3105-312 Exp. 12 - lab report PDF

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Isabelle Smola Lab Partner-Matthew Walraven 11.19.2020 CHEM 3105-312 TA-Koto Babaguchi Photochemical Isomerization of an Alkene Purpose: To be able to demonstrate the isomerization of an trans alkene to the corresponding cis isomer. To successfully preform and monitor a photoreaction by TLC. Reactions and Physical Properties Table:

Figure 1: The formation of Vitamin-D3 is promoted the absorption by UV light by 7-DHC.

Figure 3: Molecule shape changes as result of isomerization as a result of trigonal planar bonding about the double bonds

Figure 2: Molecule shape changing due to isomerization (bent to linear)

Figure 4: Endothermic reaction between molecules that does not obtain equilibrium

338.73 44.59

-114.1

Table 1.

Safety: The following is data obtained from the safety and documentation information made available at fishersci.com and kovaintl.com. Trans-1,2-dibenzoylethylene Risk Statements: Highly Flammable Skin/Eye Irritant Specific Target Organ Toxicity Safety Statement: If in contact with skin take off immediately all contaminated clothing and rinse skin with water/shower. If in contact with eyes rinse thoroughly with plenty of water for at least 15 minutes. If a fire occurs use powder or sand for extinction.

Ethanol Risk Statements: Flammable liquid Corrosive to metals Serious Skin Corrosion/Irritation Serious Eye Damage/Irritation Specific Target Organ Toxicity Safety Statements: If inhaled remove victim to fresh air and keep at rest in a position comfortable for breathing. If on skin take off immediately all contaminated clothing and rinse skin with water/shower. If in eyes rinse cautiously with water for several minutes. In case of fire use CO2, dry chemical, or foam for extinction. Physical safety: Be cautious using the hot plate as metal looks the same heated and cooled. Dispose of all chemicals to the proper waste container. Procedure: 1. Weigh out 0.025 g of trans-1,2-dibenzoylethylene and add it to the provided vials with black screw top lids. Add 3.5 mL of ethanol. 2. Warm on a hot plate with swirling to dissolve the yellow crystals. Once the crystals are dissolved lightly cap the vial and place it in the photoreactor. 3. Monitor the photoreaction by TLC approximately every 30 minutes for 1.5 hrs. More than one spot can be placed per TLC plate. Remember to rinse your spotting capillary between spotting spots. A sample solution of the starting material will be provided for comparison. The TLC plates can be developed using dichloromethane (CH2Cl2). 4. Measure the Rf values of the two isomers. Make note of any color change of the solution over time and any changes in the relative intensity of the spots on the TLC plates over the course of the reaction. 5. At the conclusion of the experiment put the solution mixture in the specially provided container. The cis isomer can be converted back to the trans isomer for use in future experiments. Return the vial to the hood for the next class.

Data/Observations: Length of TLC solvent- 3.7cm 1. 2.7cm 2. 2.3cm Length of TLC solvent- 4cm

1. 3.2cm 2. 2.8cm Length of TLC solvent- 4cm 1. 3.5cm 2. 3cm TLC plates were created during the experiment to determine the correct Rf value of the spotting solutions. A small amount of ethanol and a small amount of trans-1,2-dibenzoylethylene were mixed together and heated on a hot plate until the yellow crystals dissolved. A small amount of the solution was taken by using capillary tubes and then put into a dot of the TLC plate. Once the small dot was filled, the TLC plate was placed into the solution with the side dot closest to the solvent. The plate was pulled out of the solvent after the solvent traveled up the plate and reached 0.5 cm away from the top. After the plate was given time to dry the Ultraviolet light was then used to find how much further the spots had moved from each chemical up the TLC plate. After the solution in the vial was put back into the photoreactor for another 30 minutes and then the TLC plate recording was repeated. TLC was measured a total of 3 times every 30 minutes. Each time TLC was measured the solution gradually become more colorless. It started off yellow, became a light yellowish color, and then had no color. The one spot on the TLC plates created two spots after the procedure was performed. The first spot was larger while the second spot was smaller and gradually got a tiny amount smaller. Calculations and Results: To calculate Rf values =

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Rf Values- Solvent 3.7 cm 1.

0.234 0.534

,

5.2'6 5.2'6

Rf Values- Solvent 4 cm 2.

5.034 0.834 7'6

,

7'6

Rf Values- Solvent 4 cm 5.934 534 3. 7'6 , 7'6

Conclusion/Discussion: The transformation from a trans isomer to a cis isomer was successful in this experiment because on the TLC plate we were able to track the progress of the solvent. The spot went from one to two on each TLC plate which shows the transformation from a trans isomer to a cis isomer; one spot being trans and two spots being cis. On the last TLC plate there was a slight smudge of one

of the spots and this could be due to the capillary tube not being properly cleaned in between the TLC plates being performed on. The reaction demonstrates the relative abundance and ease of the process of photoisomerization that occurs in organic molecules. The experiment also demonstrates that different diastereomers have different physical properties. The photochemical excitement that occurred during this experiment changes the pi double bonds that are present to sigma single bonds and this allows the structure to hold together. As the solution sat longer in the lamp the bonds became more alerted and animated and this is why the dots that were lefts under the lamp longer traveled up more than the initial dots taken.

Post lab Questions:

1. Resveratrol undergoes photoisomerization when exposed to UV light. Answer the following: • Draw the isomer produced in the following reaction? • Identify which isomer (starting material or product) is trans and which is cis? • Which isomer is most stable?

OH

H H The isomer that is the starting material is the one in the trans-form and the isomer that is the product is the cis-form. The most stable isomer would be the trans isomer because there are two bulky groups on the opposite sides which makes the repulsion less.

2. The following compound undergoes photoisomerization upon exposure to light. What is the product of the following reaction?

OH

N

H2N

N

N

3. The pi electrons of an alkene can act as a base (see example below). Explain why the cisdibenzoylethylene produced in this experiment can be isomerized back into transdibenzoylethylene by heating an ethanol solution of cis-dibenzoylethylene with a few drops of H2SO4.

The cis alkene reacts with ethanol to form an alcohol product which in the presence of sulfuric acid is converted into a trans alkene and then acts as a dehydrating agent, so the isomers are possible. The cis can be isomerized back into trans because of the carbon-carbon single bond rotation and the trans is more stable than the cis....