last updated Monday, October 15, 2007

Problems Set #6

ATTN: answers to the below questions are due at the start of your lab period of Meeting 6; these answers should be part of your pre-lab write-up.

Announcements:

1. Make sure that you make progress on your formal report as well in terms of literature research and writing the parts that you completed already in the lab. The draft version of paper is due on 10-26-2007 at 5 pm. The draft version is worth 15 points, the final paper 45 points. For more information what the draft is supposed to contain see here. Late submission will receive less credit. The more of your work you submit here, the more feedback you will get, which will beneficial for the final version. We will try to return the reports by Monday.

2. The next quiz will be administered on Monday, October 15, 2007 at 10 am covering the resolution step and the ligand synthesis. Please make sure that you show up on time. Otherwise you will not be allowed to take the quiz.

1. Referring to the catalyst design, answer the following questions.

a. What is the function of the tert.-butyl groups in the catalyst?

b. The main function of the diamine bridge is control the approach of the alkene during the epoxidation step. One could make the point that the hydrogen atom is not much of a hindrance. However, when replaced by larger groups i.e. methyl the selectivity often is greatly reduced. Rationalize this observation.

c. Some newer variations of the catalyst bind the catalyst to a support like a polymer or silica. Which advantages and disadvantages do you see in this?

d. Why are reactions carried out a lower temperatures (T= -78 ^oC) often times more stereoselective than reaction carried out at room temperature?

e. Some of the catalyst variations possess methoxy groups on the benzene ring. In which way do these groups improve the catalyst?

2. Referring to the epoxidation reaction carried out in the lab, answer the following questions.

a. Why is the reaction mixture stirred vigorously?

b. What is the function of Na₂HPO₄ in this reaction?

c. How are the alkene and the epoxide visualized in the TLC experiment? Outline a brief procedure.

d. The student starts off with a solvent mixture (dichloromethane:hexane=1:1) for TLC. The procedure asks to optimize the mixture? What does this means and how is this accomplished?

e. Why is hexane added after the reaction is completed?

f. How does the crude product appear?