Experiment 7 Lab Report PDF

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Experiment 7: Dehydration of 2-Methylcyclohexanol, Tests for Unsaturation & Gas Chromatography Performed on 10/23/13 by Juliet Hammer; Due Date 10/30/13 Purpose: There were three parts to this experiment. The purpose of the first part of the experiment was to perform the dehydration of 2-methylcyclohexanol and collect the product. The purpose of the second part of the experiment was to observe the results of two different tests for unsaturation on the product obtained in the first part of the experiment. The purpose of the third part of the experiment was to perform gas chromatography on the product and determine the percent composition of the alkenes in the product. Reactions: Overall Reaction of Acid-Catalyzed Dehydration of 2-methylcyclohexanol (Pictures taken from University of Virginia Organic Chemistry Lab Manual)

This is an elimination reaction catalyzed by phosphoric acid in which a carbocation intermediate is formed. There is a major (1-methylcyclohexene) and a minor (3-methylcyclohexene) product formed in the reaction. This reaction was performed under kinetic control and thus the reverse reaction did not take place. Step 1ǂ ǂ

In the first step of the reaction, the carbocation is formed. The alcohol (OH) group on 2-methylcyclohexanol is protonated by the phosphoric acid creating a positive charge on the oxygen molecule. Because oxygen is an electronegative atom, a positive charge is not favorable and the oxygen withdraws electrons from the carbon atom it is bonded to. This weakens the bond and eventually the bond is broken with the electrons going to the oxygen and the carbon now has a positive charge. This is the carbocation.

ǂǂ

Step 2

Step 2Aǂ ǂ

ǂǂ In the second step of the reaction, a hydrogen atom is pulled from a carbon adjacent to the carbocation either by the conjugate base or water (shown in this mechanism) and a new pi bond is formed. However, there are two different hydrogen atoms (circled in red) that can be pulled off. This results in two products being formed, one formed in a larger abundance (major product) than the other (minor product). In this reaction performed under kinetic control, the major product formed will have the transition state with lower energy.

Transition State

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ǂǂǂ

In this reaction of water with the carbocation intermediate, 1-methylcyclohexane is formed as the hydrogen removed is from the carbon that is also bonded to the methyl group. This shows the formation of the major product. This is because the transition state of this reaction (shown to the right of the mechanism) is lower in energy than the transition state of the formation of 3methylcyclohexane. This is because the hydrogen was removed from a tertiary carbon rather than a secondary carbon. The tertiary carbon is better stabilized by hyperconjugation thus creating a lower energy transition state. Transition State

Step 2Bǂ ǂ ǂ ǂ ǂǂǂǂ In this reaction of water with the carbocation intermediate, 3-methylcyclo hexane is formed as the hydrogen removed is from the carbon that is not bonded to the methyl group. This is the formation of the minor product. 3-methylcyclohexane is the minor product because its transition state is higher in energy than the transition state of the formation of 1-methylcyclohexane. The removal of hydrogen from a secondary carbon is less favorable than the removal of hydrogen from a tertiary carbon because the transition state is less stable. Less 3-methylcyclohexane is formed than 1-methylcyclohexane.

Procedure: In the first part of the experiment, a microdistillation apparatus was set up and a boiling stone, 1 mL of 2-methylcyclohexanol, and 2 mL of 85% phosphoric acid were added to the bottom of the flask. The sample was heated and the temperature of the vapor was kept below 100°C. The contents were distilled until 0.5-0.7 mL of liquid was collected and the product was dried with calcium chloride. In the second part of the experiment, 2-3 drops of starting material were added to two test tubes and 2-3 drops of the dried product were added to two additional test tubes. Bromine solution was added to one test tube of the starting material and one of the dried product. The results were observed and recorded. Potassium permanganate solution was added to one test tube of the starting material and one of the dried product. The results were observed and recorded. In the third part of the experiment, 0.5 μL of the dried product was injected into the gas chromatograph and the injection point was marked. The materials were allowed to elute through the column. The peaks on the chromatogram were identified and the retention time (tr) and the number of theoretical plates (N) were determined for each peak. The percent composition of the alkenes in the product was then determined. Calculations: 1) Calculation of Percent Composition

Area underneathindividual alken e' s curve ∗100 % Total area 0.7875 cm2 ∗100 % =78.8 % Percent c omposition of 1−methylcyclohexane= 2 1.0125 cm Percent composition=

Results:

*Table 1 shows that the retention time of 1-methylcyclohexane was greater than the retention time of 3-methylcyclohexane. The number of plates of 1-methylcyclohexane was also greater than the number of plates of 3-methylcyclohexane. This is because 3methylcyclohexane has a lower boiling point (104°C) than 1-methylcyclohexane (110°C) and eluted through the column faster. The product was composed of 78.8% 1-methylcyclohexane and 22.2% 3-methylcyclohexane making 1-methylcyclohexane the major product and 3-methylcyclohexane the minor product of the acid-catalyzed dehydration of 2-methylcyclohexanol. Calculations of retention time and number of theoretical plates are shown in Appendix C. §

Graph 1§

Graph 1 compares the abundance of each product (y-axis) to the progress of the reaction (xaxis). The graph shows two major peaks. The two bigger peaks represent the two products formed from the dehydration of 2-methylcyclohexanol. The first peak represents 3methylcylohexene because this product has the lower boiling point. The product with the lower boiling point elutes through the gas chromatograph faster and is thus recorded first. The second Abundance peak represents 1-methylcyclohexene. The area under the second peak (0.788 cm2) is greater than the area under the first peak (0.225 cm2) showing that 1-methylcyclohexene was present in a greater abundance than 3-methylcyclohexene.

