General Announcement (Please read!)

1.It has come to my attention that some students are not clear about certain policies in this course. There is no such policy in this lab course which discourages students from asking questions. The reason why you are here is to learn. This should also result in questions from the the student's side, which are welcome by the teaching assistants and the instructor. However, it is expected that the student comes prepared to the lab section in order to be able to complete the experiment in a timely fashion. The majority of the questions regarding the experiment (theory and practical issues) can be answered during lecture and office hours. A question generally should demonstrate the student's ability to analyze the problem and be as specific as possible in his/her inquiry.

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 reaction carried out in the lab, answer the following questions. Show pertinent equations where appropriate.

a. Student A decides to use 95% ethanol for his reaction. Is this a good choice of solvent?

b. A hydroxy group is usually a very poor leaving group. Why does it serve as leaving group in this reaction?

c. Why is it possible to use a relatively weak base like KOH to remove the benzylic protons from dibenzylketone?

d. Why is it necessary to reflux the reaction mixture?

2. A student isolated 1.5 g of a crude product. In order to purify it, he has three solvents accessible for recrystallization.

a. Which solvent should the student choose for recrystallization? Rationalize your choice.

b. How much of the purified compound (in g and %) would the student recover if she performs the recrystallization correctly?

c. What can be said about the polarity of the product?

d. Why can it be an advantage to use a solvent mixture in some cases as compared to a single solvent?

4. The researcher is given the following TLC from his assistant, who carries out the following reaction: A + B --> C.

stationary phase: C18 mobile phase: propanol

a. Calculate the R_f-values for all spots.

b. What can be said about the progress of the reaction?

c. Is the chromatogram correctly obtained?

5. Student C has to obtain the UV-Vis spectrum for his compound, which is polar. In the literature he finds the following information: 230 (25000), 400 (2500), 500 (1500). He is asked to measure the range from 200-600 nm.

a. Sketch the spectrum assuming the largest peak has an absorbance of A=0.75 A.U. Make sure to adjust the other peaks appropriately to their e-values.

b. Which concentration(s) should he use in order to determine the molar extinction coefficient for all peaks in this range?

b. Which solvent would be appropriate here?

c. Which type of cuvette should he use?