6AL Micelle Lab

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University of California, Santa Barbara *

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6AL

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Chemistry

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Jan 9, 2024

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pdf

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1. What are three areas in which the reaction you performed adhered to the principles of green chemistry? Explain your reasoning. a. One of the primary ways the reaction performed in lab adhered to the principles of green chemistry is through the use of the surfactant TPGS-750-M. The surfactant is used to generate vesicles that contain insoluble reagents or the reduce the amount of organic solvent used. This way of carrying out a reaction greatly reduces the amount of solvent or and reagent being used, thus reduces the environmental impact due to common chemical practices. This step adheres to the principle of "safer solvents and auxiliaries." Additionally, carrying out reactions in a reaction vesicle brings the reagents closer together and increases the chance they will react with each other thus increasing the yield of the reaction. The use of lower amounts of solvents while at the same time obtaining greater amounts of desired product follows the principle of " reducing derivatives" as it reduces the need for other reagents that may pose a threat to the health of the experimenter and the environment. The third principle of green chemistry that the reaction performed in lab follows is that of "real-time analysis" which says that in-line analytical techniques should be performed during a reaction to detect changes in reaction conditions to prevent reactions from getting out of control. This proactive approach to chemical design allows experimenters to catch potential runaway reactions and correct them before they have a chance to get out of hand. Runaway reactions could lead to leakage into the environment of pose health risks to those involved. This is a common technique used in large scale manufacturing. In this experiment, TLC plates were used to track the progress of the reaction and demonstrates the principle of real time analysis. By first determining the reactant that proceeded the fastest first, we eliminated an intermediate step that would require even more use of reagents and solvents. The use of TLC analysis allowed us to minimize the use of excess materials. 2. What are two areas in which the reaction you performed did not adhere to the principles of green chemistry? a. One area of the reaction that did not adhere to the principles of green chemistry is the prolonged use of a hot plate. This does not demonstrate the principle of "design for energy efficiency." The reaction could potentially have been done in a room temperature vesicle provided there was a way to conduct the reaction albeit at a lower rate. The use of the hot plate is likely the result of the time constraint placed on students to complete the lab in the allotted time period, so a more relaxed environment might facilitate better adherence to the principles of green chemistry. Most of the U.S. power grid is supplied by the burning of fossil fuels, and so contributes to the addition of pollutants into the atmosphere as energy is produced at demand, rather than being stored for a later date. The other area that the reaction performed in lab does not adhere to the principles of green chemistry is the principle of "catalysis." Instead of using a stoichiometric catalyst like sodium carbonate, a reusable catalyst might have been better suited to reduce the amount of base used, especially considering that excess base was used.

3. What evidence do you have that TPGS-750M increases the reaction rate? How does TPGS-750M act to increase the reaction rate? a. The evidence that the addition of TPG-750M increased the rate of the reaction comes from the second TLC plate that was done. By comparing the reaction mixture of vial 1 against vial 3, a comparison between the addition of the surfactant (vial 1) or water (vial 3) can be done and a conclusion on the effectiveness on the addition of a surfactant can be drawn. The product this lab was seeking to produce is a non-polar compound, while one of the reactants, benzylamine, is a polar compound. The TLC plate used a 20:80 mixture of ethyl acetate and hexanes, thus polar compound will show up lower down, on the plate, and non-polar compound will travel further up the plate with the mostly non-polar solvent. The three spots represent benzylamine, 2-bromo-5-nitropyridine, and the product. Benzylamine had the lowest number of opportunities for hydrogen bonding with the slica gel, so it will travel the furthest up the TLC place. This means that the spot seen further up the plate likely represents benzylamine. Furthermore, the TLC analysis conducted shows a larger spot from vial 3 (control vial), representing more of the starting material being present, and thus a slower reaction rate. TPGS-750M increases the rate of the reaction by forming reaction vesicles in the solvent that brings the reagents closer together and increases the likelihood that they will react with each other. The addition of a surfactant allows for decreased solvent use and increases product yield while also allowing reaction that would normally be limited by the reagent solubility or reaction inefficiency to go closer to completion. 4. Did you correctly predict which 2-halo-5-nitropyridine would react the fastest? Discuss which substrate had the faster reaction rate and provide a plausible explanation, including which step in the mechanism is the rate limiting step. Which is more important in determining the rate of the reaction, the halogen's size or electronegativity? a. The prediction that 2-flouro-5-nitropyridine would react that fastest was correct. This is due to fluorine being a much more electronegative atom and will go through the SnAr reaction faster than bromine as the fluorine will more readily draw electrons away from the benzene ring making it a better electrophile and facilitate nucleophilic attack by benzylamine at a faster rate. The nucleophilic attack is the rate limiting step in this reaction. The halogen's electronegativity is the more important factor when predicting reaction rate as it has a larger affect on the rate limiting step of the reaction by affecting electrophilicity of the electrophile. In general, larger atoms are better leaving groups, however, a halogen's ability to withdraw electrons has a larger effect on the SnAr reaction due to the fact that the rate limiting step is dependent on the electrophilicity of the electrophile, which is more affected by the halogen's ability to withdraw electrons. 5. What was the yield for your isolated product? Draw the structure of your final product and state (and analyze) the evidence you have that you correctly isolated this product. As part of this discussion, you should print off and annotate your IR, as well as the spectra. a. The mass yield of the final product was 0.089 grams. This represents a percent yield of 39.2% compared to the theoretical yield of 0.227 grams calculated based on the amount of fluorinated reagent used. The annotated IR spectra below provides evidence for the identity of the product collected.

b. 6. State and analyze the evidence you have that your product is pure. a. On the basis of purity of the isolated compound, one of the largest pieces of evidence supporting it is that the product is pure is the thin melting point range. The melting point range obtained was 131-135 degrees celcius. With only a 4 degree range, the product

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isolated is likely very pure with very little impurities present in the crystalline structure. Additionally, the melting point range may be slightly inaccurate, as time is typically a constraint for obtaining a truly accurate melting point range, so the ideal rise of 2 degree celcius per minute is too slow for a reasonable trial during the allotted lab time slot. If time was not a limiting factor, the obtained melting range would likely be thinner. Furthermore, the IR spectra obtained lines up with the predicted product with multiple peaks present that support the conclusion that the isolated product was true to the prediction. If the reagents used were true to their identity, then the presence of the IR peak at ~3200 provides further support for the idea that the product obtained was a product of an SnAr reaction between benzylamine and the fluorinated reagant. A single broad peak here indicates the presence of a secondary amine.