The Aldol Reaction – Preparation Of Trans-Benzalacetaphenone Experiment Report

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The goal of the experiment was to prepare trans-benzalacetophenone, benzalacetophenone with aniline, and benzalacetaphenone dibromide. During the experiment some observations were made, such as colour change and crystal formation. The melting points and percent yields were measured, and IR spectrum was studied. Introduction The purpose of the experiment was to study the aldol reaction. In order to do that the unsaturated ketone was prepared through the aldol dehydration. Another name for unsaturated ketone is enol. It is a molecule that contains a bond between and carbons.

The aldol condensation reaction is used to react the enol and a carbonyl compound. The unsaturated ketone will be formed as the result. The conjugate enone forms after because of the dehydration reaction. In the field of organic synthesis, the aldol reactions are very popular because of their ability to make carbon-carbon bonds. There is one type aldol condensation which is cross-aldol dehydration where the non-enolizable ketone/aldehyde which has sterically unhindered carbonyl carbon reacts with enolizable ketone/aldehyde. It reacts in the existence of a catalyst to make unsaturated ketone or aldehyde. This experiment actually showed the reaction of benzaldehyde with acetophenone, and it’s mechanism.

The simplest aromatic ketone is acetophenone. This ketone is used in many chemical areas, for example drug making. One of the drugs that uses this molecule is dextropropoxyphene, which is designed to treat mild pain. The simplest aromatic aldehyde is benzaldehyde. It is highly popular in the food industry because it is used to make an almond taste. In this experiments trans-benzalacetaphenone, aniline benzalacetaphenone, and benzalacetaphenone dibromide will be made. The melting point and percent yield will be measure for each of them. The IR spectrum will be studied for each of them as well. The structures of aniline benzalacetaphenone and benzalacetaphenone dibromide follows accordingly:

Preparation of trans-benzalacetaphenone

The 50 mL flask was used to mix 6.2 mL of 3 M NaOH, 3.8 mL of 95 % ethanol, and 0.0125 mol of acetophenone. The mixture was cooled in the ice-bath. Then, while swirling on the magnetic stir the equimolar amount of benzaldehyde was added. The solution was kept in the room temperature while swirling for an hour. After, two spatula tips of seed crystals of benzalacetaphenone were added. The solution was heated up on the steam bath and cooled again. The crystallization appeared. The crystals were washed with cold water and cold 95% ethanol. Around 0.5 g of the solution was left in order to use it for the next part of the experiment. The rest of the solution was used in order to find the melting point and to determine the IR spectra.

Preparation of aniline benzalacetaphenone

0.5 g of trans – benzalacetophenone was dissolved in 10 mL of 95 % ethanol. The equimolar amount of aniline was added to the solution. The solution was well shacked and left for 1 week to form the crystals. When the crystals formed they were washed, collected, and melting point was measured. Some of the crystals were used to obtain the IR spectra. Preparation of benzalacetaphenone dibromide 0.5 g of dry trans-benzalacetaphenone were dissolved in 2 mL of dichloromethane in a flask. The solution was cooled in the ice-bath, and bromide was a added dropwise until the colour became dark orange and permanent. The mixture was heated on the steam bath, until all the dichloromethane was evaporated. Then, the sides of the flask were scratched with the spatula, until the crystals formed. The crystals were washed and dried. The melting point was obtained as well as IR spectra.

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Discussion

The purpose of the experiment was to study the aldol reactions. Trans-benzalacetophenone was prepared as well as benzalacetophenone dibromide. Dehydration reaction was happening in order to prepare the (alpha, beta)-unsaturated ketone. First, ethanol was mixed with sodium hydroxide and acetophenone. This was an acid-base reaction where NaOH was a strong base which has reacted with hydrogen that was alpha to a carbonyl group of acetophenone. The resonance stabilization happened and acetophenone enolate formed right after. The reaction completion was observed when the colour changed from clear to pale yellow. After the benzaldehyde was added to this solution, and was attacked by the enolate. Enolate attacked the benzaldehyde at the carbonyl group. Ketones are less electrophilic than aldehydes, so that resulted in nucleophilic acetophenone to react only with aldehyde. Benzalacetophenone was produced with the addition of aldol. The reaction was performed under the equilibrating conditions, so the trans alkenes were produced. Also, the lower energy alkene was obtained. The completion of the reaction was noticed by the change of colour from pale yellow to white. Ice-water bath was used to cool the reaction, and when the reaction was cooled the crystallization took place. Ice cold ethanol and cold water were used to wash the (alpha, beta)-unsaturated carbonyl compound. Ethanol was used for the purification purposes, and water was used for the better precipitation. The mass of the collected crystals was measured to be 0.97 g which is 18.46 % of the theoretical yield of 5.2 g. Some experimental errors leaded to the law percent yield.

