1. Chapter 17 Stereoisomerism

2. ©2005 Mark S. Davis Goals Distinguish between Constitutional and Stereoisomers Distinguish between Chiral and Achiral molecules Identify tetrahedral stereocenters Describe optical activity of chiral compounds Draw Enantiomers

3. ©2005 Mark S. Davis Biological Importance Biological systems can only use one stereoisomer Perception in light, Box 12.2 Insect pheromones, Box 12.3

4. ©2005 Mark S. Davis Review of Isomerism Constitutional isomers (Structural Isomers) –Same formula –Different connectivity 1. 2.Different 3.Different

5. ©2005 Mark S. Davis Stereoisomers Same connectivity, different Enantiomers – Diastereomers – –cis- and trans- isomers

6. ©2005 Mark S. Davis Enantiomers Must meet two requirements 1.The molecules are 2.They are not If the molecule meets the two conditions, it is said to be –Hand, This is called

7. ©2005 Mark S. Davis Ultimately A pair of enantiomers is possible only when a compound contains a

8. ©2005 Mark S. Davis Ways to draw Wedge-bond Fischer projection - rules –Horizontal = –Vertical =

9. ©2005 Mark S. Davis Naming Enantiomers Use prefix, either –These are confirmed with x-ray crystallography RULES – must be followed 1.The tetrahedral stereocenter must have 1. 2.A 3.

10. ©2005 Mark S. Davis Naming Continued 2.Fischer projections with 1.R groups pointed 2.R 1, the most substituted Carbon (least # of H) is pointed 3.R 2 is pointed 3.The D form has the heteroatom on the 4.The L form has the heteroatom on the

11. ©2005 Mark S. Davis Properties of Enantiomers Chiral compounds are different than achiral compounds in – – A pair of enantiomers have the same physical properties, mp, bp, solubility They differ in

12. ©2005 Mark S. Davis Optical Activity Ability to rotate Use a polarimeter and a solution of the desired enantiomer Light is either rotated

13. ©2005 Mark S. Davis Optical Activity Note that designations are based on Fischer drawings and NOT on the rotation of light. Some A mixture of equal concentrations of

14. ©2005 Mark S. Davis Chiral Recognition Enantiomers have the same reactivity with Enantiomers have large –See figure 17.12, p. 509 –This is called

15. ©2005 Mark S. Davis Chirality in Organisms Humans and most organisms use and synthesize only Reactions in living cells produce Reactions in laboratory produce a

16. ©2005 Mark S. Davis Chirality Different enantiomers can produce different effects in the body. See box 17.1 page 511 Box 17.2 page 512 – Dopamine, L- Dopa and D-Dopa

17. ©2005 Mark S. Davis Review… Enantiomers are The only difference in physical properties is No reactivity difference to achiral compounds Large differences in reactivity to chiral compounds or chiral enzymes

18. ©2005 Mark S. Davis Two or more Stereocenters Increases the number of stereoisomers If there are D-Glucose has Starch (many glucose connected) has hundreds of stereocenters

19. ©2005 Mark S. Davis Consider… 2,3-pentanediol –

20. ©2005 Mark S. Davis Nutrasweet (Aspartame) Two stereocenters,

21. ©2005 Mark S. Davis Consider… 2,3-butanediol –

22. ©2005 Mark S. Davis Cyclic stereoisomers Cyclic compounds can be enantiomers –Carvone from box 17.1 p 511 A ring carbon is a tetrahedral stereocenter if: –