An Observation of Catalytic Hydrogenation

Catalytic Hydrogenation

Introduction

Hetergenous catalysts are used to perform many chemical reactions. They are used in refineries to break long chains of carbons in to shorter octane fragments that make up gasoline. The catalyst in this case is at a different phase than the substrate. Meaning that a solid catalyst on a liquid substrate or a solid catalyst on a gas substrate.

In this experiment Pd will be used to convert cyclohexene into benzene and cyclohexane. In this reaction cyclohexene will be heated with palladium on carbon. The cyclohexene will act as a hydrogen donor and a hydrogen acceptor to make benzene and cyclohexane. It can be predicted that for every mol of benzene produced two moles of cyclohexane will be produced.

Experimental Procedure

To begin weigh out 0.100 grams of palladium/carbon and place in a test-tube. Then add 1.6 mL of cyclohexene to the test tube along with two boiling chips. The test tube must then be attached to an air condenser, which is then wrapped in a wet paper towel. Lower the setup into a sand –bath that is set to 3 on the variac box. Allow the test tube to heat up for 15 minutes, making sure to control the boiling from becoming to violently by raising and lowering from the sand-bath.

After 15 minutes remove the test tube and let it cool for 2-3 minutes. The catalyst must then be filtered off by using two Pasteur pipettes. The first pipette must be stuffed with a small piece of cotton and place into a weighed graduated cylinder. Then use the second pipette to remove the solution and transferred into the first. Use a rubber ball to push the solution through the cotton and into the graduated cylinder. Then reweigh the graduated cylinder to calculate the weight of the product. Finally perform gas chromatography on the solution to determine ratio of products.

Data/Calculations

The original data and calculation are included. The yield for cyclohexane was determined to be 26.38% and 33.81% for benzene. The ratio of the products was 2.56 moles of benzene to 1 mole of cyclohexane.

Results and Conclusion

The relatively low yields were most likely due to trapping the products in the cotton during filtration. Also slightly more cyclohexane being trapped in the cotton may have resulted in the slightly higher than anticipated ratio of benzene to cyclohexane.

Overall the ratio was within the anticipated amount. However, after the reaction was complete more cyclohexene was seen than either of the products. In fact a calculated 0.00517 moles were recorded in the final solution in comparison to the 0.00139 moles of cyclohexane and 0.0356 moles of benzene. More products could have been formed by allowing the reaction to progress longer before performing filtration.

Discussion Questions:

Why do you expect a 2:1 ratio of cyclohexane:benzene? (1 pts)

In this experiment cyclohexene acts as a hydrogen donor and hydrogen acceptor. For a benzene to be formed one cyclohexene must first donate two hydrogens to form the two additional double bonds. These two hydrogens are then used to create two cyclohexanes, as only hydrogen is needed for each cyclohexene to be turned into a cyclohexane. This means that for every 2 cyclohexanes formed 1 benzene is formed.

Why is the boiling point of compounds in a mixture an important consideration when running gas chromatography? How might a solid be a problem? (1 pts)

A gas chromatograph separates the components of a mixture based their volatility or how easily they turn into gas. Boiling point is a measurement of the temperature a liquid turns into gas. Meaning that a lower boiling point will make it easier for the liquid to turn into gas and it will pass through GC faster.

Gas chromatography can only be used on low volatility substances. Thus a solid, which will take much more energy to turn into a gas, will not be able to be analyzed. It would also probably mess up the machine or not even fit into the little hole.

What % conversion from cyclohexene to products did you observe? (1 pt)