Discussion: The qualitative tests for unsaturation were successful in showing that alkenes were produced from the dehydration of 2-methylcyclohexane (data shown in Appendix A). In the bromine test, when bromine was added 0 Time to the saturated starting material 2-methylcylohexane, the color of the solution was an orange-yellow which appeared to simply be a more dilute shade of the original bromine solution. However, when bromine was added to the dried product, the red color of the bromine solution was discharged and the solution was completely clear. This shows that the dried product was unsaturated because when bromine is added to a solution containing an alkene, a bromination reaction occurs and the color of the bromine molecule disappears. The second test for

unsaturation, the addition of potassium permanganate, confirmed these results. When potassium permanganate was added to the saturated starting material, the solution was simply purple liquid. This shows that no reaction took place However, when potassium permanganate was added to the dried product, a brown precipitate formed showing that a reaction occurred making it possible to conclude that the dried product contained an alkene. Although these tests showed that the reaction was successful in producing alkenes, the percent abundance of each of the products could not be determined from these tests. In order to determine the percent composition of the dried product, gas chromatography was performed. Two peaks appeared on the gas chromatograph. The first peak on the chromatograph can be identified as 3-methylcyclohexene (boiling point of 104°C) and the second peak can be identified as 1methylcylohexene (boiling point of 110°C). The first peak is 3-methycylohexene because it has the lower boiling point of the two products. A lower boiling point results in that product travelling through the column faster than the other product. Because the product with the lower boiling point spends less time in the column, its retention time is shorter and goes through fewer theoretical plates. The calculations for retention time and number of theoretical plates are shown in Appendix C.The product with the lower boiling point is recorded on the chromatograph before the product with the higher boiling point, making identification of the peaks possible. The percent abundance of 1-methylcyclohexene was calculated to be 77.8% (Calculation 1) while the percent abundance of 3-methylcyclohexene was 22.2%. This reaction was performed under kinetic control meaning that only the forward reaction was allowed to take place. Under these conditions, the formation of the product with the lower energy transition state was expected to be favored. Of the two products that were formed, 1-methylcyclohexene has the transition state with the lower energy because a hydrogen atom is removed from a tertiary carbon to form the product rather than from a secondary carbon as occurs in the formation of 3-methylcyclohexene. Thus it would be expected that the formation of 1methylcyclohexene is favored. The results are consistent with this proposed mechanism as 1methylcyclohexene was over three times more abundant than 3-methylcyclohexene. Error Analysis: In order to perform an error analysis, personal data was compared to data received from the University of Virginia (shown in Appendix B). The percent composition of the data received was 77.7% 1-methylcyclohexene and 22.3% 3-methylcyclohexene as compared to the percent composition of the personal data which was 77.8% 1-methylcyclohexene and 22.2% 3-methylcyclohexene. The results from the two data sets are extremely similar indicating that the results from this experiment are precise. Measurements of chemicals were kept precise and special care was taken to ensure that the vapor of the sample was not heated over 100°C when the reaction was being performed. Attention to these details produced good data and in future experiments, this attentiveness should be repeated. The most probable source of error in this experiment was from the physical measurements made of the gas chromatograph. The measurements were taken using a ruler and the number recorded was observed by the human eye. These measurements were taken carefully, however there is always a possibility of error when measuring using the human eye. This could have affected the calculations for retention time, number of theoretical plates, and for area under each peak and thus the percent composition calculation. In the future, more accurate measurements can be taken by uploading the chromatograph to a computer and using a program to take the measurements. However, this error was minimal and did not have a significant effect on the results. Conclusion: Gas chromatography is an effective way to separate components in a solution that have different boiling points. Also, if the components in a solution have similar molecular structures, gas chromatography is an effective way to determine the percent abundance of each component in the solution. In the dehydration of 2-methylcyclohexane under kinetic control, two products were formed: 1methylcyclohexene and 3-methylcyclohexene. Using qualitative tests, it was confirmed that an alkene was produced. Using gas chromatography, it was determined that 1-methylcyclohexene was over three times more abundant than 3-methycyclohexene in the final product of this reaction showing that the formation of 1-methylcyclohexene was heavily favored. This is consistent with the proposed reaction mechanism as 1-methylcyclohexene has a lower energy transition state.

References: Pavia, D.L.; Lampman, G.M.; Kris, G.S.; Engel, R.G. Small Scale Approach Organic Laboratory Techniques, 3rd ed.; Cenage Learning: Ohio, 2010; pp 283-318.. University of Virginia. Fall 2013 Organic Chemistry Laboratory Manual; P.S. Publishing, 2013; pp 63-73.

Appendix A: Data from Tests for Unsaturation

**Table 2 shows the results from the tests for unsaturation. It was shown that the saturated starting material did not react with bromine or potassium permanganate. However the dried product reacted with both bromine and potassium permanganate. These observations show that there was an alkene present in the dried product, though they do not describe the percent composition of the product.

Appendix B: Data Received from the University of Virginia

***Table 3 shows the results calculated from the data received from the University of Virginia. The results show that 1-methylcyclohexene was in greater abundance than 3-methylcyclohexene and thus the formation of 1-methylcyclohexene was heavily favored.

Appendix C: Additional Calculations 1) Calculation of Retention Time (tr):

t r=Distance ¿ injection point ¿ max of peak∗chart speed cm∗1 min t r ( 1−methylcyclohexane )=1.45 =0.58 min 2.5 cm

2) Calculation of Number of Theoretical Plates:

distance ¿ injection point ¿ N=16∗( max of peak ¿¿ width of peak at its base )2 2 1.45 cm N ( 1−methylcyclohexane ) =16∗ =538.42 plates .25 cm

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