Catalytic Transfer Hydrogenation Of Olive Oil

Catalytic Transfer Hydrogenation of Olive Oil

Reference: A. M. Schoffstall, B. A. Gaddis, and M. L. Druelinger, Microscale and Miniscale Organic Laboratory Experiments, 2nd Edition, McGraw-Hill: New York, 2004, Experiment 6.1C, page 241.

Introduction:

Oils are triesters derived from plants, and are usually liquids. They contain cis C=C’s. They are not uniform in structure. They are biosynthesized formed from long-chain carboxylic acids (“fatty acids”) and a triol, glycerol. An example structure is shown below.

H2C O C (CH2)7 C C (CH2)7CH3 O H H

HC O C (CH2)7 C C (CH2)7CH3 O H H

H2C O C (CH2)7 C C (CH2)7CH3 Example of an Olive Oil Molecule

Some of the C=C’s in oils are reduced to form semisolid products, which can be used in margarines, for example. These reductions are usually carried out using hydrogen gas, and a catalyst such as platinum or palladium. Since hydrogen gas is highly flammable, and the glassware to do the reductions is highly specialized, hydrogenations are usually not done in organic labs. There is another process, called catalytic transfer hydrogenation (CTH), which avoids the use of hydrogen gas. Since addition of hydrogen to an alkene is reversible, CTH uses an organic compound as the source of hydrogen. In our experiment, we will use cyclohexene as the hydrogen source. Cyclohexene is a good source of hydrogen, since it can produce the stable aromatic compound, benzene, as a byproduct when two equivalents of hydrogen are lost from it. You can learn more about the stability of benzene in chapter 16 of your lecture text. The palladium removes hydrogens from the cyclohexene, and transfers them to the olive oil. Since the palladium is just transferring hydrogens from cyclohexene to the olive oil, we only need a small amount of it, so it is functioning as a catalyst. We are using an excess of cyclohexene to drive the reaction toward products, since the reaction could be an equilibrium reaction. See the reaction scheme below.

We will semiquantitatively determine how many double bonds are reduced, using the reaction of bromine with olive oil and with your product. The more double bonds present in the oil, the fewer drops of oil will be required to react with a constant amount of bromine. An Example Reaction Scheme:

H2C O C (CH2)7 C C (CH2)7CH3 O H H Pd-C catalyst HC O C (CH ) C C (CH ) CH + 2 7 2 7 3 reflux O H H excess H2C O C (CH2)7 C C (CH2)7CH3 Example Olive Oil Molecule

H2C O C (CH2)7 CH2 CH2 (CH2)7CH3 O H H

HC O C (CH2)7 C C (CH2)7CH3 + O

H2C O C (CH2)7 CH2 CH2 (CH2)7CH3 Example Partially Hydrogenated Molecule Procedure:

To 25 mL round-bottomed flask containing a magnetic stirring bar, add 600 mg of olive oil, 2-4 mL of cyclohexene, and 40-50 mg of 10% Pd on charcoal. Fit the vial with a water condenser. Reflux the mixture using a thermowell for 50 minutes. Cool the mixture to room temperature.

Prepare a Pasteur filter pipette with cotton, and add a small portion of Celite, until you have a layer of about 2-3 cm. Place the filter pipette into a one-hole stopper that fits into your filter flask. Clamp the filter flask to a support bar. Add 1-2 mL of methyl t-butyl ether (MTBE) to the filter pipette using another pipette. Attach the hose to the aspirator to the side arm of the filter flask, and apply gently suction, to draw the MTBE through the Celite to wet it completely. Disconnect the suction hose from the side arm, and pipette 1-2 mL of the reaction mixture into the top of the filter pipette. Reattach the suction hose, and pull the solvent through the filter pipette: the black palladium on carbon catalyst should remain in the filter pipette. Continue adding the reaction mixture to the filter pipette until you have transferred all of it. Rinse the reaction flask with 1-2 mLs of MTBE, and filter it through the filter pipette.

Transfer the filtrate to a pre-weighed 50 mL round-bottomed flask (rinse your filter flask with 1- 2 mL of MTBE, and transfer that to the round-bottomed flask as well, and remove the solvents on the rotary evaporator. Weigh the flask, and determine the mass of product. Your product may be in the form of a buttery mass.

Place the filter pipette containing residual catalyst in a container in the hood labeled "Filter pipettes containing Pd catalyst."

Characterization of the Product:

Bromine in Methylene Chloride Test: Use this test to determine how much unsaturation is present in the unknown sample. Add 10 drops of a 1.0M bromine in methylene chloride solution to a 10-cm test tube. Using a pipette, add dropwise a sample of warmed liquid olive oil until the red color of the bromine disappears. Swirl the tube after each addition. Note the number of drops of olive oil required. Repeat using another 10 drops of 1.0 M bromine solution for a sample of the warmed liquid product. Record the results.

Infrared Spectroscopy: With the help of your instructor, obtain the IR spectrum of your product and compare it with the spectrum of olive oil. Absorption in the spectrum of the product at 3020 cm-1 (sp2 C-H stretching) and 1620 cm-1 (C=C stretching) should be absent if the olive oil has been completely hydrogenated. Also, because cis-alkenes absorb strongly in the 650-720 cm-1 region, this absorption is observed for olive oil, but could be absent in the product. Report Format

Title Page

1. A descriptive title with between 15-25 words. 2. Dates the experiment was performed. 3. Course and section numbers. 4. Your name 5. Your partner’s name, if you had a partner

Body of the Report

1. Observations made during the reaction. 2. Determine the percent yield of product. Show your work. 3. Drops of olive oil needed to decolorize 10 drops of 1M Bromine solution. 4. Drops of product needed to decolorize 10 drops of 1M Bromine solution. 5. Was the oil partially or fully hydrogenated? Explain.

Questions

1. Is there any evidence of any starting material present in the product? Explain. 2. Could any product be lost during the workup procedure? Explain. 3. Predict the product of catalytic hydrogenation of 1,2-dimethylcyclohexene using Pd/C in a hydrogen atmosphere. 4. Predict the product of complete catalytic transfer hydrogenation of methyl oleate with deuterated cyclohexene (C6D10).

H H Methyl Oleate C C

CH3(CH2)7 (CH2)7COOCH3