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. Why is it imperative to exclude water from the reaction here? How is this task accomplished in this reaction?
b. Why is the round-bottomed flask placed in an ice-bath prior to addition of the benzoin solution?
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. After combining the organic layers, they are extracted with sodium bicarbonate solution. Explain briefly.
e. Since lab support ran out of high-boiling petroleum ether, they provide ethyl acetate as solvent for the recrystallization. Would this work here as well?
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: toluene, ethanol and ethyl acetate. 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 hexane 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?