1. General

The draft version is due by January 22, 2016 at 4:30 pm in YH 3077E. 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 (20 points) that is as close as possible to the final version (60 points). This way you get more feedback and can correct mistakes before submitting the final version.

2. Experiment

It is imperative that the student starts the epoxidation reaction as soon as possible because the reaction can require as much as two hours to complete!

Assume a weight of 120 g/mol for the unknown alkene (styrene (ρ=0.91 g/mL), α-methylstyrene (ρ=0.91 g/mL), 3-methylstyrene (ρ=0.90 g/mL), 4-methylstyrene (ρ=0.90 g/mL), 4-chloro-α-methylstyrene (ρ=1.08 g/mL), 4-bromo-α-methylstyrene (ρ=1.36 g/mL), 4-bromostyrene (ρ=1.40 g/mL), 4-chlorostyrene (ρ=1.16 g/mL), cis-stilbene (ρ=1.01 g/mL), trans-stilbene (ρ=1.01 g/mL), indene (ρ=1.00 g/mL), 4-vinylbenzyl acetate (ρ=1.05 g/mL), 4-acetoxystyrene (ρ=1.06 g/mL) and 4-vinylbenzyl chloride (ρ=1.08 g/mL) and calculate the amount of catalyst required accordingly. The alkenes have to be measured by volume under the hood.

If your TA asks you to add pyridine N-oxide, use about twice as many moles as you used for the catalyst. Excessive amounts of the additive will lead to problems during the workup.

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 (start with EtOAc:hexane=1:4) 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 display a R_f-value about 0.4-0.5. Why?

The spotting should be performed using a capillary spotter (the student will have to make them himself). After developing the plate in the 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). Next, the plate is dipped briefly into the ceric staining solution (Careful: very corrosive!). After wiping the backside clean, the TLC plate (face up) is placed 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 take a picture) and the TLC plate then disposed in the solid waste bag. 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, the student will have to spot a little heavier in order to detect small quantities of the alkene. The reaction time should not be longer than two hours because the reaction mixture has to be worked up as well.

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 the crude is dried as thoroughly as possible. Due to the fact that the student will perform the column chromatography the following meeting, the student needs to store the crude in a vial. It might be a good idea to place a small quantity of anhydrous Na₂SO₄ in there and to cap the vial!

When boiling off the solvent, the student has to keep in mind that the products of the epoxidation are high-boiling liquids (i.e., styrene oxide: 194 ^oC (100 ^oC at 40 torr)). 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 had a higher or lower quantity of the catalyst?

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

c. Why is a biphasic 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?

f. How could the enantioselectivity of the reaction be increased?

g. Why is the use of hydrocarbon solvent not advisable here?