sample2-Carbocations+Oxidation+Discussion

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Although this lab was a little nontraditional in the sense that we were not given a set protocol to follow in order to obtain a specific desired outcome, it had many more applications to an actual research lab. The protocol we were given gave us the general direction in which to proceed, but we had to do our own background research and calculations in order to come up with a procedure to follow. Each group used a different catalyst to react either benzhydrol or benzyl alcohol. Using the guidelines from our peer leaders, the hints given on the protocol, and background research, each group was able to successfully react the alcohol with their assigned catalyst and obtain at least some of the expected, desired product.

Group one used CrO3 in Amberlite as the catalyst to oxidize benzyhdrol, a secondary alcohol, to benzohphenone, a ketone. We used 1.5 grams benzyhdrol and two molar equivalents of CrO3. However, our group was told to use one gram Amberlite. After refluxing our reaction for 60 minutes, TLC indicated that some of the benzhydrol was still present in solution. After further filtration evaporation of our solvent (dichloromethane), the TLC plate still showed a mixture of our product and reactant. The melting point of our product was taken as well. The melting point was between the melting points of benzhydrol and benzophenone. It was approximately 5 degrees Celsius below the melting point of benzhydrol, which makes us think that there may be more benzhydrol present, with the benzophenone acting as an impurity that caused the lowering of the melting point. After comparing our data to the data of another lab group who used CrO3 and three grams of Amberlite, it seems apparent that we did not use enough Amberlite in our reaction for it to proceed to (near) completion. The group that used three grams Amberlite had all product in their reaction when tested with TLC. Overall, this tells us that the ion exchange properties of Amberlite are an important part of the oxidation reaction.

Group two used pyridinium dichromate (PDC) as the oxidizing agent for benzyl alcohol (a primary alcohol). The expected product was benzaldehyde. With further oxidation, the product would then be benzoic acid. We started with 1.23 g of benzyl alcohol and 2.18 g of PDC. The mixture was a dark brown color. After refluxing the solution for approximately 35 minutes, the products were tested using thin layer chromatography. Once the TLC plates were spotted and developed, we could see that some of the reactant (benzyl alcohol) was still present in addition to the expected benzaldehyde. The product solution was then filtered using a column of silica gel. The dichloromethane was also evaporated from the solution. TLC was used again to test for the products. This TLC plate showed the same results as the previous one. Our product was a mixture of benzyl alcohol and benzaldehyde. Benzoic acid was not present. In order to obtain more of the desired product (benzaldehyde) in the future, we would have to increase the amount of PDC used and reflux the mixture for a longer time.

Group three used the performed oxidation of benzyl alcohol using the Collins reagent. The Collins reagent is a weaker oxidizing agent than CrO3 that will only oxidize primary alcohols to aldehydes rather than carboxylic acids. Therefore the expected product was benzylaldehyde. The TLC results indicate that a relatively high yield of benzylaldehyde was achieved. As mentioned in the results section, the other spots on the TLC plate most likely represented the Collins reagent and the remaining benzyl alcohol in solution that had not yet been oxidized. The origin of the streak below the benzylaldehyde spot on the stock solution side of the TLC plate is unknown, however it can be speculated that this streak represents a contamination of the pure benzylaldehyde stock. The Collins solution was allowed to react for another week to ensure that any remaining benzyl alcohol was oxidized. After gravity filtration of the Collins solution and subsequent silica gel filtration, the final liquid had a yellow tint. HCl was added to remove the pyridine from the Collins reagent/benzylaldehyde solution. When adding the Collins solution to the reaction funnel, the stopcock was open and the product spilled on the laboratory table. Efforts were made to recover the product by dabbing it with cotton balls soaked in ether. The cotton balls were placed on filter paper in a gravity filtration apparatus and were rinsed with ether, which recovered some of the product and filtered out any contaminants from the laboratory table. This incident will contribute a large source of error in this experiment. After removing the pyridine with HCl and subsequently evaporating the dichloromethane, a single drop of product remained. The small amount of product was insufficient to perform TLC, IR, and NMR, therefore TLC was not performed so that the product could be saved for IR and NMR analysis.