Meeting 8/9 Problem Set
Announcements:
1. The Grignard project will take about 1.5 lab periods. This means that most of the students will finish up the project during meeting 9. In addition, there will be also an NMR lab during this meeting which is also mandatory.
2. If you receive a three-necked flask instead of a round bottom flask, you will not need to use a Claisen adapter in your setup anymore. If you break the round bottom or three-necked flask, you will receive an automatic zero for the lab because it shows carelessness. In addition, you will have to pay for the damage!
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 in many reactions, the presence of water poses a problem in this reaction. Explain briefly why. How it is excluded from the reaction?
b. The diluted Grignard solution is placed in an ice-bath prior to addition of the benzoin solution. Rationalize this step.
c. After the reaction is completed, 10% sulfuric acid is added. Which purpose does this step serve and which precautions have to be taken here? Show pertinent chemical equations.
d. The combined the organic layers are extracted with sodium bicarbonate solution. Explain briefly.
e. The crude product can be obtained as a solid or as an oil. Rationalize this observation. How should the student proceed in either case?
f. High-boiling petroleum ether is used as solvent for recrystallization here. Rationalize this choice.
g. Why is it important that the solution is not placed in an ice-bath during the recrystallization?
2. Referring to the TLC part of the experiment, answer the following questions.
a. Assume that the following solvents are provided in the lab: hexane, propanol and diethyl ether. 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 petroleum ether as solvent and observes that all Rf-values are fairly low. 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?
d. Which solvent should be used to dissolve the samples prior to spotting?
e. How are the spots visualized after developing the TLC plate?