Meeting 8/9 Problem Set

ATTN: answers to the below questions are due at the start of your lab period; these answers should be part of your pre-lab write-up

1. Referring to the Grignard reaction, answer the following questions.

a. Why is it important to clamp the setup at the neck of the flask?

b. Why and how is water excluded from the reaction?

c. In the begininng, the solution with the Grignard reagent is placed in an ice-bath. Rationalize this step.

d. During the work-up, a student is asked to add 10% sulfuric acid to the reaction mixture. Show pertinent equations. Which precaution have to be taken here?

e. In order to recrystallize the crude, a student dissolves it in petroleum ether at room temperature, and then places the solution in an ice-bath. What would he observe?

f. Another student uses dichloromethane as solvent for the recrystallization. What would he observe?

2. Referring to the TLC part of the experiment, answer the following questions.

a. Assume that the following solvents are provided in the lab: petroleum ether, toluene, methanol and diethylether. Which single solvent would be most suitable for the TLC in order to accomplish a good separation between the product and the reactant?

b. A student uses ethanol as solvent and observes that all R_f-values are pretty high and that the separation of the compounds on the plate is very poor. What can he do to improve the separation?

c. Which compounds should be spotted on the TLC plate in the lab?

d. How are the spots visualized?

3. Which of the following solvents is suitable for Grignard reactions? Explain briefly why or why not.

a. Ethanethiol

b. Acetonitrile

c. Triethylamine