Objective: The purpose of this lab is to dehydrate an alcohol using phosphoric acid. The purity of the crude product will then be analyzed through gas chromatography and refractive index. The crude product's identity will also be tested through decolorization with bromine and oxidation with chromic acid. The success of the experiment will also be determined by calculating a percent yield based on the amount of final crude product formed and comparing that to a theoretical yield.. By doing these activities, we will gain a deeper understanding of one of the many ways an alcohol can be used how to synthesize an alkene.

Lab 2: Cyclohexene Synthesis

Compounds of Study:

Cyclohexanol

Formula: (C6H12O)

Molar Mass: 100.158 g/mol

Density:

962 kg/m^3

MP: 25.93°C

BP: 161.8 °C

Polarity: Polar

Refractive Index: 1.465

Image Source: EMD Millipore

Cyclohexene

Formula: (C6H10)

Molar Mass: 82.14 g/mol

Density:

811 kg/m^3

MP: 103.5°C

BP: 82.8 °C

Polarity: Nonpolar

Refractive Index: 1.446

Image Source: Mol-Instincts

Phosphoric Acid

Formula: (H3PO4)

Molar Mass: 97.99 g/mol

Density:

1.88 kg/m^3

MP: 42.35 °C

BP: 407 °C

Polarity: Polar

Refractive Index: 1.433

Image Source: TCI America

Pre-Lab:

1.The first week of lab, when you arrive in the lab you will first set up a simple distillation apparatus using a 50 mL RBF as a collecting flask wrapping some foil between the condenser adaptor and the collecting flask so minimize evaporation, then you will mix the reagents in another small RBF attach it to the distillation apparatus and start heating, then you will collect distillate (carefully watching the time) and then seal your distillate and clean. Using your own words, generally outline what you will be doing in the lab week 2. Your outline can be simple, like just demonstrated for a week 1 outline, following the template of, “ Then the second week of lab first we will… and then…. and then….” Your outline can be very simple.

1.) Prepare a reaction solution by mixing 20.3 mL of 0.195 mol of cyclohexanol and 7 mL of concentrated phosphoric acid and by slowly adding chilled phosphoric acid into a round bottom flask containing cyclohexanol.

2) Distil cyclohexene from the reaction mixture by simple distillation into a 50 mL round bottom flask. Distillation will be stopped when the temperature reaches 100 degrees Celsius.

3.) Wash the cyclohexene product with a saturated bicarbonate solution.

4.) Wash the cyclohexene with water and pH test the water to ensure washing is complete.

5.) Washed with a saturated salt solution and dry with anhydrous magnesium sulfate.

6.) Calculate the isolated yield.

7.) Perform analysis of the purity and identity of the product through gas chromatography, bromine test, and chromate test.

8.) Clean your work space by making a warm water soapy solution in one of the laboratory sinks and soaking all distillation glassware. Add 25 mL of water into the reaction flask, and dispose of the acid in the proper container in the chemical hood. Add a second 25 mL of water and dump it into the proper container in the chemical hood to ensure all acid is removed from class and properly disposed of. Clean all distilling glassware and place clean glasses into lab drawers. If your gloves smell, leave them in your chemical hood to air out.

9.) Choose one additional laboratory cleanup activity from the cyclohexene synthesis lab handout to complete before leaving the lab.

10.) Complete laboratory write-up and post-lab for lab notebook.

2. Draw the mechanism of the E1 reaction with cyclohexanol and phosphoric acid in single steps using curved arrow formalism. You are required to make your own drawing and you must fully draw out the structure of phosphoric acid. Copying of one another’s work is not allowed. You can draw on paper, take a picture then upload this picture.

See the image labeled "2.)" below.

3. Draw the energy diagram describing this reaction. You can draw the energy diagram on paper, take a picture and upload the picture. You are required to make your own drawing. Copying of one another’s work is not allowed.

Seethe image labeled "3.)" below.

4. Explain why distillation is stopped at not much higher than 100 oC even if significant material remains in the reaction flask.

The melting point of cyclohexene is approximately 103.5 oC, if the distillation temperature rises about the melting/boiling point of cyclohexene it will distill out, making it impossible to collect and analyze in our final product.

5. In your cyclohexene collection flask, you will notice when you are conducting the distillation that there will be two layers, cyclohexene and water. Where is the water coming from? Will the water be the top layer or the bottom layer?

The water that appears in the dehydration flask comes from the dehydration reaction. Water is formed as a byproduct of the dehydration reaction of cyclohexanol and phosphoric acid.

