1. Stereochemistry

2. Required background: Shapes of molecular structures Isomers, conformers Essential for: 1. Understanding of S N 1, S N 2, E1, E2, and other reaction mechanisms 2. Stereochemistry of carbohydrates, proteins, and nucleic acids

3. Outline 1. Chirality, optical activity 2. Fisher projections. R-,S-nomenclature 3. Stereoisomers

4. 1. Chirality, optical activity 2. Fisher projections. R-,S-nomenclature 3. Stereoisomers

5. Chirality – is a property of an object to be different from its mirror image. In other words, the object and its mirror image are noncongruent. Example: A human hand is chiral, since the right hand and the left hand are different. Enantiomers – are noncongruent mirror images of molecules Chiral objects can not contain certain elements of symmetry, for instance, plane of symmetry (most common), center of symmetry or some other elements (rare cases beyond our course). Optical activity – is the ability of chiral compounds to rotate the plane of polarization of light.

6. 1. Chirality, optical activity 2. Fisher projections. R-,S-nomenclature 3. Stereoisomers

7. Which structural elements bring chirality to a molecule? a. Chiral center (sp 3 – carbon with four different substituents) b. Other elements (beyond our course) The language for stereochemistry of molecules with chiral centers – Fisher Projections

13. Rules of handling Fisher Projections a. Rotation by 180 o in the plane of paper gives the identical compound b. Rotation by 90 o in the plane of paper gives the opposite enantiomer

14. Rules of handling Fisher Projections d. Rotation of any three substituents gives the identical compound c. Switching of any two substituents gives the opposite enantiomer

15. 1. Chirality, optical activity 2. Fisher projections. R-,S-nomenclature 3. Stereoisomers

16. Isomers Constitutional isomers (different connectivity) Stereoisomers (same connectivity) Enantiomers (mirror images) Diastereomers (not mirror images) Enantiomers have exactly same physical and chemical properties, except the sign of optical rotation and behavior toward chiral compounds. Why? Let’s consider compounds with two chiral centers

17. Stereoisomers of tartaric acid