stationary phase:
Silica
mobile phase:
chloroform
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. Why is absolute ethanol used as solvent in this reaction?
b. Protons on alkyl groups usually are not very acidic. Why is it possible to use a relatively weak base like KOH to remove the benzylic protons from dibenzylketone?
c. Why is the reaction run at an elevated temperature?
d. Why is an extended reflux during the recrystallization step not desirable?
2. A student isolated 3.2 g of a crude product. In order to purify it, he has three solvents accessible for recrystallization.
| Solvent | Temperature in oC | Solubility in g/100mL |
| Heptane | 0 | 0.15 |
| 20 | 0.18 | |
| 40 | 0.20 | |
| 60 | 0.25 | |
| 80 | 0.30 | |
| 100 | 0.40 | |
| Dichloromethane | 0 | 1 |
| 20 | 2 | |
| 40 | 4 | |
| Methanol | 0 | 4 |
| 20 | 5 | |
| 40 | 6 | |
| 60 | 8 |
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 he heats his solution to 40 oC to dissolve the crude, and afterwards allows it to cool to room temperature?
c. What can be said about the polarity of the product?
3. The researcher is given the following TLC from his assistant, who carries out the following reaction: A + B --> C.
|
stationary phase: mobile phase: |
a. Calculate the Rf-values for all spots.
b. Is the chromatogram correctly obtained? If not, how could he improve the quality of the TLC data.
4. Student C has to obtain the UV-Vis spectrum for his compound, which is polar. In the literature he finds the following information: 300 (1000), 400 (5500), 500 (3500). He is asked to measure the visible range only. (Note: The number in parathesis are the molar extinction coefficients)
a. 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?