Multistep Synthesis Chemistry Experiment

Multistep Synthesis of Tetraphenylcyclopentadienone

In this laboratory experiment a synthesis was performed through several separate steps. The purpose of the experiment was to synthesize tetraphenylcyclopentadienone from benzaldehyde and to run reactions on carbonyl containing compounds. There was a total of three steps that led up to the synthesis of the final product, tetraphenylcyclopentadienone. The first step of the experiment was the condensation of benzaldehyde to yield benzoin. Thiamine catalyst along with water and ethanol were added to the benzaldehyde, then NaOH was added until the solution turned yellow. After recrystallization, the product was benzoin. Step two was the oxidation of benzoin to benzil. Nitric acid was added to the benzoin and heated, this was followed by recrystallization to yield the benzil. In step three the benzil from step two was to be synthesized tetraphenylcyclopentadienone. The benzil and 1,3-diphenylacetone were mixed with ethanol. After heating this, ethanoic KOH was added and the solution was left to reflux. The crystals were washed and collected. This ended the multistep synthesis. The final product was not successfully synthesized due to sources of error during procedures. Therefore, a percent yield was unable to be found. The theoretical melting point for tetraphenylcyclopentadienone 218-220 degrees Celsius.

Introduction:

The first step of this experiment was a condensation reaction. Thiamine was the catalyst in this reaction. The thiazolium ion attacks the benzaldehyde. Thiazolium is made in situ by adding base, which was NaOH in this reaction. The thiazolium ion attacks the carbonyl of the benzaldehyde, this served as a nucleophile. This created a negative charge on the benzaldehyde, which attracted another molecule of benzaldehyde. The thiamine leaves the compound and a double oxygen bond is made, leaving this new compound, benzoin, neutral.

The second step was an oxidation reaction. Benzoin was oxidated to benzil. Nitric acid was the oxidating agent and it attacked the -COH bond on the benzoin, making a hydrogen leave the molecule. The molecule was then positively charged. The NHO2 then left the molecule and benzil was created. Benzil is symmetrical with two double oxygen bonds.

The final step includes a double aldol condensation reaction. 1,3-diphenylacetone is formed as an enolate anion and it attacked the benzil. This was the first condensation. Dehydration followed. This was the second condensation of the double aldol condensation. The result was the conjugated product of tetraphenylcyclopentadienone.

Experimental Section:

Benzoin: Benzoin was synthesized by the condensation of benzaldehyde. 1.485 g of benzaldehyde was placed in a round bottom flask with a magnetic stirrer. In a separate vial 225 mg of thiamine catalyst was dissolve in 0.67 ml water and 2.0 ml 95% ethanol. The benzaldehyde and thiamine solution were mixed. 3 M NaOH was added by drops and stirring until the solution turned a bright yellow color. When the solution was yellow, the flask was topped and placed in the locker to sit until the next laboratory period so the solution could react. After sitting over 24 hours, the solution in the flask had crystallized with wet, yellow crystals. The crystals were cooled in ice water to completely crystallize the product. The crystals were then filtered by using a Hirsch funnel. The crystals were washed three times with a 2 ml solution of half ice cold 50% ethanol and half water. When the crystals were washed, the yellow color of the crystals washed off, making the crystals white. The yellow color was the unreacted benzaldehyde. After the crystals were completely dry, the crystals were collected to be weighed and the melting point and IR spectrum were found.

Benzil: The benzoin from the first step was put into a round bottom flask with a magnetic stirrer. 1 mL of nitric acid was added to the benzoin. The benzoin absorbed the nitric acid. The flask of solution was then connected to an air condenser, and a vacuum was made from a tygon tube connected to a broken end of a glass pipet. The pipet was put into the top of the condenser and leaving no open spaces. The vacuum served to get rid of the nitrogen oxide gases that were formed during the oxidation reaction. The solution was heated for 30 minutes, beginning the time when the first sign of nitrogen oxide fumes were observed. After the 30 minutes, the solution was removed and cooled for a few minutes. The solution turned was a brownish-yellow color and all the crystal were dissolved, leaving a liquid. The solution was then transferred, using a Pasteur pipet, to 3 mL of water in a beaker. The reaction flask was rinsed to remove the remainder of the solution. The solution was stirred with a glass rod until room temperature of the solution was achieved. A yellow solid was to form, but instead the solution remained aqueous in the case of the specific experiment explained here. With additional scraping of the solution with a glass rod, no crystals formed at all. The next procedure, if the crystals had formed was to crush the solid with the glass rod and filter the solid until the crystals were dry. The mass would then be weighed and the crystals were to be recrystallized with 95% ethanol. The crystals were to be cooled in ice water to get full crystallization and then the crystals were to be filtered and air dried, then weighed.

Tetraphenylcyclopentadienone: Since the benzil did not form in the previous step for the, pure benzil from the stock room was used to continue with the experiment. 0.094 g of pure benzil was used and mixed with 1,3-diphenylacetone in equimolar amount (0.094 g), which were sticky yellow crystals, mixed with 1 mL ethanol in a flask and a magnetic stirrer was added to the flask. The flask was connected to a water condenser and heated until all of the solid was dissolved and when ethanol reached reflux temperature. Ethanoic KOH (0.15mL) was added through the top of the condensed using a pipet. The solution turned from dark orange to a very dark brown. The temperature of the sand bath was then increased and when a gentle refulx was observed the solution was heated for another 15 minutes.

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