6. Before washing, what will likely be the pH of the crude material considering what you have mixed into the reaction flask? After washing, what should be the pH of water wash if you have successfully washed out reactant contaminants? If the first water wash is closer to the pH of the crude material, what should you do?

Before washing, the pH of the crude product will likely be a lower number, indicating an acid is still present in the compound. After the water wash, the pH should be more neutral (close to 7) as the residual acid catalyst and any water-soluble impurities will be washed away by the saturated bicarbonate solution and water.

If after completing the water wash the pH is still acidic, the water wash should be repeated until the water wash pH becomes neutral.

7. For this synthesis lab, you need to calculate percent yield. This requires that you divide your actual yield by the theoretical yield. How do you calculate theoretical yield for a chemical reaction? You can read about this in the laboratory text but also do your own research using internet resources.

Theoretical yield is calculated by first, determining what the limiting reactant is in a reaction, converting the mass or volume of the limiting reactant into moles, finding the mole ratio between the limiting reactant and the product (i.e. 2A:3B), calculating the moles of product formed, and converting the moles of product formed into mass or volume to get the theoretical yield.

8. If a starting material had a refractive index (RI) value of 1.000 and the expected product had a RI of 2.000 and the actual lab value you measured in the lab was 1.800, what would you roughly conclude about the purity of your product even though you lack a standard calibration curve?

The value measured in lab is closer to the expected product RI than the starting material's RI. This indicates that cyclohexene is likely present within my product but likely contains impurities, such as unreacted starting material, that prevent my RI from matching the expected RI of 2.000.

9. In the O-chem I lab we regularly performed TLC. Read about Gas Chromatography either online or in your laboratory text and explain the following:

How is the sample introduced onto the resin (the solid phase) for gas chromatography?

A small amount sample is injected into the system into a gaseous "mobile phase." The sample is then vaporized and carried into the rest of the column.

What is the mobile phase for a gas chromatography instrument?

The mobile phase is a stable and non-reactive gas that carries a vaporized sample through the column. Some of the most commonly used gases are Helium and Nitrogen.

TLC separated by polarity, how does GC separate compounds?

Gas chromatography separates compounds based on their volatility (how easily a chemical turns into a gas or vapor). Compounds with lower boiling points will vaporize faster and more quickly through the column, while compounds with higher boiling points will take longer to vaporize.

Gas chromatography also separates compounds based on their polarity. If the polarity of the stationary phase (the liquid coating the inside of the column) and the compound as similar, the retention of of that gas will increase, making polar compounds linger longer in the column.

Scanned Documents.pdf

Reaction Progress.pdf

Methods:

Activity #1: Synthesis of Cyclohexene

A reaction solution of approximately 20.3 mL of cyclohexanol (0.195 mol) and approx 7 mL of concentrated phosphoric acid was prepared in a 100 mL round bottom flask.
A distillation apparatus was set up with all of the hinges greased for the collection of cyclohexane (see Image 1). Boiling stones were added to the solution flask to ensure uniform boiling within the solution. Distilled cyclohexane was collected in an ice bath in a 100 mL beaker.
The hot plate was set to a temperature to high and the overall temperature of the distillation apparatus was monitored. As cyclohexene was distilled, a foul-smelling odor was noted.
The distillation apparatus was carefully monitored to ensure the temperature did not exceed 100 degrees Celsius. Once 20 mL of distillate was obtained, the hot plate was turned off and the distillation apparatus was allowed to cool. The total distillation process took approximately 1 hour and 30 minutes to run to completion.
The distillate was sealed into a round bottom flask with a glass stopper and wrapped in parafilm.
The round bottom flask was then placed into a beaker which was then stored in a laboratory drawer until analysis for purity and identification of the distillate could be performed the following week.

Activity #2: Laboratory Cleanup:

A soapy warm water solution was created and all glassware was placed in the solution to soak.
25 mL of water was added to the reaction flask. The flask was swirled and contents were poured into the acid was container in the O Chem lab.
A second 25 mL of water was added to the reaction flask and the above step was repeated.
The glassware that was left to soak was then rinsed and returned to it's proper location within the O Chem lab.

Activity #3: Isolation and Analysis of Cyclohexanol:

The product that was isolated during last week's lab was retrieved from the laboratory drawer and placed into a separatory funnel.
The product was then washed with 5 mLs of a saturated bicarbonate solution.
The product was then washed with approx 15 mLs of DI water
The pH of the solution was tested and determined to be 6.0.
A small amount of magnesium was then added to the solution and swirled to remove the remaining water from the solution.
The volume of the final product was measured to be 13.07 mL or 13.07 g of a clear, colorless liquid with a foul-smelling aroma.
Some of the final product was then used for a bromine test - some color change of the bromine solution was observed but some solution remained unreacted.
A small amount of the solution was also used for a chromate test and a notable color change was observed.
Another small of the solution was used to run gas chromatography analysis.
The remainder of the final product was added to in the “Cyclohexene student prep” bottle in the lab.

