Organic Chemistry Chem 220a Problem Set 4   Chapter 5   Due: Monday, October 4, 1999

Michelangelo-The Creation of Man 1508-12 Sistine Chapel http://sunsite.auc.dk/cgfa/michelan/p-michela7.htm

1) Read the stereochemistry module in the StudyAids and do the exercises. There is no need to record answers on your homework. http://classes.yale.edu/chem220a/studyaids.html#Stereoisomers

2) Assign R,S-configurations to the remaining stereogenic centers in androstendione (on the right). Why is C8 of the R-configuration? How many stereoisomers of this structure are possible (excluding double bond geometrical isomers)?

3) There are twelve possible Fischer projections for a given enantiomer of lactic acid. Why? (S)-(+)-Lactic acid, the cause of cramping after vigorous physical exercise, was isolated from human muscle by Berzelius. Which of the following Fischer projections are (S)-(+)-lactic acid?

4) (R)-(+)-Limonene, which appeared on exam 1, is reduced in the presence of two equivalents of H₂ to a

mixture of two optically inactive compounds. How is the loss of optical activity possible? Illustrate and

explain. Draw (R)-(+)-limonene. How is it possible that (R)-limonene and (S)-lactic acid (#2 above) are

both dextrorotatory and yet have opposite R,S-configurations?

5) Two students measure the rotation a of a compound A. She uses a 0.028 M methanolic solution of A and

a 1 dm cell. The observed rotation a is +110.8^o; the investigator concludes that A is dextrorotatory and

optically active. He uses the same cell (cleaned, of course) and a 0.091 M methanolic solution of A and

concludes that the compound is achiral or racemic because there is no observed rotation. Explain.

6) "Natural" tartaric acid is of the R, R-configuration and it has an [a]_D = +12 ^o. What is the specific rotation

of the R, S-isomer? What is the specific rotation of a mixture that is a 2:1 mixture of R,S and R,R tartaric

acid, respectively?

7) Levorotatory a -pinene 1, a constituent of turpentine, has a specific rotation of 51.2 o. A sample of a-pinene contaminated with its enantiomer has an [a]D= -33.5 o. How much of each enantiomer is present in the mixture? What is the configuration of the two asymmetric carbons in 1? What is the configuration of the enantiomer of 1? The diastereomer of 1 (or its enantiomer) does not exist. Why? Justify your answer.

8) A mixture of enantiomers ( [a]_D = -35^o) is shown to be a 3.5:1 mixture upon chromatographic resolution.

What is the principle involved in the resolution? Which enantiomer is in excess? What are the rotations of

the pure enantiomers?