Lab Report - Experiment 13

pdf

School

University of Nebraska, Lincoln *

*We aren’t endorsed by this school

Course

253

Subject

Chemistry

Date

Apr 3, 2024

Type

pdf

Pages

Uploaded by AdmiralElement9961

Madeline Stephens Partner: Ashlyn Gregory Chem 253-003 - TA: Stephanie Berg November 7, 2023 Experiment 13: Formation of Cyclohexene from Cyclohexanol Purpose To perform an acid-catalyzed dehydration of cyclohexanol using phosphoric acid by using simple distillation, and use gas chromatography (GC) and proton nuclear magnetic resonance (HNMR) to determine the purity and number of unique hydrogens present in the cyclohexene. Theory Commonly, cyclohexanol is synthesized in large quantities every year all around the world. The most popular use for cyclohexanol is to manufacture nylon. To transform cyclohexanol into cyclohexene, the process follows an E1 elimination reaction. This kind of reaction commonly competes with an SN1 reaction. To avoid an SN1 reaction, the E1 reaction can be made the predominant pathway by choosing the proper reaction conditions. This condition is by synthesizing the alkenes by the acid-catalyzed dehydration of an alcohol. Therefore, for a successful E1 reaction to take place, various things are required. The first is the absence of any substance capable of acting as a good nucleophile to discourage competing SN1 reactions. The second is the removal of water during the reaction to drive the equilibria to the right. E1 reactions are similar to SN1 reactions, the formation of the carbocation is the rate-determining step. In turn, substrates that form stable carbocations react faster than substrates that form less stable carbocations. When these reactions occur, a potential energy graph is able to be made. These diagrams help determine whether the reaction is reversible or not. To determine this, the activation energies are of similar magnitude or both activation energies are small relative to the amount of energy available to the system. If both are true, the reaction is likely to be reversible. Aside from the reaction, simple distillation is ideal for this experiment, as the compounds that are being separated have boiling points that are very far from each other. To determine the purity of the final product, GC will be used. In GC, the sample passes over a heated wire in which electrical resistance varies with temperature. The temperature changes upon going from pure gas to a gaseous solution of the solution. The electrical charge can be computed and plotted. In this plot, a series of peaks will be presented. Peak areas can be explained as the measure of the concentration of the compound. The larger the area, the higher the concentration present. HNMR will be used to determine the number of unique hydrogens present. HNMR will present one peak per unique hydrogen. The peaks can also assist in determining chemical shifts in the compound. To determine if this experiment was successful, the concentration will be determined from the GC, the number of unique protons will be determined from HNMR, and a high percent recovery will be obtained.

Methodology To conduct this experiment, pages 99-100 in the lab manual were followed. There were no changes to the procedure; however, at the beginning of the lab, the bottle of cyclohexanol needed to be placed in a hot water bath because it was frozen. Reaction Cyclohexanol + Phosphoric Acid → Cyclohexene + Water Data Table 1: Experiment 13 Raw Data Volume of Cyclohexanol (mL) 10.0 mL Volume of 85% Phosphoric Acid (mL) 2.5 mL Mass of 50 mL Erlenmeyer Flask (g) 41.109 g Mass of Final Cyclohexene and Erlenmeyer Flask (g) 43.138 g Mass of Final Cyclohexene (g) 2.029 g Boiling Point of Cyclohexene ( ℃ ) 65 ℃ - 76 ℃ GC Vial # 644 NMR Vial # 27 The boiling point relates closely to the boiling point of cyclohexene. This boiling point is provided in the lab procedure.

Observations/Results Qualitative Observations: During the distillation, the distillate came out in two layers. The top layer was a cloudy white while the bottom appeared clear. The undistilled liquid turned a yellow/orange color after being heated. The distillation process was a little slow. When separating the two layers, the cloudy one was the organic layer and the more clear one was the aqueous layer. When adding the NaCl to dry the distillate, anything that was not organic formed a crystal-like substance. During the second distillation, the organic layer foamed after about 5 seconds of being heated. This time, the distillate came out fairly fast. Again, when drying with NaCl, anything that was impure turned into a crystal-like substance. Even though the experiment was performed under the hood, the product gave off a very strong, displeasing smell. Cyclohexanol GC Results: Cyclohexene GC Results:

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

Sample GC Results: NMR Results:

