1. General

Review the following topics: recrystallization theory, vacuum filtration, GC chromatography

DO NOT DISCARD ANYTHING INTO THE WASTE CONTAINER BEFORE YOU ISOLATED YOUR FINAL PRODUCT.


2. Experiment

Due to the moisture sensitivity of the Grignard reagent it is very important that you dry all your glassware very thoroughly. All bottles should be closed if not in use to keep the moisture out of the solvents and reagents.

Check the label of the Grignard solution for its concentration. You will need ~12 mmol of Grignard reagent for the reaction. Make sure that your setup is already assembled before the TA dispenses the reagent with the syringe provided. The lab support provides a 1.4 M solution of MeMgBr in toluene/THF this quarter.

During the work-up, make sure that you add the 10% sulfuric acid solution slowly especially in the beginning. Why?

After you separated the organic and the aeqeous layer the first time (be careful here!), the aeqeous layer has to be extracted again with the more ether. Why?

The combined organic layer have to be dried over a minimum quantity of anhydrous MgSO₄.

In some cases, two crystallizations will be required in order to get a relatively pure compound. In rare cases, one crystallization will be enough. Make sure not to place the solution in an ice bath. What are you trying to remove at this point? How do you know if you are done purifying the sample?

Submit two GC samples (~3 mg/mL) for evaluation. (The samples should not contain any visible amounts of water since this will ruin the column and the separation will be rather poor as well.) One from the crude product and a second one once you isolated your final product in order to evaluate its purity. Make sure that you write down the bar code number (1-999) of your vials. Sign in your name and vial numbers in the list that the TA provides. The spectra can be picked up from the instructors office usually in the afternoon of the following day.

Leave a little of your crude product behind for TLC.

Do not dispose of your compound since you will need it again during the next meeting to obtain a TLC spectrum.


3. Things to think about

a. In many cases the solution turns purple/brown during the recrystallization. Why?

b. Which compounds can be found in the crude product?

c. Why is it important not to use too much drying agent in this experiment?

d. How many signals do you expect to see for the product in the ¹H and ¹³C-NMR spectrum?

e. Why are 1,2-dibromides not good starting materials for Grignard reactions?