Organic Chemistry Lab 3: The Dehydration of p-Menthan-3-ol(Menthol) PDF

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Organic Chemistry Lab 3: The Dehydration of p-Menthan-3-ol(Menthol)...

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The Dehydration of p-Menthan-3-ol(Menthol)

Abstract: The objective of this experiment is to synthesize menthene(a cyclohexene) from menthol(a cyclohexanol) via an alcohol dehydration reaction. The most common catalyst that is used to create a cyclohexene from a cyclohexanol is with the use of an acid catalyst; in this case phosphoric acid was used. Introduction: Many alkenes can be prepared from alcohols by an acid catalyzed dehydration reaction. When a secondary alcohol such as cyclohexanol, is heated with a concentrated acid, such as phosphoric acid, the protonated alcohol can lose water to generate a carbocation. Loss of hydrogen ion by the carbocation will give the corresponding alkene. Commercially available menthol (p-menthan-3-ol) is a racemate (DL-menthol) can be dehydrated in the presence of a strong acid (phosphoric acid) to produce a mixture of alkenes.

The dehydration reaction of alcohols is an elimination reaction done by heating the alcohol in the presence of a strong acid to a high temperature. Because cyclohexanol is a secondary alcohol it should be heated between 100 and 140 for the alcohol to be sufficiently dehydrated. When this secondary alcohol is heated the alcohol becomes protonated by the acid and loses water to generate a carbocation. This loss of water is the dehydration. The carbocation then loses a hydrogen ion and gives in to the formation of the alkene. Since the boiling points of cyclohexene and cyclohexanol are different, the product is separated by using simple distillation. All of the reactions in the reaction are reversible, with the use of hydration. So removing the product as it forms drives the reaction towards product

formation. To maximize product yield, an additional solvent, toluene, can be added to recover all of the cyclohexene. However, the toluene chaser was not used in this experiment.

Table1. Physical and Chemical Properties of Chemicals Used Throughout the Experiment

Compound

Molecular Weight (g/mol)

Density (g/mL)

Boiling Point (°C)

Amount Used

DL-p-menthan-3-ol 156.27

0.89

212.0

10g

p-menth-3-ene

138.25

0.80

173.5

N/A

p-menth-2-ene

138.25

0.80

165.2

N/A

p-menth-1-ene

138.25

0.80

176.0

N/A

Phosphoric Acid

98.9

1.685

N/A

5mL

Experimental:

Cyclohexene was obtained through the use of dehydration from cyclohexanol. 10 grams of cyclohexanol and 5 milliliters of concentrated phosphoric acid were transferred to a 50 mL round-bottomed flask. The flask was then swirled to mix the layers. The mixing of the two liquids will cause the mixture to produce heat. A few boiling chips were added to the flask and and a West condenser to reflux the mixture. After refluxing the mixture for 20 min the set up was changed for a simple distillation The previously refluxed mixture was distilled until at least 5 mL of distillate was collected, noting the temperature range over which the distillate was collected. The distillate was then put into a separatory funnel and mixed with with 15 mL of 5%

aqueous solution of sodium bicarbonate to neutralize the distillate. The lower aqueous layer was removed and the organic layer was washed with distilled water. Once the organic layer was transferred into a clean flask, 0.8g of Anhydrous Sodium Sulfate was added to remove water from the organic layer. The clear liquid was then decanted into a dry 50mL round bottom flask. Finally, a bromine test was performed on the collected sample

Results: Yield Report: Weight of Menthol

10.0g

Moles of Menthol

0.064 mol

Theoretical Yield of Menthene

0.064 mol

Theoretical Yield of Menthene

8.85g

Actual Yield of Menthene

8.0g

Percent Yield

90.4%

Boiling Point(Range) of Product

135 - 148 °C

Calculations: Theoretical yield: No. of moles of reactants: n = 10g/156.27g/ml = 0.064mol ∴Theoretical yield of product: m = 0.064 x 138.35 g/mol = 8.85g

Percent Yield = Actual Yield/Theoretical Yield x 100% Percent Yield = 8.0g/8.85g x 100% = 90.4%

Discussion: The objective of this experiment is to synthesize menthene(a cyclohexene) from menthol(a cyclohexanol) via an alcohol dehydration reaction.  To be sure that the product formed was an alkene a bromine test was conducted. Bromine reacts with alkenes to generate dihaloalkanes through electrophilic addition. The product of the bromine test is colorless. When the bromine test was conducted on the sample obtained, the solution became colorless. Bromine does not react with alcohol, so the solution remains brown in color. The solution gradually became colorless when the brownish dye was added, implying that there was some residual cyclohexanol in the solution that would possibly prevent the solution from becoming colorless immediately. The observed boiling point range of the product also suggests the impurity of the alkene obtained. Zaitsev’s rule predicts that the major product to be formed from the dehydration of menthol is p-menth-3-ene which has a boiling point of 173.5°C, the lower boiling point range observed in this experiment suggests the presence of some residual cyclohexanol which has a lower boiling point og 156.7 °C of p-menth-2-ene which has a boiling point of 165.2°C.

Conclusion: The purpose of this experiment was to make an menthene (a cyclohexene) from a menthol (a cyclohexanol) through a dehydration reaction. Based on the bromine test, a cyclohexene was successfully synthesized in this dehydration reaction....