2-Methylcyclohexanol Dehydration

Dehydration of Methylcyclohexanols
Balie Zander
Date Conducted: March 27, 2013
Date Submitted: April 3, 2013

Introduction:
The purpose of this experiment was to demonstrate the application of Zaitzev’s rule, the Evelyn Effect and the occurrence of an E1 mechanism in an alcohol dehydration reaction. Specifically this experiment involved the dehydration of 2-methylcylcohexanol by heating this alcohol in the presence of phosphoric acid and then to based on the results of the gas chromatography of the product to determine whether the alcohol exhibited the Evelyn Effect. The reaction taking place for both the Cis and the Trans isomers for Methylcyclohexanol are as follows: E1 Mechanism for Cis Methylcyclohexanol

E1 Mechanism for Trans methylcyclohexanol
Calculations:

2-Methylcyclohexanol

Mass: (20mmol/1)(1mol/1000mmol)(114.2g/1mol) = 2.284g Volume: (2.284g)(1mL/.930g) = 2.46 mL

Theoretical Yield: (.02mol) (1mol/1mol) (96.1702g/mol) = 1.924g methycyclohexene

Actual Percent Yield:

Distillate 1: Mass: 24.543g – 24.130g = .413g Percent Yield: (.413)/(1.924) X 100 = 21.47%

Distillate 2: Mass: 39.075g – 38.825g = .25g

Percent Yield: (.25)/(1.924) X100 = 12.1 %

Data, Results and Observations:

Table 1: Measurements T1 | 75° C | T2 | 90° C | T3 | 99° C | Beaker 1 | 24.130g | Beaker 2 | 38.825g | Beaker 1 w/ Product | 24.543g | Beaker 2 w/ Product | 39.075g |

Table 2: Gas Chromatography for Distillate 1

Peak # | Ret. Time | Area | 1 | 1.711 | 5.8029% | 2 | 1.758 | 5.2536% | 3 | 1.784 | 6.6098% | 4 | 1.830 | 5.5547% | 5 | 2.031 | 1.6967% | 6 | 2.110 | 20.7735% | 7 | 2.183 | 32.8520% | 8 | 2.229 | 21.3453% |

Table 3: Gas Chromatography for Distillate 2

Peak # | Ret. Time | Area | 1 | 1.774 | .5729% | 2 | 1.821 | 9.7434% | 3 | 2.073 | 7.7451% | 4 | 2.148 | 40.7443% | 5 | 2.205 | 41.1523% | 6 | 2.391 | .0419% |

Discussion:

This experiment involved the dehydration of 2-Methylcyclohexanol,which was done by heating the alcohol in the presence of phosphoric acid.
The 2-methylcyclohexanol alcohol used in this experiment was a mixture of both cis and trans isomers. The way this reaction occurs is as follows. Through protonation, the acid-catalyst converts the already present, poor leaving group (-OH) to a much better leaving group (H2O+). Then once the H2O and the H+ are eliminated the reaction yields an alkene in which the un-protonated alcohol serves as the reaction solvent. This experiment then called for the distillation of this mixture, using a distillation apparatus. As this distillation took place the products, alkenes and water continuously distilled from the reaction mixture into a Hickman still as they were formed. The removal of these elements shifted the equilibrium to the right, ultimately increasing the yield of alkene, this is also known as Le Chatelier’s Principle. Then both fractions were washed with 1.5mL of 5% aqueous sodium bicarbonate, the reason or this is that the sodium bicarbonate gets rid of the excess acid present. In order to analyze the fractions, both were put through gas
chromatographer. This experiment called for the determination of whether or not the Evelyn Effect was seen in the dehydration reaction. The Evelyn effect is a phenomenon of the difference in ratios of the two fractions as the reaction proceeds. During the dehydration of 2-Methylcyclohexanol when the Evelyn Effect was discovered the first fraction consisted of primarily the cis iomer (molecules on the same side of the plane) while the second fraction consisted primarily of the trans isomer(molecules on opposite sides of the plane). Meaning the for the Evelyn effect to be present in this reaction there should have been more cis isomer in gas chromatography results for fraction (or distillate) 1 and more trans isomer in the gas chromatography results for fraction (or distillate) 2. For 1, peak 7 which was the trans isomer showed 32.8520% while peak 8 which was the cis isomer showed 21.3453%. Because these results show more of the trans isomer present, not the cis isomer the Evelyn Effect is not supported by this data. For 2, peak number 4 the trans isomer showed 40.7443% while peak 5, the cis isomer showed 41.1523%. Once again the data does not support the Evelyn Effect as there should have been more trans than cis in this fraction but the data shows there was more cis than trans. The results of this experiment were not to be expected and do not support the Evelyn Effect. This is most likely due to any number of possible errors that could have occurred throughout this experiment. Samples could have been mixed up at any time during the experiment which would account for the results being the opposite of what they should have been.
Conclusion:
In conclusion, this experiment called for the dehydration of 2-Methylcyclohexanol through the heating of the alcohol in the presence of phosphoric acid. The goal of this experiment was to determine the presence of the Evelyn Effect for 2-Methylcyclohexanol, which is the difference in product ratios as the reaction runs its course. According to the Evelyn Effect there should have been more cis present in fraction 1 and more trans present in fraction 2. According to the results of the gas chromatography of the two fraction the Evelyn Effect was not supports as the first fraction contained more trans while the second fraction contained more cis.
Exercises:
3) If the phosphoric acid had not been added to the mixture before the distillation process the reaction would not have taken place because the phosphoric acid acts as both the acid-catalyst and the solvent. The alcohol leaving group would not have been protonated and therefore would not have been a good leaving group.
5) Three possible bases present in the reaction are: H20, H2PO4- and 2-Methylcyclohexanol itself.