1. Chemistry 2100 Chapter 15

3. Enantiomers Enantiomers: Enantiomers: Nonsuperposable mirror images. –As an example of a molecule that exists as a pair of enantiomers, consider 2-butanol.

4. Enantiomers

5. chiral asymmetric alanine

6. plane of symmetry achiral glycine

7. Enantiomers chiral Objects that are nonsuperposable on their mirror images are chiral (from the Greek: cheir, hand). –They show handedness. The most common cause of enantiomerism in organic molecules is the presence of a carbon with four different groups bonded to it. stereocenter –A carbon with four different groups bonded to it is called a stereocenter.

8. Drawing Enantiomers

9. * * Enantiomers matter! *

10. The R,S System Because enantiomers are different compounds, each must have a different name. –Here are the enantiomers of the over-the-counter drug ibuprofen. –The R,S system is a way to distinguish between enantiomers without having to draw them and point to one or the other.

11. The R,S System The first step in assigning an R or S configuration to a stereocenter is to arrange the groups on the stereocenter in order of priority. –Priority is based on atomic number. –The higher the atomic number, the higher the priority.

12. R,S Priority of Some Groups

13. The R,S System To assign an R or S configuration: 1. Assign a priority from 1 (highest) to 4 (lowest) to each group bonded to the stereocenter. 2. Orient the molecule in space so that the group of lowest priority (4) is directed away from you. The three groups of higher priority (1-3) then project toward you. 3. Read the three groups projecting toward you in order from highest (1) to lowest (3) priority. R S 4. If reading the groups 1-2-3 is clockwise, the configuration is R. If reading them is counterclockwise, the configuration is S.

14. Properties of Enantiomers

15. Optical activity

16. plane-polarized light

21. Fig. 15-6, p. 435

22. (±) or (dl) optically inactive raceme levorotatory (l) (–) rotation "left-hand isomer" dextrorotatory (d) (+) rotation "right-hand isomer" Assigning Rotation

23. Multiple Chiral Centers n 2 n For a molecule with n stereocenters, the maximum number of possible stereoisomers is 2 n. –We have already verified that, for a molecule with one stereocenter, 2 1 = 2 stereoisomers (one pair of enantiomers) are possible. –For a molecule with two stereocenters, a maximum of 2 2 = 4 stereoisomers (two pair of enantiomers) are possible. –For a molecule with three stereocenters, a maximum of 2 3 = 8 stereoisomers (four pairs of enantiomers) are possible, and so forth.

24. 2R,3S 2S,3R 2R,3R 2S,3S The Four Stereoisomers of a Simple sugar

25. Fischer Projections

26. Threose and Erythrose, Fischer-ized

27. meso COOH Tartaric Acid

28. meso COOH

29. meso Meso form

30. **

31. **

32. Stereochemical Reactions

33. * pyruvic acidlactic acid

34. * pyruvic acid lactic acid

35. (–) (+)

36. (–) (+)

37. (–) (+)

38. (–) (+)

39. (–) (+)

40. (–) (+)

41. (–) (+)

42. (–) (+)

43. But, Biologically…

44. (+)

45. 

46. 

47. 

48.   

49.   

50. H+H+   

51. H+H+   

52. Chirality of Biomolecules –Because interactions between molecules in living systems take place in a chiral environment, a molecule and its enantiomer or one of its diastereomers elicit different physiological responses. –As we have seen, (S)-ibuprofen is active as a pain and fever reliever, while its R enantiomer is inactive. –The S enantiomer of naproxen is the active pain reliever, but its R enantiomer is a liver toxin!