Aldol condensations are a way to make carbon-carbon bonds, thus they are important in organic synthesis. The aldol condensation involves the formation of an enolate anion, that is formed from an aldehyde or ketone. This enolate anion can act as a nucleophile due to resonance stability, and attack a molecule that is either the same compound or a different compound. The nucleophile attacks the electrophilic carbonyl carbon of an aldehyde or ketone. This will form a β-hydroxyaldehyde or β- hydroxyketone. Then, this can be dehydrated to form a conjugated system. 1
The aldol reaction can either be acid-catalyzed or base-catalyzed. The base catalysis is more commonly used. The most important step in the base-catalyzed version is the nucleophilic addition of the enolate anion to the carbonyl group, which forms the tetrahedral carbonyl addition intermediate. 1
In this reaction, carbon-carbon bonds were formed by the method of a base- catalyzed aldol condensation reaction. Acetone, benzaldehyde, and hydroxide were reacted to form dibenzalacetone. During this reaction, the acetone anion acts as a nucleophile, and attacks the carbonyl carbon on benzaldehyde. This forms a β- hydroxyketone. Hydroxide leaves and a double bond is formed. Then, this intermediate product acts as a nucleophile and attacks another benzaldehyde. Another β- hydroxyketone is formed. Hydroxide leaves again and forms another double bond. This forms the final product, dibenzalacetone. This reaction is shown in figure 1.
A downside to this reaction is the possibility of side products. In this reaction, side products can be formed by the side reaction called the Cannizzarro reaction. This reaction involves a simultaneous reduction and oxidation of the benzaldehyde-acetone tetrahedral intermediate. 2 A hydride ion acts as the leaving group, then a second molecule of benzaldehyde accepts the hydride ion in another nucleophilic addition. This forms two unfavorable side products, which are benzoic acid and benzyl alcohol. This reaction is shown in figure 2. All chemicals used in this experiment are shown in table 1.
Figure 1: Figure 1 shows the base-catalyzed aldol condensation reaction. Acetone, benzaldehyde, and hydroxide react to form dibenzalacetone.
In this experiment a base-catalyzed aldol condensation reaction was used to react acetone and benzaldehyde to form dibenzalacetone. A solid precipitate product was obtained from this reaction and the percent yield was calculated. The limiting reagent was calculated to be benzaldehyde. This calculation is shown as equation 1. Next, the actual yield was calculated. This equation is shown as equation 2. Finally, the percent yield was calculated, which was 176%. This is shown as equation 3.
Next, the pH of the final product solution was neutralized to a pH of about 7. Then, the melting point of the dried solid product was obtained. The melting point was a range from 111°C to 114°C. A summary of the product data collected is shown in table 2.
Equation 1 : Equation 1 shows the two calculations to determine the limiting reagent. Benzaldehyde is the limiting reagent, and 1 x 10-3 mol is the theoretical yield.
Equation 2: Equation 2 shows the calculation for the actual yield.
Equation 3 : Equation 3 shows the formula and the calculation for percent yield.
Table 2: Data for Dibenzalacetone
pH Mass (g) Percent Yield (%) Melting Point (°C)
~7 0 176 111 — 114.
Acetone, benzaldehyde and hydroxide were reacted to form dibenzalacetone. The product obtained from this reaction was a light yellow-white solid. Pure dibenzalacetone is a pale yellow solid, so the obtained data agrees with this. 4 If there were side products formed from the Cannizzaro reaction, the product would appear to be a thick white substance, but this did not occur. 5
The pH of the final solution was neutralized with water to ensure that all enolates and hydroxide ions present were neutral. 2 The final product solution was indicated to be neutral by testing with pH paper, which was an orange color, indicating a pH of around 7.
Next, the percent yield for the product was calculated. The percent yield was determined to be 176%. Several errors could have occurred to give this high and inaccurate percent yield. During the drying process, not all of the water and ethanol could have been dried from the product. This percent yield could have included some of the water and ethanol still in the product. Another error could have been the inaccurate weighing of the starting reactants and the final product, thus, giving a high percent yield.
Lastly, the melting point was taken on the final product. The melting point was a range from 111°C to 114°C. The melting point of pure benzalacetone is a range from 110°C to 111°C. Several factors could give this higher and broadened melting point range. Impurities such as water and ethanol could increase the melting point. 6 Also, if any side products were present this could have also increased the melting point.
During this experiment, an aldol condensation reaction was done with the reactants acetone and benzaldehyde. This reaction forms the product benzalacetone. A side reaction, called the Cannizzaro reaction, can occur during this reaction. This reaction would give the two products benzoic acid and benzyl alcohol.
In conclusion, benzalacetone was determined to be the major product. No major side products from the Cannizzaro reaction were observed. The visual appearance of the final product and the melting point confirmed this. The percent yield that was calculated was inaccurate due to errors.
More analysis tests could be done on the product, such as IR spectroscopy. This would show if side products from the Cannizzaro reaction, ethanol, or water were present in the final product. A way to improve this experiment would be to improve the drying method. This could be done by putting the product in a heat source to allow the product to dry completely.
