Photochromism Lab Report 2

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University of Cincinnati, Main Campus *

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2040L

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Chemistry

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Dec 6, 2023

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Photochromism Lab Report Step 1- Aldol Condensation: Results Table and Mechanism: Compound Weight (g) Mols Theoretical Yield Percent Yield Metling Range Literature Melting Range Trans-4- nitrochalcon e 1.12 g 0.0044354 0.838 g 134% 160.1 o C – 160.7 o C 158 o C - 160 o C Discussion: The purpose of step 1 of the experiment was to use an aldol condensation reaction to synthesize a chalcone product. This was done by mixing 95% ethanol, 4-nitrobenzakdehyde, and acetophenone. Then, sodium hydroxide was added after mixing and the product was vacuum filtered. The ending product mass of 1.12 g was more than the theoretical amount of 0.838 g, which makes the percent yield of 134%. A melting range of 160.1 o C – 160.7 o C was taken of the largest crop. This melting range was slightly higher than the literature melting range, so there could be an indication of impurities. Some errors that occurred in this step could be the reason for impurities and a percent yield of over 100%. One error that occurred was a melting range was only taken of the final product, which was the combination of all the crops. Each crop should have been tested before combing, to eliminate impurities. The melting range was slightly over the literature melting range. Letting the reaction mix longer would ensure the reaction was completed before filtering. Step 2- Trans-Addition of Bromine: Results Tables and Mechanism:

Compound Weight (g) Mols Theoretical Yield Percent Yield Melting Range Dibromide 1.12 g 0.0027115 0.773 g 145% 148.5 o C – 152.8 o C TLC Plate: TLC Plate Distance Traveled (cm) Rf Value Solvent Front 5.8 N/A Chalcone .4 0.069 Dibromide 1.8 0.31 Dibromide Product 2.3 0.40 Discussion: The purpose of step 2 of this experiment was to use the chalcone, created in step 1, to create a dibromide product. This was done by mixing the chalcone from step 1 with chloroform and 2.5% bromine. Then, this ran for 45 minutes, and taken to a rotary evaporator to create the final product of this step. The final product weighed 1.12 g and was well over the theoretical amount of 0.773 g. The percent yield of this step was 145%. A TLC plate was run and the Rf values were calculated (shown above). The melting range taken of the product was within and slightly above the literature melting range, meaning there was some impurity, but the product was pure. The error and probable impurities were likely from the first step. The TLC plate shows a slight impurity, although it was run twice due to an error on my part. Making sure the solution was the same color of the bromine solution would have created less error, and letting the reaction fully take place before testing would have shown less error on the TLC plate. Step 3- Formation of an Aziridine: Results Table and Mechanism:

Compound Weight (g) Mols Theoretical Yield Percent Yield Melting Range Aziridine 0.224 g 0.00083 0.752 g 29.8% 132.1 o C - 136.7 o C Discussion: The purpose of step 3 of this experiment was to use the dibromide product, from step 2, to synthesize an aziridine product. This was done by combining the dibromide product with 95% ethanol and ammonia hydroxide. This was then left to react for an entire week (in the locker). The solution was vacuum filtered and recrystallized, ensuring the aziridine was isolated. The mass of the final product of step 3 was 0.224 g, which was less than the theoretical amount of 0.752g. The percent yield was 29.8%, which is significantly less than the last two steps, indicating error occurred. The melting range of the product was close to the literature melting point of 136 o C, meaning the minimal amount of product was pure. The error likely occurred during the recrystallization step. This step removed the impurities that were created in the last 2 steps which resulted in less product. Also, the methanol was not boiling/ bubbling when it was added to the product, so the product likely did not recrystallize quickly. This was also a reason for the loss of the product. Making sure the methanol was boiling would have led to an increase in product. Step 4- A Photochromic Imine: Results Table and Mechanism: Compound Weight (g) Mols Theoretical Yield Percent Yield Melting Range Photochromi c Imine .062 g 0.00079 0.187 g 33.2% 56.5 o C - 57.6 o C

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Discussion: The purpose of step 4 of this experiment was to use the aziridine product, from step 3, to synthesize a photochromatic imine. This was done by combining the aziridine with absolute ethanol, ammonium bromide, and benzaldehyde. That product was then combined with anhydrous ammonia and left to react, in the dark, for a week. The final solution was then vacuum filtered and recrystallized. The mass of the final product, photochromic imine, was 0.062 g, which was significantly less than the theoretical amount of 0.187 g. The percent yield was 33.2%. The meting range taken was close to the literature melting range of 57 o C . This indicates the product was pure. Any error that occurred likely happened before step 4 of this experiment. The low percent yield was likely because the amount of product was low in step 3. Another reason for error was during the saturation of the anhydrous ammonia, the flow of air was higher so the reaction could not have been saturated before it was capped. Some product could have been lost before covering and sealing the cap of the round bottom flask. Step 5- Photoproduction: Results and Mechanism:

Discussion: The purpose of step 5 of this experiment was to use the photochromatic imine, formed in step 4, to react with UV light. A negative was placed on top of the filter paper, and the light was placed on top of both for 30 seconds. This was done by dissolving the photochromic imine, from step 4, in boiling ethanol. Then the product was recrystallized and dissolved in dichloromethane. The product was absorbed with filter paper. 2 pieces of filter paper were used to absorb the product, as shown in the photo above, but one absorbed more than the other. The color of the photo reproduction reaction was dark blue, as it should have been. The color could have been darker but was not due to probable exposure to light. The hood I was working in was in the only part of the room that had a light on. I covered the product with foil as often as possible to avoid exposure to the light. Questions: 1. a. Being able to isolate the intermediate will indicate what the rate determining step is, also by looking at the slowest reaction in the mechanism. b. The rate determining step is the slow step, which is the elimination step of dehydration after the beta-hydroxyketone formation. c. The rate determining step can be sped up by increasing the concentration of the reactants. d. After the attacking of the benzaldehyde, the ring becomes a better nucleophile and will increase the rate of the reaction. By adding an electron donating group (like OH, OR, or R) to the para position, the reaction could be sped up even more. 2.

a. b. There would only be a single spot on the TLC plate because the enantiomers have extremely similar properties and have the same polarity. They will travel the same distance on the plate because they are equally attracted to the mobile and stationary phase. c. d. It is not possible to form the stereoisomers above, because the bromine always adds in the trans position. Because of that, the stereoisomer would not be allowed to be formed. 3. The bromonation reaction is stereoselective. Only one diastereomer will form because the cis-isomer is more sterically hindered meaning more of the trans-isomer is formed. 4. 5. We use ammonium bromide because it is a weak acid and NH 3 is its conjugate base. No new species would be created in the reaction because NH 3 is one of the reactants in step 4 of the reaction. If a stronger acid was used, it would probably protonate the aziridine reactant, making a whole different reaction occur.

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