The main job for this meeting will be to purify the crude epoxide by column chromatography, and to submit an NMR sample.

Setup:

a. Dry method

Obtain a 25 mL burette that is not be chipped on the top! Clamp it securely in the hood. Place a small cotton ball on the bottom. Then add 0.5 cm of sand on top of it. Level the sand and then add ~20 cm of silica gel (~up to the 10 mL mark). Level the powder and then add another 0.5 cm of sand on top. Prepare ~250 mL of the eluent mixture that you plan to use as mobile phase. Carefully fill the column with the eluent to approximately 1cm from the top. Open the stopcock at the bottom of the column and place a big beaker underneath. Then place a thermometer adapter (or rubber stopper) with a glass tube (has to be checked out from the lab support as well) of appropriate diameter on the top. Then open the regulator for the air supply slowly. Hold on to the adapter. This will generate a pressure that pushes down the solvent faster. When the solvent front is 1 cm away from the sand at the head of the column, take the tubing out and add more solvent. Repeat this procedure until the entire column is wetted. After this happened, allow the solvent level to go down to the sand level.

b. Wet method (slurry)

Obtain a 25 mL burette, and clamp it securely in the hood. Place a small cotton ball on the bottom. Then add 0.5 cm of sand on top of it. Then pour ~10 mL of the eluent into the column, before you add a slurry of the stationary phase (silica gel) slowly using a powder funnel. Allow the solid to settle down. Make sure that there are no gas bubbles in the column. If this is not the case, add ~0.5 cm of sand on the top. After this happened, allow the solvent level to go down to the sand level.

c. Why two methods?

Although the first method is usually faster, many researchers prefer the second method since it limits the formation of gas pockets and cracks. Some more polar solvents can react with the stationary phase and virtually start to boil inside the column. This will lead to the formation of gas bubbles, which will deteriorate your separation quality and the elution speed. In either way, the final product should look like the picture on the left.

Conditioning

Since silica is usually slightly acidic, and the epoxide is very sensitive towards acids, the acidic residue has to be neutralized first. This is done by applying a 1% solution of triethylamine to the column prior to applying the sample.

Running the column

The crude product is dissolved in 2-3 mL of the eluent (or a non-polar solvent). This mixture is then added carefully to the top of the column. The mixture is pushed into the column using an adapter (consisting of a thermometer adapter with a glass tube with a tight seal). The container is rinsed with 2-3 mL of eluent and then loaded onto the column as well. The column is carefully filled with eluent and the air is then used to force it through at a rate of ~10 mL per minute. If the drip rate appears to slow, TA or instructor should be consulted for help. 10 mL fractions are collected in clean test tubes.

Identification of product fractions

Upon completion of the column chromatography step, TLC is used again to identify the fractions that contain the desired product. Starting with fraction 2, a TLC is obtained for every other fraction. Three spots can be placed on one plate. If the proper fractions (fraction range) is identified, the fraction before and after the identified fractions are examined as well. The fraction containing the product are combined and the solvent is carefullly removed.

Analysis

1. A NMR sample of the product is prepared. If enough material is available, ~0.1 mL of the compound is dissolved in 0.5 mL of deuterated chloroform. The solution placed in a clean NMR tube, which is labeled on the top part with a Sharpie (Name, Chem 30CL, Section, Unknown). The sample has to be signed in on a log-sheet provided by the TA. Proper labebling is important in order to provide the correct NMR spectrum to the student later on, and for recovery in terms of further identification e.g. GC/MS.

2. The GC/MS is acquired in a dilute solution. One drop of the epoxide is dissolved in 2 mL of diethylether. 1 mL of this solution is injected into the GC/MS instrument.

3. The small sample quantity usually does not permit to acquire an IR spectrum for the compound.

Make sure that you have all the data that you need to write your formal report as well.