In this chapter, we study the three-dimensional structures of molecules to understand their stereochemical relationships. We compare the various types of stereoisomers and study ways to differentiate among stereoisomers. In future chapters, we will see how stereochemistry plays a major role in the properties and reactions of organic compounds. | Chapter 5 © 2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Stereochemistry Chapter 5 Chirality “Handedness”: Right glove doesn’t fit the left hand. Mirror-image object is different from the original object. Chapter 5 Achiral Objects that can be superposed are achiral. Chapter 5 Stereoisomers Enantiomers: Nonsuperimposable mirror images, different molecules with different properties. Chapter 5 Chiral Carbons Carbons with four different groups attached are chiral. It’s mirror image will be a different compound (enantiomer). Chapter 5 Achiral Compounds Take this mirror image and try to superimpose it on the one to the left matching all the atoms. Everything will match. When the images can be superposed the compound is achiral. Chapter 5 Planes of Symmetry A molecule that has a plane of symmetry is achiral. Chapter 5 Cis and Trans Cyclic Compounds Cis-1,2-dichlorocyclohexane is achiral because the molecule has . | Chapter 5 © 2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. Stereochemistry Chapter 5 Chirality “Handedness”: Right glove doesn’t fit the left hand. Mirror-image object is different from the original object. Chapter 5 Achiral Objects that can be superposed are achiral. Chapter 5 Stereoisomers Enantiomers: Nonsuperimposable mirror images, different molecules with different properties. Chapter 5 Chiral Carbons Carbons with four different groups attached are chiral. It’s mirror image will be a different compound (enantiomer). Chapter 5 Achiral Compounds Take this mirror image and try to superimpose it on the one to the left matching all the atoms. Everything will match. When the images can be superposed the compound is achiral. Chapter 5 Planes of Symmetry A molecule that has a plane of symmetry is achiral. Chapter 5 Cis and Trans Cyclic Compounds Cis-1,2-dichlorocyclohexane is achiral because the molecule has an internal plane of symmetry. Both structures above can be superimposed. Trans-1,2-dichlorocyclohexane does not have a plane of symmetry so the images are nonsuperimposable and the molecule will have two enantiomers. Chapter 5 (R) and (S) Nomenclature Different molecules (enantiomers) must have different names. Usually only one enantiomer will be biologically active. Configuration around the chiral carbon is specified with (R) and (S). Chapter 5 Cahn–Ingold–Prelog Rules Assign a priority number to each group attached to the chiral carbon. Priority is assigned according to atomic number. The highest atomic number assigned is the highest priority #1. In case of ties, look at the next atoms along the chain. Double and triple bonds are treated like bonds to duplicate atoms. Chapter 5 Assign (R) or (S) Working in 3-D, rotate the molecule so that the lowest priority group is in back. Draw an arrow from highest to lowest priority group. Clockwise = (R), .
