Dehydration of 2-Methylcyclohexanol Lab Report

Abstract

In Experiment 9, the dehydration of 2-methylcyclohexanol was carried out using a 4:1 mixture of phosphoric and sulfuric acid as a catalyst.

Gas Chromatography was employed to measure the products, resulting in the formation of two major products, 3-methylcyclohexene and 1-methylcyclohexene. The theoretical yield of the alkene mixture was 3.914 g, while the actual yield was 0.73 g, resulting in a percent yield of 18.65%. The Gas Chromatography data confirmed that the major product, 1-methylcyclohexene, aligned with Zaitsev's rule.

Introduction

The experiment aimed to perform the dehydration of 2-methylcyclohexanol using a 4:1 mixture of sulfuric and phosphoric acid as a catalyst. This reaction is an elimination reaction in which water is eliminated, and alkenes are formed as products. The reaction follows Zaitsev's rule, which states that the more highly substituted or stable double bond will be the dominant product.

2-methylcyclohexanol has a poor leaving group, the hydroxyl group (OH-), but when a strong acid such as H+ protonates the oxygen in the hydroxyl group, it becomes a good leaving group in the form of water.

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This leaving group departs as water, forming a carbocation intermediate under the reaction conditions. With no other nucleophile present besides water, the intermediate undergoes elimination to achieve stability. This results in the formation of two major alkenes: 3-methylcyclohexene and 1-methylcyclohexene. According to Zaitsev's rule, 1-methylcyclohexene should be the major product.

Reaction Scheme

2-methylcyclohexanol → 3-Methylcyclohexene + 1-Methylcyclohexene

Table of Physical Constants

Experimental Procedure

Part 1: Distillation

1. Set up a simple distillation apparatus using a 25 mL round bottom flask.

2. Add 5 mL of 2-methylcyclohexanol and 1 mL of a 4:1 phosphoric acid: sulfuric acid solution to the round bottom flask. Swirl to mix reagents.

3. Heat the flask, maintaining a temperature below 100°C.

4. Continue distillation until approximately 3 mL of distillate is collected.

5. Weigh a beaker containing sodium sulfate.

6. Add the collected distillate to the beaker, cover, and leave for a week.

Part 2: Gas Chromatography

1. Weigh a vial.

2. Retrieve the distillate from the beaker using a pipette and transfer it to the vial.

3. Weigh the vial with the distillate.

4. Obtain 10 microliters of distillate with a syringe and inject it into the Gas Chromatograph.

5. Collect and analyze the results from the computer-generated graph.

Calculations

Part 1

Mass of 2-methylcyclohexanol (C7H14O) = density (0.9304 g/mL) x volume (5 mL) = 4.652 g

Moles of 2-methylcyclohexanol (C7H14O) = mass / molar mass = 4.652 g / 114.19 g/mol = 0.0407 mol

Theoretical Yield of Alkene Mixture (g) = moles of limiting reagent x molar mass of product = moles of C7H14O x molar mass of C7H12 = 0.0407 mol x 96.17 g/mol = 3.914 g

Actual Yield of Alkene Mixture (C7H12) = (mass of liquid + vial) – mass of vial = 9.65 g – 8.92 g = 0.73 g

Percent Yield = (Actual Yield (g) / Theoretical Yield (g)) x 100 = (0.73 g / 3.914 g) x 100 = 18.65%

Part 2

Area of Peak A = ? (1.2 mm) x 0.5 mm = 0.3 mm²

Area of Peak B = ? (2.3 mm) x 5.4 mm = 6.21 mm²

Percentage of Product A = (Area of Peak A) / (Area of Peak A + Area of Peak B) = (0.3 mm²) / (0.3 mm² + 6.21 mm²) = 4.6%

Percentage of Product B = (Area of Peak B) / (Area of Peak A + Area of Peak B) = (6.21 mm²) / (0.3 mm² + 6.21 mm²) = 95.39%

Results and Discussion

The experimental results demonstrated that the dehydration of 2-methylcyclohexanol using a 4:1 mixture of phosphoric and sulfuric acid yielded two major products, 3-methylcyclohexene and 1-methylcyclohexene, in accordance with Zaitsev's rule. The theoretical yield of the alkene mixture was calculated to be 3.914 g, but the actual yield obtained was 0.73 g, resulting in a percent yield of 18.65%. The lower-than-expected yield could be attributed to incomplete distillation and potential product loss during extraction.

Gas Chromatography analysis confirmed the presence of two major products. Peak A represented 3-methylcyclohexene, with a percentage of 4.6%, while peak B represented 1-methylcyclohexene, with a percentage of 95.39%. These results were consistent with Zaitsev's rule, as 1-methylcyclohexene, the more highly substituted alkene, was the dominant product, in agreement with theoretical expectations.

Conclusion

The experiment successfully demonstrated the dehydration of 2-methylcyclohexanol using a 4:1 mixture of phosphoric and sulfuric acid as a catalyst. The major products formed were 3-methylcyclohexene and 1-methylcyclohexene, in accordance with Zaitsev's rule. However, the actual yield of the alkene mixture was lower than the theoretical yield, resulting in an 18.65% percent yield. This lower yield may be attributed to incomplete distillation and potential product loss during extraction.

Recommendations

To improve the efficiency of the reaction and increase the yield, it is recommended to ensure complete distillation of the products. Additionally, meticulous extraction techniques should be employed to minimize product loss during the transfer of the distillate. Further experiments can be conducted to optimize the reaction conditions and enhance the overall yield of the desired products.