The first error could come from no taking enough time to let the solution to stay in the ice bath. The solution was left in the room temperature instead, which could result in not full precipitation. Second error could come from the transferring the solution to the funnel for the filtration. Some amount of product could be left in the flask, and some of on in the filter paper. The melting temperature of the product was measured to be in the 40 0C – 50 0C range, which is a little lower than the literature value which is 55 0C – 57 0C.

The second part of the experiment was made by adding aniline to the benzalacetophenone. The solution was left for 1 week at room temperature. When the reaction was completed white crystals formed. The reaction happened between the nucleophilic amine group from aniline and beta carbon from benzalacetophenone. As a result, 3-(N-phenylamine)-1,3-diphenyl-1-propanone was made. The percent yield of the resulting product was 72.60 %, and the melting point was measured to be 165 0C – 175 0C. The lower percent yield in this case is the result of experimental errors as well. The errors could come from the not occurred measurement of reagents. The final part of the experiment was to produce benzalacetophenone dibromide. The achieved product was (alfa)-bromo-(beta)-phenyl benzalacetophenone. The solvent for this experiment was dichloromethane because it has polar properties, which makes it to evaporate quickly. The bromide reacted with the double bond in order to induce the dipole moment. The melting temperature of benzalacetophenone dibromide was measured to be 140 0C – 155 0C. The percent yield was measured to be 82.95 %. There were also some side reactions happening together with the main reaction. They were also the reason for the lower percent yields of the main products.

For this experiment one of the side reaction was the formation of flavanones from the 2’ – hydroxy functional group. This could happen because of the high base concentration. Other side reactions that could happen include the following: rearrangements, polymerization, and multiple condensations. The purity levels could also be affected by those side reactions. The IR spectrum which is attached in the “Results” part was also studied. For the first product (trans-benzalacetophenone) it shows peaks at 3083.16 cm-1, 3050.66 cm-1, and 3023.70 cm-1. Those peaks represent the C-H stretch in aromatic compounds. Another strongly marked peaks located at 490.90 cm-1, 987.86 cm-1, and 1447.01 cm-1. A peak at 1447 cm-1 shows a medium C-C bond stretch in the aromatic ring. The peak at 686.16 cm-1 represents C-H bonds in aromatic compounds.

Overall, the IR spectrum showed that trans-benzalacetaphenone is an aromatic compound. Second IR spectra was made for the compound achieved from addition of aniline to benzalacetophenone. The peak at 3384.18 cm-1 showed the presence of amine (N-H bond) in the compound. There are also couple peaks at the 3000 cm-1 region, which represent the presence of C-H stretches in the aromatic compound. The is also a strong peak at 1600.30 cm-1 which is due to C = O stretch, which shows , - unsaturated ketone. The C-N bond on the aromatic amine is represented by the peak at 1287.94 cm-1. Finally, C-C bond is represented by 1447.62 cm-1.

Overall, this IR spectra shows the ring structure, amine group, and presence of a ketone. This is all because of the presence of aniline. The third IR spectra was made for the benzalacetophenone dibromide. It shows a peak at 3026.76 cm-1 which is due to the presence of C-H bonds in aromatics. There is also a peak at 1601.86 cm-1 which shows C=O bond, which comes from ,-unsaturated ketone. There is another peak at 564.97 cm-1 which is due to the presence of C-Br bond. One more peak is at 1446.71 cm-1 shows the presence of C-C bond. Overall, it means that the addition of bromine to benzalacetophenone resulted in C-Br bond, aromatic compound, and presence of a ketone.

Conclusion

There were three main products obtained in this experiment: trans-benzalacetophenon, benzalacetophenon with aniline, and benzalacetaphenon dibromide. The melting points for those points were measured: 40 0C – 50 0C, 165 0C – 175 0C, 140 – 155 0C respectively. The present yield for the first product was 14 %, for the second 72.60 %, and for the third 82.95 %. The IR spectrum for all three products were studied as well.

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