Experiment
10
Synthesis of Dibenzalacetone
Objectives
1) To synthesize dibenzalacetone by a base-catalyzed condensation reaction of acetone and
benzaldehyde.
2) To recrystallize and characterize the product by measuring its melting point.
Introduction
Condensations are reactions that add together two or more molecules, often with the
loss of a small molecule such as water or an alcohol. In its simplest form, the aldol
condensation combines two carbonyl compounds to give a β-hydroxy aldehyde or ketone. The
product is also called an aldol because it contains both an aldehyde and an alcohol group. The
base-catalyzed condensation of two molecules of acetaldehyde to give 3-hydroxybutanal is
shown in Equation 1.
Hydrogens adjacent to a carbonyl group are weakly acidic. Loss of a proton leads to a
resonance-stabilized enolate ion. The enolate ion is a strong nucleophile that adds to another
carbonyl group. Aldol products can be dehydrated under either acidic or basic conditions to
give ,β-unsaturated aldehydes and ketones. Although aldol condensations are often evenly
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balanced equilibria, the dehydration is usually exothermic, therefore driving the condensation
to completion. The base-catalyzed dehydration of 3-hydroxybutanal is shown in Equation 2.
A crossed-aldol condensation is an aldol condensation in which one aldehyde or ketone
adds to the carbonyl group of a different compound. The compounds used in a crossed aldol
condensation must be carefully selected to avoid unwanted product mixtures. Crossed aldol
condensations are most effective if only one of the reactants can form an enolate ion and the
other reactant is particularly electrophilic toward enolates. Benzaldehyde is an excellent
reagent for crossed aldols because it has no -hydrogens, so it cannot form an enolate. The
base-catalyzed condensation of acetaldehyde with benzaldehyde is shown in Equation 3.
Historically, the benzylidene group (=CHPh) has been called the benzal group because it
is easily added to a carbonyl compound by aldol condensation with benzaldehyde. For example,
the formation of dibenzalcyclohexanone is shown in Equation 4.
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In this experiment, acetone is used as the enolate-forming compound, adding to
benzaldehyde followed by dehydration to form a benzal group. Like cyclohexanone, acetone
has enolizable -hydrogens on both sides of the carbonyl group, so acetone can add to two
molecules of benzaldehyde. The condensation of acetone with two molecules of benzaldehyde
to give dibenzalacetone is shown is Equation 5. The systematic name for dibenzalacetone is 1,5diphenyl-1,4-pentadien-3-one.
Experimental Procedure
1) Place 0.9 g of benzaldehyde into a pre-weighed 10 mL round bottom flask. Record the
actual weight to at least the second decimal place.
2) Carefully add 0.3 mL of acetone from a burette.
3) Add 5 mL of 95% ethanol. Stir the mixture in an ice-water bath.
4) Over a period of 15 minutes, slowly add 2 mL of 40% NaOH solution to the mixture while
stirring thoroughly with a glass rod.
5) Remove the ice-water bath. Stir the mixture at room temperature for another 10 minutes.
6) Place approximately 10 g of crushed ice in a small beaker. Pour the reaction mixture into
the ice and stir until the precipitation occurs.
7) Collect the precipitate by vacuum filtration.
8) Wash the filtrate with distilled water. Test the last drop of water wash with red litmus
paper, continue the washing if it is still basic.
9) Transfer the precipitate into an Erlenmeyer flask.
10) Recrystallize the product using ethanol and water. Add 2-3 mL of ethanol and swirl the flask
in a steam bath. If the precipitate completely dissolves, add water dropwise until cloudiness
occurs. Re-heat the mixture and allow it to cool to room temperature. If the precipitate
does not dissolve completely, add more ethanol until a clear solution is obtained.
11) As the solution reaches to room temperature, the crystals should appear. Cool the flask in
ice-water bath for 5-10 minutes.
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12) Collect the crystals by vacuum filtration. Dry the crystals by spreading them thinly over a
clean watch glass. Stir the crystals occasionally to allow any remaining ethanol to evaporate.
13) Place the product in a pre-weighed plastic bag and record the weight of your product.
14) Determine the melting-point of your crystals.
Laboratory Safety Precaution
1)
Wear safety goggles and lab coat at all times while working in the laboratory.
2)
Acetone and ethanol are flammable and irritating. Keep away from flame or heat sources.
3)
Benzaldehyde is toxic and a suspected carcinogen.
4)
Aqueous sodium hydroxide is corrosive and toxic. Prevent eye, skin, and clothing contact.
5)
Wash your hands thoroughly with soap or detergent before leaving the laboratory.
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