1. General

The draft version is due on April 23, 2010 at 4:30 pm in YH3077E. Submit submit a printed version to the instructor office. Do not submit the draft version on turnitin.com!

It is on your own interest to provide a draft version (15 points) that is as close as possible to the final version (45 points). This way you get more feedback and can correct mistakes before submitting the final version.

2. Experiment

Assume a weight of 120 g/mol for the unknown alkene (Styrene, a-Methyl-styrene, 3-Methyl-styrene, 4-Methyl-styrene, 4-Chloro-a-methylstyrene, 4-Bromo-a-methylstyrene, 4-Bromo-styrene, 4-Chloro-styrene) and calculate the amount of catalyst required accordingly. If your TA asks you to add pyridine N-oxide, use about twice as many moles as you used for the catalyst.

Make sure that you close the flask with some parafilm during the reaction. The hotplate has to be cold. Why?

TLC is used to monitor the reaction and to optimize the solvent used in the column chromatography. A good solvent (mixture) should show a good separation of the epoxide and the alkene (difference in R_f-values ~0.3-0.4). The epoxide should have a R_f-value about 0.4-0.5. Why?

The spotting should be performed using a capillary spotter (you will have to make them yourself). After you developed the plate in your TLC chamber, the TLC plate should be dried briefly and then placed under the UV-light to visualize the alkene (Circle the spot with pencil). Then dip it briefly into the ceric staining solution, wipe the backside clean and then place it on a warm hot plate. The Mo(VI)-compound in the solution reacts with polar compounds (epoxides, aldehydes, ketones) to form a dark blue Mo(V)-complex. A blue spot appears on the plate. The sketch of the resulting plate is drawn in the notebook and the TLC plate then disposed of. Do not take the TLC plate home. The silica and the chemicals will be all over the place.

The reaction should be terminated if the alkene is not (or barely) visible on the TLC plate anymore. Towards the end of the reaction, you will have to spot a little heavier in order to detect small quantities of the alkene.

As much as possible of the catalyst should be removed during the extraction and solvent removal. This will make it easier during the subsequent flash chromatography step.

It is very important that your crude product is dried as good as possible. Due to the fact that you will perform the column chromatography the following meeting, you need to store the crude in a vial. It might be a good idea to place a small quantity of Na₂SO₄ in there and to cap the vial!

When you boil off the solvent, keep in mind that your product is a liquid. Therefore you should not boil it off to dryness. Generally, a small quantity of a oily residue with a characteristic odor will remain.

3. Things to think about

a. What would change if you have a higher or lower quantity of the catalyst?

b. Why do you need two techniques to visualize your compounds?

c. Why is a two phase system used in this reaction?

d. How does the promoter work?

e. Why should the epoxide have a R_f-value of 0.4-0.5 instead of 0.1-0.2?