Lab 12 PDF

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Jason Chau Lab Partner – Brandon Broyles 10/17/14, CHEM 3105-390 TA – Yunxiang Li Experiment #12: Photochemical Isomerization of an Alkene Purpose: Illustrates the ease of cis-trans isomerization in organic molecules and, specifically in this case, demonstrate the isomerization of a trans alkene to the corresponding cis isomer via photochemical excitation.

Reaction & Physical Properties Table: Compound Trans-1, 2Dibenzoylethen e Ethanol

MW 236.27

Amount 25 mg

3.5 mL

Mmol 0.11

MP 111

BP

78.5

Safety: Ethanol Risk Statements: Irritating to skin, highly flammable, risk of serious damage to eyes. Safety Statements: Keep in a cool place, keep container dry, keep away from heat, and keep locked up. Trans-1,2-Dibenzoylethene Risk statements: Irritating to skin, highly flammable, risk of serious damage to eyes. Safety Statements: Keep goggles on at all times, keep away from skin, and don’t allow contact with eyes.

Procedure: 1. Add 25 mg. of Trans-1,2-Dibenzoylethene and 3 mL of 95% ethanol to a 50 mL beaker. a. LABEL the beaker 2. Warm this mixture gently until a homogeneous solution is obtained. 3. Put the beaker directly under the sunlamp (with help from TA) 4. Irradiate the solution for about 1hour. a. ***The progress of the isomerization will be followed by thin-layer chromatography analysis every 30 minutes.*** 5. Remove the beaker from the light source and let the solution cool to room temperature. 6. Move the remaining mixture to an ice bath to finish the crystallization of the clear cis-1,2-Dibenzoylethene product. 7. Collect the crystals by vacuum filtration with a Hirsch funnel. 8. Wash them with 0.5 mL of COLD ethanol. 9. Air-dry the crystals on a clay plate/filter paper. 10. Weigh the dried product and calculate percent yield. 11. Find the melting point, then compare results with the literature value. 12. Get the IR spectra of the cis and trans isomers and compare them to the figures given. 13. Discuss the similarities and differences of the experimentally derived spectral data to the reference spectra.

Data/Observations: The mixture didn’t take long to become homogeneous and was put under the sunlamp rather quickly. The Trans-1, 2-Dibenzoylethene was a solid yellow powder before mixing with the ethanol. The trans reaction changed to cis, when the bright yellow mixture began turning colorless. Distance Traveled on TLC Solvent: O min Dibenzoylethene: 30 min Dibenzoylethene: 60 min Dibenzoylethene: Rf Values for each Dibenzoylethene 0 min 30 min 60 min

4.2 cm 4.1 cm 3.1 cm & 3.8 cm 3.2 cm & 3.9 cm

0.976 0.738 & 0.905 0.762 & 0.929

Calculations: Rf value

Rf= d₁/d

d₁=distance traveled by analgesic d=distance traveled by solvent

Conclusion: After the completion of the experiment, the transformation from trans to cis was successful. We wanted to see the reaction from trans to cis and were able to track its progress with the TLC plates. On the TLC plates it went from one to two spots on the TLC showing that it transferred form trans to cis. One spot being trans and two spots being cis. Post Lab Questions 6-38. What are some clues that tell you that you have made a different isomer product in this reaction? - Some clues would be that the cis isomer is much more soluble in water, and melts at a lower temperature. The initial trans isomer shows different qualities than the cis. 6-39. Suggest a reason why the starting material (trans-1,4-diphenyl-2-butene-1,4-dione) should be recrystallized prior to use… - A reason may be that because if it is not crystallized, then few parts may change from cis, causing cis to give off different product as compared to trans. 6-40. cis and trans-1,4-diphenyl-2-butene-1,4-dione are stereoisomers. What is the specific relationship between the two of them? - They are geometric isomers. 6-41. Explain how this photochemical isomerization allows the production of the thermodynamically less stable cis isomer. In other words, why is the trans isomer exclusively converted to the cis isomer during short reaction periods and not vise-versa? Is it possible, under these conditions that the trans and cis isomers are in equilibrium with one another - The two isomers can be switched with photochemical isomerization. The Cis isomers are less stable than the trans isomers because of steric strain between the two substituents on the same side of the double bond. Because of this, the cis-isomer will relax back to the trans via cis-to-trans isomerization. The trans isomer is more stable by approximately 5x10^4 J/mol, and the barrier to photo-isomerization is approximately 2x10^5 J/mol....