Problems Set - Meeting #4
Announcements:
1. Make sure that you make progress on your formal report as well in terms of literature search and writing the parts that you completed already in the lab.
2. The next quiz will be administered on Monday, October 16, 2006 at 12 pm. Please make sure that you show up on time.
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. Histidine is found in fruits such as bananas and grapes, meat and poultry, and milk and milk products. It is also found in root vegetables and all green vegetables, though in lesser quantities. It is part of the several catalytic triads where is often serves as proton shuttle. A student isolates Histidine from one of the sources above. He observes an optical rotation of a=(-1.92o) for a solution of 0.225 g in 4.5 mL of water in a 100 mm polarimetry cell.
a. Determine the optical purity of the sample and the absolute stereochemistry (R/S and D/L) of the histidine isolated.
b. A second student prepares a solution in hydrochloric acid and observes a significantly lower value for the specific optical rotation for a solution that has the same concentration. Rationalize this observation.
2. Referring to the synthesis and characterization of the Jacobsen ligand, answer the following questions.
a. Potassium carbonate and the ammonium tartrate salt are dissolved in water. Provide a balanced chemical equation for this reaction.
b. A slight excess of the aldehyde is used in the reaction. Why?
c. Some of the students will use a rotavap to remove the solvent from the ligand. Which advantage does this technique have?
d. The ligand still possesses two phenolic functions, which would usually appear around 4-8 ppm in the 1H-NMR spectrum. However, they are observed at ~13.6 ppm in the 1H-NMR spectrum of the ligand. Rationalize this observation.
e. Which solvent is used to acquire the optical rotation for the ligand? What is the expected value for the specific optical rotation of the ligand?