Aldol Condensation Lab Report PDF

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Aldol Condensation Report. Made a 20/22. ...

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Aldol Condensation Synthesis of Dibenzalacetone Lead Author: Olivia Rye Reviewer: Kristen Rutledge Editor: Christina Dabit

Introduction An aldol condensation is a condensation reaction that involves an enol or an enolate ion reacting with a carbonyl compound that will form two products, an aldehyde or a ketone.1 Aldol condensation is good because they lead to the formation of new carbon-carbon bonds. The formation of new carbon-carbon bonds is usually very hard to accomplish. In the aldol condensation reaction, there are hydrogens, called alpha-hydrogens. They are located on the carbon that is connected to the carbonyl carbon.2 In order to remove the alpha-hydrogen, a base is needed because hydrogens are highly acidic, due to the resonance present with the carbonyl group. The conjugate base has an anion on the carbon that assists with stability. Acetone has alpha hydrogens on both sides to the carbon, thus when it reacts with benzaldehyde in a strong base like sodium hydroxide, it deprotonates to form the enolate anion. The intermediate produced is alkoxide. Once the intermediate is protonated by a beta-hydroxyketone, this is known as a base catalyzed dehydration.3 The aldol condensation reaction using acetone, sodium hydroxide, and benzaldehyde forms a yellow precipitate when done correctly in the proper order. If the reaction is not carried out correctly the by-product Cannizzaro can form.1 Cannizzaro forms a white precipitate. The aldol condensation reaction can be found in figure 1, with the mechanism for the reaction performed in figure 2, and the Cannizzaro reaction in figure 3. Figure 1. Aldol Condensation Reaction

Figure 1 demonstrates the chemical equation for an aldol condensation reaction.

Figure 2. Aldol Condensation Mechanism

Figure 2 shows the mechanism for the aldol condensation reaction. Figure 3. Cannizzaro Reaction

Figure 3 shows the equation for the formation of Cannizzaro, which is not a favored product in the reaction. Experimental To begin the experiment 0.1 mL of acetone and 3 mL of aqueous ethanolic NaOH were added to a small Erlenmeyer flask. The flask was then swirled for 3 to 4 minutes. 0.3 mL of benzaldehyde, a spin bar, and a cap with a stopper was added and was stirred for 30 minutes. A yellow precipitate formed and was filtered out by vacuum filtration. A 25 mL sidearm flask and a Hirsch funnel were used to separate the solid. The solid was then washed at least three times with 1 mL of water. After rinsing the water wash pH was tested for basicity. If the water wash was not neutral the rinsing steps were repeated. The solid was then allowed to dry and weighed to determine the percent yield, the melting point, and to run an IR spectroscopy.

Results

Table 1. Melting points Name Melting Point (oC) (cis, cis) Dibenzalacetone Below 25 (cis, trans) Dibenzalacetone 60 (trans, trans) Dibenzalacetone 110-11 Product 107.6-110.4 Table 2 shows the melting points of the isomers of dibenzalacetone that could have been formed during the experiment along with the product that was produced.

Discussion Most experiments only give one opportunity to see if the right product formed, but in an aldol condensation, there are two opportunities to see if the correct product has formed. Usually, the melting point, NMR, or IR are primarily used to check. For the aldol condensation, the first way to check to see if the right product was produced occurred while the mixture was being stirred. During the stirring process, a yellow precipitate formed, confirming the correct product was produced. The incorrect product would have produced a white precipitate, meaning the byproduct Cannizzaro was formed. The white product is not favored so that is an early way to tell the experiment has gone wrong and the solutions were not added in the correct order. Once the solid formed it was vacuumed with a Hirsch funnel to separate the product. The solid was then washed with ethanol and water. These two solutions were used to separate the pure dibenzalacetone, which is insoluble in these solutions, but the impurities are not. The impurities that come from side reaction are soluble in these solutions and can be separated. Conclusion The yellow solid that formed was identified as (trans, trans) dibenzalacetone. The physical/qualitative properties during the experiment led to the determination of the final

product. The yellow precipitate that formed indicated the aldol condensation took place correctly because the white solid did not form. After the solid was separated and purified the melting point was taken and recorded as 107.6-110.4oC. Since the melting point was a little under the specified range, it was understood that there were some impurities. Also, the percent yield was over 100% further indicating there were remaining impurities.

References 1

Hill, R.; Barbaro, J.; Experiments in Organic Chemistry. 3rdEd.; Contemporary Publishing Company of Raleigh: Raleigh, NC, 2005; pp. E14-1 – E14-7

2

Whal, George, et al. “Experiement 6-Aldol Condensations.” Advanced Industrial systems, Inc. 87.7 (2011): 714-716.

3

“Aldol Synthesis of Dibenzalacetone, an Organic Sun Screen”. Chemguide. Nov. 2015. Web. Accessed 28 Oct 2016....