Experiment 8 Lab Report PDF

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Experiment 8: Hydroboration-Oxidation of 1-Octene Performed on 10/30/13 by Juliet Hammer; Due Date 11/6/13 Purpose: The purpose of this experiment was to perform the hydroboration-oxidation of 1-octene and to determine the percent yield of the entire product and the percent abundance of each component in the product. Reactions: Hydroboration-oxidation is a reaction that occurs in two parts: the first part is hydroboration and the second part is oxidation of the product obtained from hydroboration. This reaction was performed under kinetic control and thus the most important part of the reaction is the transition states of the major and minor products during hydroboration. The rest of the mechanism can be found in Appendix A. Overall Reaction (All pictures taken from the U. Va. Organic Chemistry Lab Manual): Hydroboration:

Hydroboration is a concerted, syn, addition that is regioselective for the Anti-Markonikov product.

Oxidation is a substitution reaction in which each boron group on the molecule is replaced by an alcohol group. Th transition states of the two products: 1-octanol and 2-octanol. The transition state of 1-octanol demonstrates an Anti-Markovnikov addition of borane to the Transition State of 1-octanol

Transition State of 2-octanol

alkene; that is, the bigger group (borane) was added to the less substituted carbon (carbon 1). The transition state of 2-octanol demonstrates a Markovnikov addition in which the bigger group (borane) is added to the more substituted carbon (carbon 2). The anti-Markovnikov transition state results in the formation of the major product, 1-octanol. This is because the anti-Markovnikov addition of borane minimizes steric interactions between the groups on the molecule. When the borane adds to the more substituted carbon, the two large substituents have a greater chance of interacting and clashing. This results in a transition state that is less stable and higher in energy. Because of this, the transition state with the anti-Markovnikov addition has a more stable transition state that is lower in energy. Thus, under kinetic control the formation of 1-octanol is preferred. Procedure: A 10mL round bottom flask was flame dried and immediately equipped with a drying tube. The flask was cooled in an ice bath for 2-4 minutes after which the drying tube was quickly removed. 1 mL of 1-octene was added to the round-bottom flask and the flask was capped with a rubber septum. 2.1 mL of 1M borane-THF solution was obtained using a syringe equipped with a needle. The needle of this syringe was inserted into the rubber septum and a second needle was also inserted in order to vent the apparatus. The borane solution was added dropwise and the solution was swirled throughout the addition. Once the addition was complete, the apparatus was placed in an ice bath and was allowed to react for 1520 minutes. After the reaction was over, the cap was removed and a 30% solution of H 2O2 was added dropwise to the flask. Once addition was complete, 3M NaOH solution was added until the pH of the reaction solution was near 8. 4 mL of deionized water was added to the reaction mixture and the solution was transferred to a small separatory flask.

The aqueous solution was extracted three times with 3 mL aliquots of ethyl acetate and the organic layer from each extraction was added to a 25 mL Erlenmeyer flask. The combined organic layers were washed with 3 mL of a saturated solution of NaHCO3 and the organic layer was added to another 25 mL Erlenmeyer flask. The organic layer was dried over solid magnesium sulfate over a period of 10 minutes. The liquid was decanted into a 25 mL tared round-bottom flask and the solvent was evaporated using a rotary evaporator. The yield of the reaction was then determined. Gas chromatography was not performed in this experiment; however, data was received from the University of Virginia in order to analyze a gas chromatograph of the reaction performed. Calculations: 1) Percent Abundance:

Areaunderneath individual curve ∗100 % Totalarea under all curves 3.28 cm2 Percent abundance of 1−octanol= ∗100 %=93.97 % 3.49 cm2 Percent abundance=

2) Expected Yield:

Expected Yield=Moles of Limiting Reagent∗Molecular weight of Product g =0.82 g Expected yield of 1−octanol ∧2−octanol =6.3 mmol∗130.23 mol 3) Percent Yield:

Percent Yield=

Observed Yield ∗100 % Expected Yield

Percent Yield of 1−octanol ∧2− octanol=

0.41 g ∗100 %=50 % 0.82 g

Results:

*Table 1 shows that the formation of 1-octanol (percent abundance of 93.97%) was heavily favored over the formation of 2octanol (percent abundance of 6.03%). This can be explained by the fact that the transition state in the formation of 1-octanol is much lower in energy than the transition state in the formation of 2-octanol. Because this experiment was run under kinetic control, the product with the lower energy transition state is formed in a greater abundance. The percent yield of this experiment was rather low. Some of the yield loss is most likely due to spillage; however, most of the loss is most likely due to the reaction not going to full completion. The pH of the solution was not raised to 8; rather it was between 7 and 8 causing the reaction to occur more slowly than would be ideal. The reaction was also not allowed to take place for a full 40 minutes and was instead reacted for 15-20 minutes. When the product was collected, the reaction most likely had not gone to full completion resulting in a lower yield than expected.

