CHEM 322: Crossed Aldol Condensation:
Synthesis of Dibenzalacetone (1,5-Diphenyl-1,4-pentadien-3-one)
Introduction:
In this experiment, you will perform a type of base-catalyzed crossed aldol condensation called
the Claisen-Schmidt reaction. The aldol reaction is used extensively to synthesize new C-C
bonds. In a crossed aldol synthesis, two different aldehydes or ketones (or one ketone and one
aldehyde) react in the presence of dilute base to yield β-hydroxyaldehydes or β-hydroxyketones.
In most base-catalyzed aldol reactions, the end product is an α,β-unsaturated aldehyde (or
ketone) and a separate molecule of water. The initial product results from the attack of an α-
carbon (in the form of an enolate ion) of aldehyde (or ketone) on an electropositive carbonyl of a
second aldehyde (or ketone). A Claisen-Schmidt reaction product always has at least one double
bond that is conjugated to both a carbonyl group and an aromatic ring. In this particular
experiment, you will be preparing dibenzalacetone, a compound that once was used as an active
ingredient in some sunscreen formulations because of its ability to absorb UV radiation. See the
overall reaction below:
O H
2+
ONaOH
H
H
O
H
H
Ethanol
The two organic starting materials have many uses outside of chemistry. Benzaldehyde is found
in almonds and almond paste which can be used in lots of cookies and nutritional bars recipes.
Some uses of acetone are 1) a paint and varnish remover; 2) a solvent in many varnishes, rubber
cements, lacquers, etc.; and 3) an indicator of metabolic disorders, like diabetes, if present at
high concentrations in the bloodstream because acetone is a natural metabolic byproduct
normally found in limited quantity in the body.
Normally, crossed aldol condensations produce 4 different products. However, several
precautions have been taken to make this reaction successful.
2.2 moles of benzaldehyde to 1 mole of acetone will be used instead of a 2.0 : 1 molar
ratio of benzaldehyde to acetone as required by the stoichiometry. A slight excess is
important because:
1) The benzaldehyde may be contaminated from partial oxidation to benzoic acid;
2) Enough benzaldehyde must be present to react with both methyl groups of
acetone, preventing contamination by monobenzalacetone.
Only one of the starting materials can undergo α-hydrogen abstraction to produce a
carbanion which can be stabilized via keto-enol tautomerism.
Conditions are right for a base-catalyzed dehydration of the β-dihydroxyketone formed as
one of the intermediates in the reaction. This dehydration is very fast due to the
stabilization gained by forming an extensive conjugated π-system.
