Conclusion expod. lab PDF

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Discussion explaining observations, equations, mechanism, sources of errors for the different aspects and techniques, conclusion. For the epoxidation lab, our purpose was to understand and demonstrate the synthesis of 5a, 6a-Epoxycholestan-3B-ol via epoxidation of cholesterol. The experiment was an overall success since we understand why each observation looks after a procedural step and shows the equations and mechanisms that will be listed on the last page. For the epoxidation lab, the appearance of each starting material is that cholesterol is a white powdery solid weighed at 66 mg and MCPBA is a white flaky solid weighed at 39 mg. The 1st step is to combine cholesterol and MCPBA into a single test tube then add dichloromethane to the test tube with cholesterol and MCPBA then make sure the components are dissolved by gently swirling the solution. The observation when dissolved in methylene chloride is a clear colorless solution. Next, take a TLC capillary and spot the reaction mixture which is going to be spot 1. There might be repeated applications that are necessary to let the initial spot dry before spotting an additional time. Next, the reaction will run at 40 degrees C for 30 mins where the reaction mixture will be a clear, colorless solution. Some solvent loss may occur if the temperature reaches significantly higher than the boiling point of dichloromethane which is 40 degrees C. Now, we would spot the final reaction mixture on the TLC plate which is spot 2 after heating the test tube. One spot is necessary due to previous results where two spots were too concentrated. It is important to note that the boiling point of methylene chloride is low and highly volatile since it evaporates quickly when reaction mixture is spotted. Next, we will weigh out the mass of the two empty test tubes which is test tube 1 being 2.107 g and test tube 2 being 2.073 g. Next, we would pipette columns for purification of products where the column removes 3-chlorobenzoic acid. We would spot the chromatograph product which is spot 3 on TLC plate where the fractions are clear and colorless. The final/purified product that is spotted should be fairly dilute in this solution where multiple spotting applications will be needed even though efforts are made to concentrate the solution by evaporating some of the ether. After evaporating ether from fractions, the combined difference of weights for % yield calculation is that test tube 1 has residue of 2.127 g and test tube 2 with residue is 2.095g. To find the amount of residue, subtract the empty test tube weight for 1 and 2 from test tube 1 and 2 with residue. We would then develop the TLC plate with the eluent as tert butyl methyl ether where the observation is shown by the sketch and measurements shown in TLC data. The proper setup of the chamber will include needing filter paper with enough solvent but not too much (4 mm high). TLC plate should be placed straight in and level on the bottom and not be touching filter paper. Chamber should have a cover and let the solvent go up to determine the solvent front. Then, put under UV visualization for conjugated compounds where the visualization is with iodine chamber. We would place the TLC plate face down in the iodine/silica mixture and cap the chamber. We let sit for 5 minutes for unconjugated compounds. We would compare and contrast TLC spots where spots b,c,f are visible under UV and spots a,d,e,g,h visible with I2 vapor. Spots of starting material and products from TLC plate also indicated polarity. MCPBA and 3-chlorobenzoic acid are UV active while cholesterol and

cholesterol epoxide is not UV active and need I2 vapor. The last step is to perform recrystallization to remove excess starting materials and any side product using a solvent pair which is acetone and water. Acetone is a better solvent due to low boiling point. Add water to the crystal solution in acetone until first signs of cloudiness and cool slowly to room temperature. Lastly, place in ice to collect, wash, dry and weigh then take the melting point. The recrystallized residue from test tube 2 is weighed to be 17 mg after dry crystals looking white needles like crystalline solid. The melting point of the crystal is 145-148 degrees C. In this epoxidation experiment, a source of error is having collecting fractions in preweighed test tubes and eluent being tert-butyl methyl ether where we cannot recover all products. Some sources of error for TLC are contamination and error in techniques. Some examples are making spots that are too large that can lead to streaking, using the wrong solvent (too low or high polarity) as eluent, not using a mixture that is soluble in elute, using a pen to mark spots instead of a pencil, not using capillary tubes as a spotter, the pair of solvents are immiscible, the sample is too concentrated, spots too close to edge, flaking adsorbent that can lead to a gap forming that may stop the flow of the eluent or cause the eluent to run crookedly and lead to inaccurate Rfs. Sources of error for deviation of melting point values can result in inaccuracy of the identity of the product because the melting point in the experiment is lower than the literature melting point. In the epoxidation experiment, the observed melting point is 145-148 degrees C but the literature melting point for pure cholesterol epoxide is 148-150 degrees C. This means that there are impurities in the melting point in the experiment. Some sources of error for melting point can be impurities crashing out of solution from fast cooling, when cooling solution is distributed, crystals are too going and proper drying is not adhered to, using old starting material (not brand new), 40 minutes not adhered too, and crystals didn’t go to completion. Our melting point can also not agree with literature temperature due to moisture in the air absorbed by the sample, contamination of the sample, determination made too fast (heating), and criteria for filling melting point capillaries not adhered to. Sources of error for % yield due to side products could be the formation of cholesterol epoxide with wedges or diastereomeric b-form of the cholesterol epoxide (5a, 6a-Epoxycholestan-3B-ol) which is a diastereomer and reaction not going to completion due to heating or drying of crystals. Side products could have contaminated samples and affect yield. The balanced equations and mechanisms for the side products are shown below. Sources of error for % recovery in recrystallization would be mostly technique errors, if crystal solution is not cooled down in ice, if wet, solute adheres to filter paper, if solute did not precipitate completely from cold solution, and if minimum amount of solvent was not used (i.e., solution not saturated). Another reason why we would not recover the maximum amount of product is not cooling crystals to room temperature and not placing test tubes in ice. Sources of error for purity include formation of side products that contaminated product samples, reaction not going to completion due to heating too fast, and mostly purification techniques. Other sources of error for purity includes if impurities cannot be removed due to solvent not having proper characteristics, if impurities formed due to cooling solution being disturbed, if solvent not completely removed, if impurities crashed out of solution due to fast cooling, and if wet, filter paper could adhere to crystals as they are scraped off with a spatula. In conclusion, the

experiment for epoxidation is an overall success since we understand our sources of error for different techniques and we achieved our purpose in understanding the synthesis of 5a, 6aEpoxycholestan-3B-ol via epoxidation of cholesterol. Observations that resulted in reaction: Equations, reactions, and mechanisms: 1. Major Product → 5a, 6a-Epoxycholestan-3B-ol 2. Side Product → diastereomeric b-form of the cholesterol epoxide (5a, 6aEpoxycholestan-3B-ol)...