Image 1 - Distillation Apparatus

This image shows the distillation apparatus that was used in activity one for the synthesis of cyclohexene.

Results:

Cyclohexene:

Theoretical Yield: 16.02 g

Isolated Yield: 13.07 g of a foul-smelling product.

Percent Yield: 13.07 g/16.02 g = 81% yield.

Chromate Test: Positive

Bromine Test: Inconclusive - slight color change, however, some of the solution was unreacted.

Gas Chromatography:

Peak Height: 89.1 mv

Retention Time: 8.725 min

RI: 1.4495

Image 2 - Gas Chromatography

This image shows the Signal (mV) vs Time (min) graph for gas chromatography of final cyclohexene product.

Discussion:

This experiment resulted in an 81% percent yield of a clear, foul-smelling liquid consistent with the characteristics and appearance of cyclohexene. This yield is considered successful as cyclohexene formation is produced through the E1 reaction mechanism and directly competes with Sn1. The effectiveness of this reaction was increased by running the reaction at an elevated temperature.

The final product was tested using a chromic acid test. A color change was noted from orange to green indicating a positive result and the presence of an alcohol group within the final product. This indicates that there is likely some unreacted cyclohexanol within the system.

A bromine test was also run which showed some of the solution decolorized while some remained unreacted. This indicates that there are likely some products with alkene character in the solution but still some unreacted cyclohexanol, which is consistent with the prior chromic acid test results.

The product was tested via gas chromatography which resulted in a peak height of 89.1 mv and a retention time of 8.725 min. The standard retention time of cyclohexene is 4.53 min and 9.58 minutes for cyclohexanol. This reveals inconclusive data indicating that there may be both cyclohexene, cyclohexanol, or another contaminant within the final product.

Finally, the product was tested based on its RI value. An RI value of 1.4495 was found. This value is greater than the literature value for cyclohexanol (1.465) and cyclohexene (1.446), indicating that the product measured is slightly denser than cyclohexene and cyclohexanol and may contain contaminants.

Conclusion: This lab was somewhat effective in it's synthesis of cyclohexene as the qualitative testing reveals the presence of both cyclohexene and cyclohexanol within the final product. The results of the bromine test and gas chromatography are inconclusive and with further distillation and purification, it is possible that more cyclohexene would be present upon analysis.

Reflection: In this lab I learned about gas chromatography and it's used in identifying unknown compounds. I continued to practice qualitative tests such as the bromine test to determine the presences of alkenes, alkanes, and alkenes within a compound. If I were to repeat this experiment, I would allow the distillation process to run for longer at a higher temperature to hopefully aid in the purification and transformation of a greater amount of cyclohexanol into cyclohexene.

Post-Lab:

1. In good detail explain how La Chalier’s principle was used to increase the yield for the reaction.

Pulling off the product as it's made, La Chalier's principle helps chemistry progress forward toward the products rather than causing the chemistry to move in the "reverse" direction toward the reactants. Heat was used to help ensure the reaction had a higher yield.

2. As explained, normally GC would be used to analyze a liquid product. Read the section on GC in your laboratory text and list three ways that Gas Chromatography (GC) differs from Liquid Column Chromatography (LC). As a reminder, Liquid Column Chromatography is what you performed when you were using the chromatography columns packed with silica gel.

If you do not have your own copy of the laboratory manual, there is one in the chemistry coaching center, there is one setting on the shelves just outside 430, also one that can be borrowed during office hours.

1.) Gas chromatography uses a gas like helium or nitrogen as the mobile phase while liquid chromatography uses a liquid as the mobile phase.

2.) Gas chromatography uses a liquid as the stable phase while liquid chromatography is packed with a solid.

3.) Gas chromatography works best for volatile compounds while liquid chromatography works best for non-volatile compounds with a high molecular weight.

3. What product would be obtained by the dehydration of 3-methyloctan-4-ol? Give the formal IUPAC name. You will need to remember if dehydration will be Zaitseff or Hoffman and if the reaction is more likely to form a cis- or trans-product.

(3E)3-methyloct-3-ene.

The most likely elimination follows Zaitsev's rule, and results in a trans product.