Hydroboration:Oxidation Lab report PDF

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lab report for hydroboration and oxidation lab ...

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Hydroboration/Oxidation of 1-Hexene The purpose of this experiment was to synthesize 1-hexanol from 1-hexene using the process of hydroboration, using the reactant complex BH3 – THF, then to oxidize the intermediate through oxidation, using the reactants H2O2, NaOH, and H2O, creating an alcohol. The most logical hypothesis for this reaction would be to predict that the primary alcohol is the major product, while the secondary alcohol is the minor product. This phenomenon occurs through the anti-Markovnikov rule; the bulky BH 3 group cannot be added to the most substituted carbon, so therefore the hydrogen will instead be attracted to that carbon. Steric strain inhibits the bulky group, so it then must bond to the less substituted carbon. Once hydroboration determines the major product to be the primary alcohol, oxidation simply replaces the BH 2 group with an alcohol on the same carbon, retaining the primary substitution. To execute the experiment, collect the stir bar, a septum, and a heated 50 mL round bottom flask. Insert the stir bar into the flask and cover it with the septum. Pierce the septum with the needle from the nitrogen supply. It is important to have nitrogen gas instead of atmospheric air because impurities might be introduced or unwanted reactions could occur. Next, inject .6 mL of 1-hexene with 2 mL THF into the round bottom flask using separate needles. Add 3 mL of 1 M borane-tetrahydrofuran complex into the flask. Ensure that the flask and its contents are placed in an ice bath because the oxidation of hydrocarbons is exothermic, which could possibly cause burns, but also, it would ensure that the reaction occurs faster according to Le Châtelier’s principle. After the reaction has run for 30 minutes, add 1 mL of water to the flask. The purpose of this step is to deplete any extra borane, while creating hydrogen gas. Using a syringe, add .6 mL 3 M NaOH to the mixture. After, add .6 mL 30% H 2O2 to the mixture, also using a syringe. It is important to be aware that the H2O2 concentrated to the point that it can cause lasting damage. Guarantee that the H2O2 does not make contact with skin. After both

reactants have been added to the mixture, run the reaction for 15 minutes. After the final product has been made, isolate it. Add potassium carbonate until the solution is saturated, and it appears to display the snowball effect; that is, until all potassium carbonate has bound with all the water, and now excess potassium carbonate is in the solution. Insert the solution in a separatory funnel and rinse with diethyl ether. After the layers have separated, drain the aqueous layer separately from the organic layer, and repeat this process 2x more with the aqueous layer. Dry the organic layer with magnesium sulfate to ensure the mixture is completely dry. Gravity filter the magnesium sulfate out of the solution using filter paper. Next, concentrate the solution using the rotary evaporator. This is done because it allows for clearer results in the GC and IR spectra. Finally, weigh the flask to establish the percent yield. With the final product, conduct a GC and IR analysis. Data: Mass of flask+product – mass of flask= mass of product Percent yield=(actual yield)/(expected yield) * 100% Actual moles hexanol produced Actual moles 1-hexene used

41.734 g - 41.54 g =.194 g hexanol (.0019 mol)/(.0048 mol) * 100% = 39.6 % yield .194 g * (102.17 g / 1 mol)= .0019 mol .6 mL * (.67 g/1 mL)(1 mol / 84.16 g)=.0048

mol The goals of this experiment were to synthesize 1-hexanol from 1-hexene using the processes of hydroboration and oxidation. As described before, the BH 2 group would be added to the least substituted carbon through the anti-Markovnikov rule while the hydrogen would be added to the more substituted carbon. After the hydroboration, H 2O2, H2O, and NaOH would be responsible for the oxidation while adding an alcohol group in place of the BH 2 group. The following mechanism (with chemical structures) would occur in these reactions:

The indicated transition state for the first step is lower in energy because the BH 2 group is too bulky to bind to the more substituted carbon, due to steric hindrance by the multiple hydrogens. The bulky group binds to the less substituted carbon more easily because there is a lack of steric hindrance from multiple atoms. Also, the positive charge would be most optimally placed on the more substituted carbon, so this also influences the bulky BH 2 to bind to the primary carbon.

Similarly, if 1-hexene is treated with H2SO4 and H2O, it will also form an alcohol, but only 2-hexanol is formed instead. This is the only product formed because when the pi bond of the alkene attacks the hydronium ion produced by the H 2SO4 and H2O, the positive charge will naturally go to the more substituted carbon (a secondary carbon) because it can be stabilized more due to hyperconjugation, compared to a primary carbon. Then the water will only attack the positive charge on the secondary carbon. The intermediates are not stable because they are all charged species; therefore, they can easily react with a nucleophilic species. The mechanism, transition state, and energy diagram is shown below:

The data suggests that 1-hexanol was the major product. Firstly, the IR spectrum supports this claim. An IR spectrum of the starting material, 1-hexene, does not display the broad peak expected of an alcohol at 3200-3400 cm-1. After the reaction has reached completion, an IR spectrum of the final product, a hexanol, shows the acquirement of the alcohol group at 3344 cm-1. This support the data that the reaction did reach completion, and form the expected alcohol. Also, this broad peak at 3344 cm-1 is not related to water, due to the fact that the transmittance value would be much larger. Furthermore, the starting 1-hexene IR displays a peak at 1653 cm -1, which is attributed to the alkene. This peak does not show up in the product, also supporting the claim that the hexene was reacted to form an hexanol. Now, there is an issue of what hexanol was formed, 1-hexanol or 2-hexanol. For this, one must analyze a secondary source; in this case, GC data. The GC data displays 3 peaks; one with a retention time at 5.28 min and an area of 10974, a large peak at 6.16 min and an area of 223874, and one with a retention time 8.15 min and an area of 21616. According to the given retention times, dichloromethane, used to dilute the samples, is the very large peak with the 223874 area. Also, the given retention times do not match any data over the GC done for this experiment, so one must analyze the order in which the peaks appear. Using the information that 2-hexanol has a retention time at 3.32 min, which is before 1-hexanol at 3.80, one can logically assume that this relationship still exists, even if the retention times don’t match. Using this assumption, it is safe to say 2-hexanol appears at 5.28 min with an area % of 4.9. Then, 1-hexanol would appear after 2-hexanol, in this case, at 8.15 min and an area % of 9.66. Because 1-hexanol has a larger area percentage, this correlates to the amount of the substance in the sample, almost at 2x as much. Thus, 1-hexanol is the most likely major product, analyzed through the IR data and GC data.

The largest problem for this experiment was that the final product was not thoroughly filtered, so extra magnesium sulfate was still in the flask. This meant that a GC could have not been performed because the solid particles could skew the results and possibly ruin the machine. The observations made during the experiment indicate that the work up only lasted for 30 minutes. With more time, the work up could have produced a higher percent yield. The data suggests that a 39.6 % yield of hexanol was produced. A 100 % yield might have not been obtained because of how the experiment was performed, such as the reaction was not at an optimal temperature or that the experiment was not allowed to run long enough to completion. A by-product, H2, made during the reaction could have possibly been filtered out through the nitrogen system that was used during the workup.

Extra credit reaction mechanism:...