WEEK 10:
ALDOL CONDENSATION REACTION
PURPOSE:
This experiment will introduce the student to one of the
many condensation reactions in Organic Chemistry. This
synthesis usually gives good yields with little difficulty.
IMPORTANT REACTION:
2
O
O
C
C
H
Benzaldehyde
(M.Mass 106.13
bp 178oC)
+
H3C
NaOH
CH3
Acetone
(M.Mass 58.08
bp 56oC)
O
H H
C C C C C
H H
dibenzalacetone
(M.Mass 234.30
mp 110.5-112oC)
BACKGROUND INFORMATION:
The aldol addition reaction is one in which two aldehyde
molecules react with one another to give a product which
contains both an aldehyde and alcohol function group, hence the
name ‘aldol’. A molecule of water can easily be lost from this
product giving a conjugated alpha-beta unsaturated aldehyde. If
this dehydration occurs, the reaction is called a condensation
reaction. The overall reaction in that case would be called an
aldol condensation. There are very many variations of reactions
that seem to have an aldol flavor and such reactions are often
called aldol reactions even though they are technically
different. The reaction in this experiment is really a ClaisenSchmidt reaction because it is a reaction of an aldehyde with a
ketone. Aldol type reactions will be thoroughly presented in
the lecture part of this course.
This broad class of reactions has a common mechanism. One
of the molecules has a mildly acidic hydrogen which can be
removed by a strong base to give a carbanion. The resulting
carbanion acts like a nucleophile and attacks a partially
positively charged carbon in a second molecule. The negative
charge on this addition product is removed in some way depending
on the nature of the starting molecules. One way would be to
add a proton. Another would be to eliminate an anion.
Aldol and related reactions are extremely important in
organic synthesis as they allow the chemist to construct larger
molecules from smaller ones. In these reactions, a carboncarbon bond is formed. Also of great importance is that the
product molecule can have many different useful functional
groups which can be transformed in subsequent reactions to other
useful products.
The strength of the base that is used is determined by the
acidity of the active hydrogen compound. Often the base of
choice is sodium hydroxide or sodium ethoxide which are
effective in removing a hydrogen that has a pKa in the 17-20
range. The alpha hydrogen to a carbonyl or ester group often
has such a pKa due, in part, to the resonance stabilization of
the resulting anion.
In the experiment to be done today, the alpha hydrogen on
an acetone molecule is removed by the hydroxide ion giving a
carbanion which attacks the carbonyl of a benzaldehyde molecule.
A hydrogen ion then adds to neutralize the adduct. A water
molecule is lost from this adduct to extend the conjugated
system. Remember that conjugated systems increase the stability
of a compound. An alpha hydrogen on the other methyl group of
the acetone molecule repeats the process described above with a
second benzaldehyde molecule giving the final product,
dibenzalacetone. Two molecules of water are formed as products
in this reaction. The final product is a nicely crystalline
yellow solid.
EXPERIMENTAL PROCEDURE:
Place sodium hydroxide (2.5 g) in a 100 mL Erlenmayer flask
and add water (25 mL) and ethanol (20 mL). Add a magnetic
stirrer bar and stir until all the NaOH has dissolved. To a
second reaction tube, add benzaldehyde (2.65 g, 0.025 mol) and
acetone (0.725 g, 0.0125 mol). Add approximately half of the
aldehyde-ketone solution to the sodium hydroxide solution, and
stir for 10 minutes with a watch glass covering the top of the
flask. After this time, add the remainder and stir for a further
10 mins. Filter off the product using a Buchner funnel and wash
the product with water until the washings no longer appear to be
basic (test using pH paper). The best way to do this is to
remove the vacuum tube from the filter flask, fill the Buchner
funnel with water, carefully stir the product with a glass rod
and re-apply the vacuum. Each time the funnel is filled with
water, add some litmus paper to the solution to test the pH.
When all traces of NaOH have been removed via water washing, dry
the product and record the mass and melting point of the crude
product. Then calculate the percent yield. Recrystallize the
product from the minimum amount of hot ethanol, recording the
mass, melting point and percent yield of the pure product.
IMPORTANT INFORMATION ABOUT THE REPORT:
The report for this experiment will follow the usual format
for synthesis experiments. Be sure the percent yield
calculation is carefully done. Be sure to pay attention to the
coefficients in the balanced equation when determining which
starting material is the limiting reagent. Record the melting
point range of the final product and compare that melting point
to the reported melting point of dibenzalacetone. Using these
data, discuss the relative success on the experiment.
END OF EXPERIMENT.
© 2007 STEPHEN ANDERSON AND ROBERT SHINE