SDN Members don't see this ad. (About Ads) I'm having difficulties in understanding two questions of the following passage. Could someone here please help me with it? The passage is from Princeton Review MCAT Science Workbook Ochem - passage 4 (advanced passage) "A research chemist synthesized Compound A, which was shown to be a powerful analgesic upon clinical testing. Further evidence indicated only one stereoisomer of A was the acitve agent. The researcher was instructed to separate Compound A into its different stereoisomers to help determine which one was the analgesic. Compound A, which showed an optical rotation of 0 degree, was treated with the R stereoisomer of Compound B (melting point 55C). Two easily separated solids, C and D, whose melting points were 101C and 90C, respectively, were isolated from the reaction mixture. Compound C and D were then independently treated with warm aqueous acid. After workup, Compound C gave Compounds E and B, which Compound D yielded F and B. Testing of Compounds E and F determined that E was the active analgesic, while F was biologically inactive." Question: 1. Compound A can be described as: a. a meso compound b. a racemic mixture c. an equal mixture of two diastereomers d. an optically pure compound Answer is B. The explanation given is "from the information given, Compound A is a mixture of two stereoisomers. With an optical rotation of 0 degree, it must be a mixture of enantiomers, which is racemic mixture". My concern is, I could tell only from the text that it is a kinda mixture. However, if consider calling it "compound A", shouldn't it be a pure chemical? 2. Compound C and D are best described as: a. enantiomers b. tautomers c. diastereomers d. rotamers Answer is C. Explanation: "Compound C has configuration R, R while compound D has configuration S, R. They are different at only one center and are therefore diastereomers. My concern is that I couldn't tell how to figure out the configuration of C and D just based on the passage. I could only guess they are probably diastereomers. Thank you very much!