Dehydration of 2-methylcyclohexanol Lab Report PDF

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Dehydration of 2-methylcyclohexanol Lab Report, Spring 2021...

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Dehydration of 2-methylcyclohexanol Lab Report

Nora Cipkowski - Writer Matthew Estacio - Reviewer Maddie Looney - Editor

Introduction: The dehydration of 2-methylcyclohexanol involved learning new laboratory techniques and concepts. To determine the products formed in the reaction, a gas chromatograph was used. Boiling points were compared to relative retention time, and the relative sample production vs area of peak was graphed. During the experiment, the percent yield of the alkene product was calculated, and compounds in the gas chromatography were identified using relative retention times and boiling points. Relative peak integration values were used to determine the percent yield of each compound, and the results were compared to Zaitsev’s rule (Reference 1). The reaction mechanism was drawn for the three products formed. The experiment involved heating 2-methylcyclohexanol, an alcohol. This was done in the presence of phosphoric acid to dehydrate the alcohol, causing water to leave and forming three potential alkene products: 1-methylcyclohexene, 3-methylcyclohexene, and methylenecyclohexane (Reference 2). The reaction mechanism begins with the protonation of the alcohol group by phosphoric acid. This forms a secondary carbocation when the protonated alcohol group leaves as water (Figure 1).

Mechanism:

Figure 1: The formation of a carbocation intermediate during the first steps of the dehydration of 2-methylcyclohexanol. For the two products formed, water is used as a base to remove the β-hydrogen from the carbocation. An alkene is formed by the electrons in the C-H bond shifting to the carbon of the carbocation (Figure 2).

Figure 2: The formation of 1-methylcyclohexene and 3-methylcyclohexene through an E1 mechanism. A secondary carbocation is formed and stabilized by hyperconjugation from the two adjacent carbons. A hydrogen from the adjacent tertiary carbon shifts to the carbon of the carbocation to form a more stable tertiary carbocation. This carbocation is more substituted, and stability is increased from the hyperconjugation of the three adjacent carbon atoms. The methyl β-hydrogen is removed through an E1 mechanism, forming the third alkene product (Figure 3).

Figure 3: The formation of methylenecyclohexane through the formation of a more stable tertiary carbocation and an E1 elimination at the methyl group β to the carbocation. Similar to the formation of the third alkene product (Figure 3), 1-methylcyclohexene can also be formed through an E1 elimination from the tertiary carbocation (Figure 4).

Figure 4: Alternate formation of 1-methylcyclohexene through the formation of a more stable carbocation intermediate.

A carbocation that can arrange to increase stability through substitution tends to be produced in greater amounts compared to a less stable carbocation.

Table 1: Table of Reagents Compound

MW (g/mol)

BP (℃)

MP (℃)

Density (g/mL)

2-methylcyclohexanol

114.2

163-166

n/a

0.930

1-methylcyclohexene

96.2

110-111

-126.3

0.813

3-methylcyclohexene

96.2

104

-124

0.801

methylenecyclohexane

96.2

102-103

-107

~0.8

Calcium chloride

110.98

1935

772

2.15

Phosphoric acid

97.994

158

42.35

1.685

Water

18.015

100.0

0.0

1.00

Experimental: A similar setup as the glassware used for the simple distillation lab was used for the dehydration of 2-methylcyclohexanol. 1.25 mL of 2-methylcyclohexanol was placed in a 5-mL long-neck round-bottom flask in addition to a boiling chip and clamped in place (Reference 3). 0.25 mL of 85% phosphoric acid was slowly added, loosening the clamp as needed, and the contents of the flask were gently mixed. The distillation head was fitted with a thermometer adapter and thermometer, then inserted firmly into the top of the round bottom flask. A small Erlenmeyer flask was placed inside a beaker with ice water, and a three-finger clamp was used to clamp the beaker. The arm of the distillation head was wrapped with a wet paper towel, and the sand bath was plugged into the Variac. The Variac was set to a maximum of 40 and not plugged directly into the wall. The sand bath temperature was slowly raised to about 100-105℃, and the liquid was distilled until about 0.5 mL remained (not distilled to dryness). When isolating the product, the Erlenmeyer flask was first checked for the appearance of two separate layers. The bottom layer was carefully removed with a pipette. Then, the remaining liquid was pipetted out into a vial and a few beads of calcium chloride were added to dry the product. The vial was capped to prevent evaporation and allowed to sit for five minutes. The

calcium chloride “dried” the liquid product by removing residual water during this time. A clean, dry sample vial was obtained and weighed empty before transferring the dry product to the vial. The vial was capped and weighed to determine the amount of alkene obtained and the percent yield. Afterward, the product was submitted for analysis by gas chromatography after checking to ensure that the sample was clear, with no residual solid or water.

Results: Empty vial - 16.31g Vial with product - 16.47g Alkene mass - (16.47 - 16.31) = 0.16g

Area of first peak = 476.96 Area of second peak = 8172.68 Area of third peak = 34152.85 Total area of peaks = 44018

Discussion: The reaction performed in this experiment can yield 3 possible products depending on the pathway taken. The three possible products, 1-methylcyclohexene, 3-methylcyclohexene, and methylenecyclohexane all have different boiling points, the lowest of which being methylenecyclohexane and the highest of which being 1-methylcyclohexane. The difference in boiling points is useful because it allows for analysis through gas chromatography. The different boiling points cause the different substances to register at specific places in the gas chromatograph, and by analyzing the sizes of the peaks it is possible to determine the relative amounts of each product. Determining which product is most abundant in turn shows which

mechanism is preferred in the reaction. The largest peak in the gas chromatograph is the last peak with an area of 34152.85, meaning of the 3 products it has the highest boiling point. This shows that 1-methylcyclohexane is the most abundant product whereas the compound with the lowest of the 3 boiling points, methylenecyclohexane, is the least abundant. This makes sense because in step 3 of the reaction (Figure 2) either a secondary or tertiary carbon has to give up a hydrogen to form a double bond. Tertiary carbons are more stable than secondary carbons, so the hydrogen is more likely to be removed from the tertiary carbon than the secondary carbon. This results in 1-methylcyclohexene.

Conclusion: The key for this experiment to work is the difference in boiling points of the possible products. They are all different and therefore it is easier to tell them apart. As a result, gas chromatography can also be used to show the different boiling points. While a percent yield of 13.76% is not good, that was not the goal of the experiment. The true goal is to simply make the product as pure as possible. Ways to improve this experiment would be to have a proper set up before starting the experiment and to insulate the distillation column to keep a higher temperature throughout.

References: 1.) Kahn, K. (2007). PC gamess TUTORIAL: Dehydration Reaction, Part 1. Retrieved February 12, 2021, from https://people.chem.ucsb.edu/kahn/kalju/chem226/public/pcgamess_tutorial_A1.html

2.) Moores, B. (2010). Background. Retrieved February 12, 2021, from http://www.thecatalyst.org/experiments/Moores/Moores.html 3.) University of Houston. (2009, October 20). Dehydration. Retrieved February 12, 2021, from https://www.slideshare.net/chem3221/dehydration-2296257...