Synthesis of Dibenzalacetone by the Aldol Condensation
Anna Shahrour
Jocelyn Barraza
TA Yong Wang
Section 5
Introduction
Aldol condensation
An aldol condensation is an organic reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to give a conjugated enone (Mahrwald, 2004).
The following equation is an example of an Aldol condensation:
Aldol condensations are important in organic synthesis, providing a good way to form carbon–carbon bonds. The Robinson annulation reaction sequence features an aldol condensation; the Wieland-Miescher ketone product is an important starting material for many organic syntheses. Aldol condensations are also commonly discussed in university level organic chemistry classes as a good bond-forming reaction that demonstrates important reaction mechanisms.In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β-hydroxy ketone, or "aldol" (aldehyde and alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals (Wade, 2005).
The name aldol condensation is also commonly used, especially in biochemistry, to refer to just the first (addition) stage of the process—the aldol reaction itself—as catalyzed by aldolases. However, the aldol reaction is not formally a condensation reaction because it does not involve the loss of a small molecule (Smith, 2001).
Mechanism of experiment (Weldegirma, 2012)
side reactions (Weldegirma, 2012)
Experimental section
In medium sized test tube: add 4 mL of 3M NaOH, 3 mL of 95% ethanol, 0.410 mL of benzaldehyde, 0.140 of acetone.
Take a TLC using hexanes:EtOAc (4:1) solvent and record the Rf.
Cap the tube and shake vigorously. Shake occasionally for 30 minutes. Take a TLC at the end of the period.
Remove the liquid from the tube and discard. Add 2 mL of H2O into the tube to wash the crystals and remove using a pipette. Repeat 3 times.
Collect the solid via vacuum filtration. Dry the crystals with squeezing between two filter papers
Recrystallize the dibenzalacetone with 2 mL HOT 70:30 ethanol-water mixture and dissolve.
Take the test tube from the steam bath and allow to cool to room temperature, and then cool in ice bath for 10 minutes.
Collect the product in vacuum filtration, and wash with 0.5 mL of ice-cold ethanol.
Weigh the product and obtain the melting point. Run an IR to determine the quality of the product.
Table of Chemicals
Chemical
Structure
Molar mass g.mol-1
Melting point °C
Hazards
Sodium hydroxide
40
318
Corrosive, alkaline
Ethanol
46
-114
Flammable
Benzaldehyde
106
-26
Irritant
Acetone
58
-95
Flammable, harmful
Dibenzalacetone
234
110
Irritant
Results
Melting point of product dibenzalacetone: 74°C
Weight of product: 0.313 g
Calculating the percent yield:
Every 2 moles (212 g) of benzaldehyde produce 1 mole dibenzalacetone (234 g), therefore 0.410 mL (×1.0415 g/mL = 0.427 g) will produce 0.471 g product
Every 1 mole (58 g) of acetone produce 1 mole dibenzalacetone (234 g), therefore 0.140 mL (×0.791 g/mL= 0.11074 g) will produce 0.447 g product
The limiting reactant is acetone, and the theoretical yield is 0.447g
Percent yield = actual yield / theoretical yield × 100%
Percent yield = 0.313 / 0.447 × 100% = 70%
Calculating Rf values: Rf = distance traveld by compound / distance traveled by solvent
Rf at the beginning of experiment (Benzaldehyde): distance traveled by compound =2.3 cm, distance traveled by solvent = 6 cm—
Rf=2.3/6= 0.383
Rf at the end of experiment (Dibenzalacetone): distance traveled by compound =4.5 cm, distance traveled by solvent = 6 cm—
Rf=4.5/6= 0.75
IR graph
Discussion
The melting point (74 °C) is very low compared to the literature value of dibenzalacetone melting point (110 °C). This could be an indication of low product quality and the presence of impurities in the crystals of the product.
The percent yield (70%) is fairly good and means the experiment was sufficiently efficient; however, the presence of impurities may mean that the actual yield may be lower than 70% taking the weight of the impurities into consideration.
Taking Rf values during the performance of the experiment helps track the progress of the reaction and determine the ending point of reaction. The fact that the Rf value increased from the beginning to the end of the experiment indicates the occurance of the reaction since the product (dibenzalacetone) is less polar than the reactants (acetone and benzaldehyde).
The IR spectrum indicates the presence of the desired product. The strong peak at 1700 cm-1 is an indication of the presence of the carbonyl functional group. The peak at 3000 cm-1 is an indication of the presence of the double bonds in the product, and the next weak signals are indicators of the presence of aromatic rings.
Question 1 page 121 of manual:
Conclusion
Dibenzalacetone can be synthesized from benzaldehyde and acetone by Aldol condensation. The experiment was fairly successful as the percent yield indicated the experiment was sufficient. The IR graph and the Rf value results also supported the completion of the reaction.
References:
Mahrwald, R. (2004). Modern Aldol Reactions 1, 2. Weinheim, Germany: Wiley-VCH. pp. 1218–1223.
Smith, M. B.; March, J. (2001). Advanced Organic Chemistry (5th ed.). New York: Wiley Interscience. pp. 1218–1223.
Wade, L. G. (2005). Organic Chemistry (6th ed.). Upper Saddle River, NJ: Prentice Hall. pp. 1056–1066.
Weldegirma, S. (2012). Experimental Organic Chemistry. Tampa, Florida: Cengage learning.
