Problem set #2 (Meeting 2)

Announcements

1. You are not allowed to use any reader in the lab anymore starting meeting 2. All information that is needed to carry out the lab should be in your pre-lab report.

2. You will not be permitted to carry any experiments without proper attire!

ATTN: answers to the below questions are due at the start of meeting 2; these answers should be part of your pre-lab write-up. Show pertinent equations where appropriate.

1. Referring to the benzoin condensation, answer the following questions.

a. Why is the thiamine hydrochloride first dissolve in water and 95% ethanol is added afterwards?

b. What observation does the student make upon addition of the sodium hydroxide solution to the solution above?

c. A student starts off with 0.50 g of thiamine hydrochloride and 5 mL of 0.20 N sodium hydroxide solution. Afterwards, he adds 4.00 mL of benzaldehyde to the solution. After the work-up, he isolates 3.60 g of white crystals. Determine the yield for the reaction. Show your calculations.

2. Vacuum filtration

a. Draw a sketch for the setup used for the filtration of the benzoin and label all parts needed.

b. Why is it important to use a clean filter flask for the filtration?

c. After the filtration, the crystals are rinsed with ice-cold water and ice-cold 95% ethanol. Which purpose does this step serve? What does the student observe?

d. Why is it important that the vacuum trap and the filter flask are clamped down?

3. Referring to the elimination reaction, answer the following questions.

a. The reaction mixture is heated during the course of the reaction. Explain briefly why.

b. After the reaction is completed, saturated sodium chloride solution is used to collect the sample. As a result, two layers are observed. Suggest two methods how the the student identifies the layer containing his target compound.

c. What is the function of the sodium sulfate used during the work-up? Why does it have to be removed prior to the distillation step?

d. Why can a distillation be used to separate the cyclohexene from cyclohexanol?

e. One of qualitative tests that is performed to verify the presence of an C-C double bond is the reaction with bromine. Show the pertinent chemical equation (including proper stereochemistry). What does the student observe in this reaction?

4. Infrared Spectroscopy

a. How is the IR spectrum for the final product of the elimination obtained in the lab? Outline a brief procedure.

b. What are the most important changes in the IR spectrum going from cyclohexanol to cyclohexene?

c. How would the infrared spectrum look like if the student did isolate the wrong layer in the separation step?

5. a. Using PC Spartan 2002, perform the following calculations for the alkenes: terpinolene, limonene, a-terpinene, isoterpinolene. Make sure that all hydrogen atoms are present and there are also no extra ones either.

b. Select Calculations from the Setup menu.

The following window should appear. Select the options shown.

Verify that the Charge is Neutral and Multiplicity is Singlet. Click OK.

Select Submit from Setup menu.

Then click OK to exit dialog. Close the previous window.

Select Submit from Setup menu. When the calculation is completed you will be notified. It should not take more than one minute for the calculation to complete under normal circumstances. Make sure that you have only one window open at this point. The messages are usually displayed in the first window, and often not in the window you are working in at this point.

Under the Display menu, select Properties. Record the value for the Energy.

Analysis:

1. Based in the observed trends in trends in the heat of formation (=energy), which alkene is the most stable and which one is the least stable one? Can you rationalize these observations? Do the obtained data match with the expectation? If not, what is the conclusion?

2. Can you explain the product distribution observed for the elimination of water from a-terpinol (see reader page 20)?

Problems with calculations: Check here