetone and
its melting point, and calculate the percentage yield.
In a typical experiment the melting point should range
from l10.5 to 112°C.
In the last step of the aldol condensation, loss of
water from the β-hydroxyketone can form molecules in
which the alkene hydrogen atoms are either cis or trans
to each other. The name dibenzalacetone does not
completely characterize the molecules made in this
experiment. There are 3 isomeric dibenzalacetones, one
melting at 100-110 °C, λmax 330 nm, ε 34,300; another
melting at 60 °C, λmax 295 nm, ε 20,000; and a third, a
liquid with λmax 287 nm ε 11,000. Both the melting
points and the UV spectral data give some hints
regarding the structures of these molecules. The first
one is very symmetrical and can pack well into a
crystal lattice. The long wavelength of the ultraviolet
light absorption maximum and the high value of the
molar absorbance ε indicate a long, planar conjugated
system. The other two molecules are increasingly less
able to pack nicely into a crystal lattice or to have a
planar conjugated system.
In your Final Report, include the structures of the
three geometric isomers of dibenzalacetone (cis-cis;
cis-trans; trans-trans), and assign each one to the
three molecules described in the previous paragraph.
Post Lab Questions
1. If you take an IR of your product, you will notice a
C=O peak ~1639 cm-1. Why is the C=O peak showing up at
lower wavenumbers (1639 cm-1) than standard C=O
frequency (1715 cm-1)?
2. How would you change the procedures in this
experiment if you wished to synthesize benzalacetone,
C6H5CH=CHCOCH3? Benzalacetophenone, C6H5CH=CHCOC6H5?
3. The product formed has a pale yellow color. Why?