Meeting 8/9 Problem Set
Announcements:
1. This week's project will take about 1.5 lab periods. This means that most of the students will finish up the project during week 9. In addition, there will be also an NMR lab during week 9 which is also mandatory.
2. If you receive a three-necked flask instead of a round bottom flask, you will not require a Claisen adapter in your setup. If you break the round bottom or three-necked flask, you will receive an automatic zero for the lab because it shows carelessness.
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. Show pertinent chemical equations where appropriate.
a. Like often in this lab course, the presence of water poses a problem. In which way does water interfere with the desired reaction and how is it excluded from the reaction?
b. What is the function of the drying tube in this reaction?
c. What is the proper use of the rubber septum?
d. Why is the flask placed in an ice-bath at the beginning of the reaction?
e. After the reaction is completed, the reaction mixture is placed in and ice bath again and chilled 10% sulfuric acid is added slowly. Rationalize these steps.
f. Why is the organic layer extracted with sodium bicarbonate solution afterwards?
g. The crude product is recrystallized from hot petroleum ether. Rationalize the choice of solvent. Which precautions have to be taken here?
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, ethanol, ethyl acetate and toluene. 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 Rf-values are fairly high. In addition, 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? Which solvent should be used to dissolve the samples?
d. How are the spots visualized?