Experiment 4_ Addition Reaction Using NBS

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Feb 20, 2024

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1 10/2//21 Experiment 4: Addition Reaction Using NBS Observing the rate of the reaction depending on the amount of NBS added. Monitoring the appearance of product(s) and the disappearance of reactant(s) using TLC. Reaction Scheme: Hypothesis : As the concentration of NBS increases, the rate of reaction will increase because the number of particles in the system increases, resulting in more frequent collisions between the molecules.

2 Reagent Table : Reagent Molar Mass (mg/mmol) mmol Quantities needed Safety Trans-stilbene (C H ) ₁₄ ₁₂ 180.25 0.2 72.1 mg Irritant N-bromosuccinimide (NBS) (C H BrNO ) ₄ ₄ ₂ 177.99 0.2 / 0.8 35.6 mg / 142.4 mg Oxidizer, corrosive, irritant, environmental hazard Succinimide 99.09 N/A N/A Irritant Water (H O) ₂ N/A N/A 〜 6-8 drops 10% DMSO (Dimethyl sulfoxide C H OS) / THF ₂ ₆ (Tetrahydrofuran C 4 H 8 O) N/A N/A 〜 6 mL Irritant, flammable, health hazard

3 Hexane (C H ) ₆ ₁₄ 86.18 N/A 〜 10.5 mL Irritant, flammable Ethyl Acetate ( C 4 H 8 O 2 ) 88.11 N/A 〜 4.5 mL Irritant, flammable Procedure : Small Group Work : 1. Working in as part of a small group, obtain the following samples for spotting on TLC plates: a. Trans-stilbene b. N-bromosuccinimide (NBS) c. Succinimide d. The THF/DMSO solvent system your reaction will be running in 2. With GSI, figure out which TLC developing solvent system your group will be responsible for testing. DS system: 100% hexane and/or 90% hexane/10% EtOAc 3. Set up two TLC chambers (one for each solvent system) with assigned developing solvent. a. May have to dissolve compound in lab if not done so already 4. Prepare two TLC plates with start lines. Then, carefully spot trans-stilbene, NBS, succinimide and the THF/DMSO mixture. Run the plates in your group’s developing solvent. 5. Visualize the TLC plates under UV light and trace each spot in pencil. Then use iodine to visualize the spots that cannot be seen under UV. 6. Calculate the Rf value for each spot in each lane. Rf = d/D = distance from start line to spot (cm) / total distance from start line to solvent line (cm)

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4 7. Develop/visualize the other plate using another method (ex. Potassium permanganate stain). 8. Record data in the data section of the lab notebook. Pair procedure : 1. Convert 0.2 mmol into “mg” for trans-stilbene (molar mass = 180.25 mg/mmol) = 0.03605 g 2. Weig h out this calculated amount of trans-stilbene and place that amount into a disposable capped (white caps) reaction vial. Repeat for a second vial and add a stir bar to each flask. 3. Add 3-4 drops of deionized water into 3 mL of the 10% DMSO/THS stock solution using a pipette. Add this solution to the reaction vial. Repeat for the second flask. Use a stir plate and stir until the trans-stilbene dissolves. 4. Label two TLC plates with three spots (shown in picture)

5 5. Spot the dissolved trans-stilbene on the “SM” mark on the TLC plate. 6. In one flask, add 1 equivalent (0.2 mmol) of NBS. In the other flask, add 4 equivalents (0.8 mmol) of NBS (molar mass = 177.99 mg/mmol). (See conversions to mg below): = 0.0356 g = 0.1424 g 7. Co ntinue stirring. The reaction will become more yellow after the NBS is added. 8. After adding NBS, wait 1 minute. Then, spot the reaction on your TLC plate for the 1 minute mark. 9. Allow the reaction to run for 30 minutes. Spot the reaction on your TLC plate. 10. Run the TLC plates in the solvent system you determined was the best. Visualize using UV and iodine as before. Best solvent system: 20% hexane, 80% ethyl acetate 11. Pour reaction into 40 mL of ice-water to stop the reaction. 12. Retrieve your stir bar, then dispose of all waste into the appropriate liquid waste container.

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7 Data :

9 Iodine stain visualization: Conclusion : Analysis for choosing the best developing solvent : When ranking the reaction compounds from the lowest to highest Rf values, the THF/DMSO solvent has the lowest Rf value, NBS has the second highest, and trans-stilbene has the highest Rf value. The more non-polar the molecule, the higher the Rf value. This is because non-polar molecules are soluble in nonpolar solvents, such as hexane, while polar molecules are more soluble in polar solvents such as NBS. The THF/DMSO spots are clearly shown still at the start line. When observed under the iodine stain, the THF/DMSO spots were the darkest, indicating that a high concentration of the solvent only stayed at the start line. The best developing solvent for resolving the compounds was 20% hexane and 80% ethyl acetate. We chose this as the best solvent because ethyl acetate is polar so it would allow the molecules to travel up the TLC plate more easily. DS stain Polar succinimid e Desired non- polar product Reactio n - only picked up by UV Nonpolar trans- stilbene

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10 NBS Analysis : If the concentration of NBS decreases over time, the spots would appear less pigmented because the reaction would shift to the right. If the concentration of NBS increases over time, the spots would be more pigmented because the reaction would shift to the left. This happens because of Le Chatelier’s principle where changes in concentration of a system will result in predictable and opposing changes in the system in order to reach a new equilibrium state. The new spots of the desired product (labeled in picture above) are shown halfway up the TLC plate. From the iodine stain, these spots are clearly identified with a light pigmentation on the 1 equivalent plate and a medium pigmentation on the 4 equivalent plate. The desired halohydrin product matches its polarity as non-polar molecules are able to move up the plate more. It sits between the polar succinimide and the non-polar trans-stilbene. Based on the TLC data, we can conclude that one of the spots is the desired product by using the process of elimination. By examining the polarities of the reactants and products, we can rule out which of the spots matches the desired product. With that, we can not however be certain in the identification of the desired product because many spots were present on the TLC plate that might have only shown up in one form of visualization method. TLC can also be inaccurate based on human error from spotting and/or the type of developing solvent. On the 4 equivalents of the NBS plate, the desired product spots appeared darker and more pigmented than the 1 equivalent plate. This shows that there was a higher concentration of product when the number of equivalents increased. This data supports my hypothesis. One thing that I would add to my hypothesis for further analysis is to include that the higher the concentration of NBS, the darker the spots, as a result of Le Chatelier’s principle.