In a round bottom flask, the alcohol (5 mmol) is dissolved in acetonitrile (5 mL). The following reagents are then added sequentially: aqueous pH 7.0 buffer (1.2 mL), 2,2,2,6-tetramethylpiperidin-1-oxyl (TEMPO) (0.5 mmol), and iodobenzene diacetate (IBD) (5.5 mmol). The reaction is stirred at room temperature, and progress is monitored at 10 minute intervals by TLC (8:2 hexane:ethyl acetate). When the reaction is complete, the mixture is poured into ethyl acetate (25 mL). The solution is washed sequentially with 10% aqueous sodium thiosulfate (2 × 15 mL) and saturated aqueous sodium bicarbonate (15 mL). The organic layer is dried over sodium sulfate, filtered, and concentrated by rotary evaporation. The product is obtained by column chromatography (9:1 petroleum ether : acetone) and characterized by IR, GC-MS, and NMR. The purified product should be saved in a scintillation vial labeled with your name and lab section.
Reminders & Hints
1. Remember that you are NOT performing this procedure as written. You must apply the procedure to your assigned alcohol (geraniol or nerol) and scale the quantities used in the reaction so that your theoretical yield is 0.152 grams.
2. You must complete the Project 3A Prelab Quiz before entering the lab to perform this procedure.
3. Recall that “washing” is the same as a liquid-liquid extraction. The term is applied when the desired substance is NOT expected to migrate into the solution being added (e.g., brine). So, you add the solution (e.g. sodium thiosulfate or brine), mix the two layers, allow the layers to separate, and drain away the undesired solution.
4. You performed column chromatography in CHEM 205L. To refresh your memory on this process, review the technique video here. In addition, a step-by-step example procedure is given below.
5. Remember that you are working on a small scale in this reaction. It is better to measure out a little LESS alcohol rather than a little too much. Even a little bit over your calculated value might throw off the relative amounts of reagents so that the alcohol is no longer the limiting reagent! Transfer small amounts of liquid with pipets rather than pouring!
6. Use your time wisely! Go ahead and set up your TLC materials and chromatography column during the 10-20 minutes your reaction is running and being monitored!
Column Chromatography Step-by-Step
1. Build the chromatography column:
a. In a 150 mL beaker, mix about 20 mL of silica gel with 9:1 petroleum ether:acetone until you have a slurry (melted milkshake consistency).
b. Clamp the column so that it is vertical, place an Erlenmeyer flask beneath it (to collect solvent that passes through), and add the slurry of silica. Allow the solids to settle for a few minutes. You want the height of the packed solids to be about 15 mL as marked on the column. If necessary, add more silica-solvent slurry as necessary to increase the height of your column.
c. Using a Pasteur pipet, add extra solvent to wash down the walls of the column. Use a swirling motion to add the solvent to the walls of the column in order to prevent disturbing the column integrity. Tap the column to make the top of the silica even.
d. Allow the solvent to drain until it reaches the top of the column.
e. Add a small layer (~1 cm) of sand on top of the silica. Wash down any sand from the walls of the column with solvent using the same swirling motion.
f. Use a Pasteur pipet to add a few milliliters of solvent to the top of the column. Drain until it reaches the top of the column.
2. Using a Pasteur pipet, add your reaction sample to the column using the same swirling/gentle addition method described above. Remember that you want a tight band of your sample at the top of the column. Add your sample in a single shot and do NOT try to wash in residue with additional solvent. Allow the sample to seep into the column.
3. Add the mobile phase to the top of the column: Start with just a thin layer and allow that pass into the column. (This will wash in any sample residue.) Then, add a larger layer and fill the column above the silica. Be sure to add gently at first to prevent disturbing the surface of the column. Once you have a few centimeters of liquid above your column, you may pour in solvent along the sides of the column.
4. Allow the solvent to pass through the column, collecting fractions in small test tubes. Refill with mobile phase solvent as necessary. You will probably need to collect about 12 fractions.
5. Perform TLC analysis on your fractions (8:2 hexane:ethyl acetate), spotting the contents of each test tube as it is filled. You can fit 5-6 fractions on a single TLC plate. Visualize the plates under UV and with p-anisaldehyde stain.
6. Use your data to decide which fractions contain pure product. Recombine these fractions in an appropriately sized pre-weighed round bottom flask. Remember that your goal is to collect a purified sample in order to characterize the product structure.
7. Remove the solvent by rotary evaporation and re-weigh the flask to determine the quantity of purified product.