Mass Calculations: Cyclohexanol: 9.62 g ?????? × ????𝑖?? = 10. 0?𝐿 × 0. 9620𝑔/?? 3 = Yield Calculations: Cyclohexanol: = 0.0960 mol 9. 62𝑔 × 1 ??? 100.16 𝑔 Theoretical yield of Cyclohexene: = 0. 0960 ??? (𝐶????ℎ??𝑎???) × 1 ??? (𝐶????ℎ?????) 1 ??? (𝐶????ℎ??𝑎???) × 82.14 𝑔 (𝐶????ℎ?????) 1 ??? (𝐶????ℎ?????) 7.8854 g Cyclohexene Actual yield of Cyclohexene: Mass after Distillation - Mass of Erlenmeyer flask = 2.029 g Cyclohexene 43. 138𝑔 − 41. 109𝑔 Percent Yield: = 25.73% yield 𝑎???𝑎? ?𝑖??? ?ℎ?????𝑖?𝑎? ?𝑖??? × 100 = 2.029𝑔 7.8854𝑔 × 100 Percent Error: = -74.27% error 𝑎???𝑎? ?𝑖???− ?ℎ?????𝑖?𝑎? ?𝑖??? ?ℎ?????𝑖?𝑎? ?𝑖??? × 100 = 2.029𝑔 − 7.8854𝑔 7.8854𝑔 × 100 GC Observed Concentrations: Observed Cyclohexene: 35.6508% Observed Cyclohexanol: 12.8118% GC Actual yield of Cyclohexene Actual yield of Cyclohexene Observed Cyclohexene × = 0.7234 g 2. 029 × 0. 356508 GC Percent Yield: = 9.18% yield 𝑎???𝑎? ?𝑖??? ?ℎ?????𝑖?𝑎? ?𝑖??? × 100 = 0.7234𝑔 7.8854𝑔 × 100 GC Percent Error: = -90.82% error 𝑎???𝑎? ?𝑖???− ?ℎ?????𝑖?𝑎? ?𝑖??? ?ℎ?????𝑖?𝑎? ?𝑖??? × 100 = 0.7234𝑔 − 7.8854𝑔 7.8854𝑔 × 100

Discussion/Conclusion Based on previous calculations, the percent yield of cyclohexene was 9.18%. It is known that the ratio of reactant to product is 1:1. With that knowledge, it can be concluded that the yield calculations will be the same for both cyclohexanol and cyclohexene. However, cyclohexene has a smaller molar mass, so the expected product will be a little less than what was started with. This is an explanation for why some mass was lost. This means that, theoretically, there cannot be a 100% yield. While this can explain a slightly lower percent yield, it is no explanation for the loss of nearly all the product. During the experiment, the Erlenmeyer flask containing the product was tipped over. While nothing appeared to be lost, the percent yield calculation indicates otherwise. If proper lab practices had been followed, a higher percent yield could have been obtained. To find the numbers to determine this yield, there was an analysis of the GC graph. The graph shows 3 peaks, all at different heights. The first peak has an area percentage of 51.5086. It appears at 1.532 minutes. Based on the given GC data, there is no explanation for the 51.5086%. The second peak is at 1.565 minutes. This peak has a percentage of 35.6508%, indicating the presence of cyclohexene. Finally, the third peak is at 1.655. This peak has a percentage of 12.8118%, indicating the presence of cyclohexanol. These results lead to the conclusion that the obtained product is not really pure at all. It is about 35% cyclohexene and 65% other product whether it be remaining cyclohexanol or other compounds. Once again, this result could be due to the loss of product when the Erlenmeyer flask was tipped over. Aside from the percent yield, the boiling point of the product was observed. This point was 65 ℃ - 76 ℃ . This relates closest to the theoretical product, cyclohexene, which has a boiling point of 83 ℃ . However, the low boiling point indicated that the cyclohexene was not pure. After observing the HNMR graph, a lot of different conclusions can be drawn. To begin, based on the results, it can be determined that there are around 9 unique hydrogens. However, knowing what the product is, that seemed incorrect. That led to investigating more about what is being shown in the graph. To start, there appears to be a peak at 7.3 ppm. This indicated an aromatic ring bonded to 1 hydrogen. This was abnormal, as an aromatic ring is not in the structure of either the reactants or products. The reasoning for this being shown on the graph is still undetermined. Moving further down, there is a peak at around 5.6 ppm. This is an indication of a C=C bond. This is reasonable, as there is a C=C in the expected product. There is only 1 peak within this peak, indicating no neighboring hydrogens. There is a third peak at about 3.6 ppm. This indicates an O-H bond. This validates that there was still some cyclohexanol in the product. Finally, there are multiple peaks between 1 and 2 ppm. This indicates the presence of C-H bonds. The NMR graph indicates that the incorrect product was formed. Unfortunately, this experiment was not successful in producing cyclohexene. After the analysis of the NMR and GC data, there was an indication that some cyclohexanol was left over and another product, other than cyclohexene, was formed. During the experiment, proper lab techniques to prevent spilling could have been used to have a successful experiment.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

Exercises 1. Write a detailed mechanism for the dehydration of cyclohexanol. a. 2. Draw a potential energy diagram for this reaction. Clearly label the relevant intermediates, transition structures, and the rate-determining step. a. 5. In the acid-catalyzed dehydration of cyclohexanol, the mechanism requires only catalytic acid, but in the experiment, we used a large excess. Think about the mechanism and suggest a reason why a large excess of strong acid is used. a. After analyzing the mechanism, it is clear that the reaction can be reversed. Because of this, a larger amount of reactants need to be used in order to move the reaction forward. So, a large excess of acid was used to get the preferred product.