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Dehydration of 2-Methylcyclohexanol

Writer: Daylon Wingate Reviewer: Daylon Wingate Editor: Daylon Wingate

Introduction The purpose of this experiment is to explore the effect of acid-catalyzed dehydration and acquire familiarity with gas chromatography analysis. Dehydration is a chemical process of removing an alcohol from an alkane to yield a more reactive alkene. The dehydration of 2methylcyclohexanol was performed to yield one or more products of three possible organic compounds.

Figure 1. General reaction and products for the dehydration of 2-methylcyclohexanol.

Dehydration reactions of alcohols are greatly affected by the substitution of the carbon that is directly bonded to the alcohol group. The secondary substitution pattern of 2methylcyclohexanol makes it prone to some rearrangement due to formation of a carbocation intermediate.

Figure 2: Mechanism for the formation of carbocation intermediate.

After formation of the intermediate the water produced from the alcohol leaving group reacts with a hydrogen neighbor to the carbocation and removes it from the ringed compound. This is based on an E1 elimination mechanism and thus, consequently produces a carbon-carbon double bond.

Figure 3: E1 elimination mechanism of neighboring hydrogen atom.

The third possible product stems from the rearrangement of the ringed compound through a hydride shift. The shift occurs to create a more stable tertiary carbocation. The carbocation creates a dipole moment for the methyl substituent and consequently becomes subjected to deprotonation by another water molecule, which yields the terminal alkene substituent.

Figure 4: Formation of terminal alkene substituent.

According the Zaitsev’s rule the major product of the experiment should be the most substituted and therefore most stable alkene. The identity of the major product can be determined through gas chromatography analysis, which will allow the calculation of the percentage of each compound. Area of Peak x 100 % of ∑ all peak areas

Compound %=

Figure 5: Gas chromatographer functional schematic.

Table 1: Physical properties of relevant compounds.

Experimental

A hot sand distillation apparatus was used during this experiment. A round bottom long neck flask was acquired prior to completing the setup of the distillation apparatus and 1.3 ml of 2-methylcyclohexanol was added to the flask. Several boiling chips were placed in the flask and 0.75 ml of phosphoric acid was added and mixed. The flask was then inserted into the apparatus and a paper towel was tightly wrapped and secured around the neck of the flask. A wet paper towel was wrapped around the collection duct to facilitate condensation. After distillation was completed, two layers of distillate were present, and the bottom layer was removed, and the remaining product was placed into a vial. Calcium chloride beads were then used to chemically dry the product. The product was chemically dried 4 separate times with increasing amounts of calcium chloride due to presence of cloudy precipitate. A vial was weighed prior to drying and was determined to have a weight of 4.9 g. The final dried product was then weighed in the same vial and was determined to have a final weight of 16.49 g. The final product was then submitted for GC analysis. Results Ultimately the percent yield was determined using the following general equation to be 2,144.34%.

Percent yield=

actual yield × 100 % theoretical yield

Gas chromatography analysis of a different experimenter’s product was acquired and analyzed to have peak areas of 476.96; 8172.68; 34,152.85. The sum of all peaks was 44,018 and the following calculations indicate the acquired compound percentage for each peak.

Compound % ( 1) =

476.96 x 100 %=1.1 % 44,018

8,172.68 Compound % ( 2) = x 100 %=18.6 % 44,018

Compound % ( 3) =

34,152.85 x 100 %=77.6 % 44,018

Figure 6: Gas chromatography results.1

Discussion The results of the experiment performed were ultimately useless. The product percent yield is irrationally high. This is likely due to several reasons such as, increased amount of phosphoric acid used during distillation, the multitude of chemical drying, but most likely the reason is due to an equipment failure of the weighing scales used. The initial mass of the empty vial was 4.9 g. When the product was dried the same scales gave a mass of 3.45 g. Clearly the

same vial could not have weighed less with an additional substance within it so multiple weights were taken using the same scales and a different result was given each time. On some occasions the scales continuously varied in its measurement and never settled on a final mass. Separate scales were then used and yielded a mass of 16.49 g consistently. The initial weight of the vial must have been inaccurate. The GC analysis acquired indicates that the compound that occurred in the greatest amount was the one with the highest boiling point. A higher boiling point itself is indicative of a structure that is more stable, thus the major product was the tertiary cycloalkene and conforms to Zaitsev’s rule. The results of this product are typical for the experimental outcome. Conclusion The most important conclusion to draw from these results is that the more substituted alkene is produced in the greatest amount due to increased stability. This is significant in prediction of possible products for other similar reactions. For future experiments, it would be best to utilize a glass pipette in addition of reagents to flasks and assuring that the equipment to be used is functioning properly to avoid possible skewed results.

1

Dr. Brock Casselman. Gas Chromatography Data Set. https://uab.instructure.com/courses/1532137/pages/sample-gc-data? module_item_id=15807450 (October 07, 2020).

2

Dr. Brock Casselman. Experiment 6: Dehydration of 2-methylcyclohexanol. https://uab.instructure.com/courses/1532137/files/64289419? module_item_id=15807095 (October 07, 2020).

3

Brown, Henry et al. Reactions of Benzene and its Derivatives. Organic Chemistry, 8th edition; Cengage Learning. Mason, Ohio, 2013....