General Announcement (Please read!)
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 for the actual reaction? What is it's common name?
b. Arrange the following three compounds (2-pentanone, pentane and 2,4-pentadione) according to increasing acidity. Rationalize your choice.
c. Why is it necessary to reflux the mixture prior to adding the KOH solution?
d. Why is a solvent mixture used for recrystallization here?
2. A student has 3.5 g of a crude product X. If the solubility of X is 70 g/100 mL at 80 oC, 50 g/100 mL at 60 oC, 40 g/100 mL at 40 oC, 30 g/100 mL at 20 oC and 20 g/100 mL at 0 oC, how many grams of the product would he recover if he conducts the recrystallization properly?
3. While running the PTC reaction, a student acquires a TLC for his reaction mixture. He observes two spots, one at a height of 35 mm, the other one at a height of 45 mm. The solvent moved 7.0 cm up the plate. He uses a plate coated with silica and a mixture of dichloromethane and hexane (2:1) as eluent.
a. Determine the Rf-values for the two spots and assign them to the compounds. Explain briefly.
b.What can be said about the progress of the reaction?
c. His neighbor did not attend lecture and decides to use a solvent mixture dichloromethane and hexane (1:2) as eluent. What would change?
d. What is used to apply the sample to the TLC plate? Explain briefly.
4. A student has to obtain the UV-Vis spectrum for compound W. The literature reports the following peaks: 270 (24500), 360 (10000), 550 (5000). He uses a quartz cuvette and absolute ethanol as the solvent. (Note: The number in parathesis are the molar extinction coefficients)
a. Determine the maximum concentration(s) for solution if he measured the UV range is only.
b. Which color does the product have?
c. Would acetone be an acceptable alterntaive as a solvent here?