Discussion: The gas chromatograph received from the University of Virginia (Appendix B) has six peaks; however, only the last two peaks which represent the products are important in this experiment. The first four peaks have been labeled 1-4. Peak 1 is air, peak 2 is ether (rather than ethyl acetate which was used in this experiment), peak 3 is THF, and peak 4 is 1-octene (the starting material). The last two peaks have been labeled 2-octanol and 1-octanol, the products of this reaction. Substances with a lower boiling point elute through a gas chromatograph more quickly making the separation and identification of components in a product very easy. Because of this, the first of these peaks can be identified as 2-octanol (boiling point of 174-181°C) because it has a lower boiling point than 1-octanol (boiling point of 195°C). According to the gas chromatograph, the formation of 1-octanol (percent abundance of 93.97%) was heavily favored over the formation of 2-octanol (percent abundance of 6.03%). This result agrees with

the proposed mechanism because this reaction was run under kinetic control meaning that the product with the lower energy transition state was favored. The transition state of 1-octanol is lower in energy than the transition state of 2-octanol because the anti-Markovnikov addition of borane to the alkene minimizes steric interactions which increases stability and lowers energy. There are multiple ways in which the experiment could be changed in order to increase or decrease the relative percent abundances of the octanol products. One method of control would be an increase or decrease of the temperature at which the reaction was carried out. If the temperature was lowered, the regioselectivity of the reaction would increase favoring even more the product with the lower energy transition state. This is because there is even less energy in the system to form the product with the higher energy transition state. Thus the percent abundance of 1-octanol would increase while the percent abundance of 2-octanol would decrease. If the temperature was increased, there would be more energy in the system and thus more of the product with the higher energy transition state would be formed. The percent abundance of 2-octanol would increase and the percent abundance of 1-octanol would decrease. However, the percent abundances would not change dramatically and 1-octanol would still be the major product while 2-octanol would still be the minor product. 2-octanol can never predominate when this reaction is run under kinetic control because of its higher energy transition state. Another possible method of control would be to use a solvent different than THF. For example, the solvent 9-BBN could be used in order to increase the selectivity of the reaction. The use of this solvent would result in an increase of the percent abundance of 1-octanol and a decrease in the percent abundance of 2-octanol. This is because 9-BBN has a substituent that is even more bulky than the substiuent in THF. This would cause the anti-Markovnikov addition of borane to be even more favorable because a Markovnikov addition would more steric clashes. Error Analysis: The percent yield of this experiment can be used in order to perform an error analysis due to the fact that no gas chromatograph was run of the product obtained. The percent yield of the products was rather low in this experiment. This loss of yield was most likely caused by two sources: spillage and the reaction not going to completion. There were many points in the procedure where spillage potentially occurred. When transferring the solution to the separatory flask and when decanting the liquid into the round-bottom flask, there was potential spillage. However, most of the yield loss most likely occurred from the reaction occurring too slowly and the reaction not going to full completion by the time the product was collected. Sodium hydroxide is added to the solution in order to make hydrogen peroxide more reactive by deprotonating it and giving hydrogen peroxide a negative charge. This makes the reaction occur at a faster rate. Sodium hydroxide is also consumed by boric acid during oxidation and must be replenished in order to drive the reaction. If not enough base is added, the reaction occurs very slowly; however, if too much base is added, an elimination reaction occurs (shown in Appendix A) and the starting material is regenerated. In this experiment, the desired pH of the solution was 8. The pH of the solution when the experiment was performed was between 7 and 8 but was closer to pH 7. Because of this, the reaction took place at a slower rate than would be ideal. The reaction was also only allowed to take place for 1520 minutes rather than a full 40 minutes. Because of this, the reaction most likely did not go to completion which resulted in a lower yield of the products. In the future, more care should be taken not to spill the material when transferring it from one apparatus to another. In addition to this, the pH of the solution should be raised to 8 rather than 7. This will allow the reaction to take place at a faster rate without over-basifying the solution. The reaction should also be allowed to react for a longer period of time in the future. This will ensure that the reaction will go to completion and the yield of the products will be much closer to 100%. Conclusion: The hydroboration-oxidation of 1-octene is a regioselective reaction that results in two products: 1-octanol and 2-octanol. Because this reaction was performed under kinetic control, the formation of the product with the lower energy transition state, 1-octanol (with a percent abundance of 93.97%), was heavily favored over the formation of 2-octanol which had a percent abundance of 6.03%. The transition state of 1-octanol is lower in energy and more stable because it minimizes steric interactions between the borane being added and the substituent attached to a carbon involved in a double

bond. The percent yield of this experiment was rather low most likely due to the reaction not going to completion. This can be solved in the future by increasing the pH of the solution to 8 and allowing enough time for the reaction to completely finish. References: University of Virginia. Fall 2013 Organic Chemistry Laboratory Manual; P.S. Publishing, 2013; pp 74-78.

Appendix A: Reaction Mechanism and Side Reactions of the Hydroboration-Oxidation of 1-octene All pictures used from the University of Virginia Organic Chemistry Lab Manual

Appendix B: Data Received from the University of Virginia

4 Graph 1** **Graph 1 displays 6 different peaks. Peak 1-4 are air, ether, THF, and 1-octene respectively. The two peaks of interest are labeled 2-octanol and 1octanol. The first peak can be identified as 2octanol because it has a lower boiling point (174181°C) than 1-octanol (boiling point of 195°C). The percent abundance of 1-octanol was 93.97% and the percent abundance of 2-octanol was 6.03% showing that the formation of 1-octanol was heavily favored. This can be explained by the proposed reaction mechanism which shows that the transition state of 1-octanol was lower in energy than the transition state of 2-octanol. Because this reaction was run under kinetic control, the formation of the product with the lower energy transition state would be favored. This proved to be the case with the data shown....