Chapter 5: Stereochemistry The three-dimensional structure of a molecule can greatly affect its physical and chemical properties. Can you give some examples? Three dimensional structure is critical in biochemistry. HOW? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Chapter 5: Stereochemistry For pharmaceuticals, slight differences in 3D spatial arrangement can make the difference between targeted treatment and undesired side-effects. WHY? Isomers that have the same connectivity between atoms but different 3D, spatial arrangement of their atoms are called STEREOisomers Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.1 Isomers Isomers are NONidentical molecules that have the same formula There are two classifications of isomers Draw a pair of molecules that are constitutional isomers Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.1 Isomers C-C bonds that are constrained in a cyclic structure can not freely rotate Although the two molecules below have the same connectivity, they are NOT identical. The naming system we have learned thus far would give them the same name, but they are NOT identical, so we need to learn additional nomenclature rules…to be continued Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.1 Isomers To maintain orbital overlap in the pi bond, C=C double bonds can not freely rotate. Although the two molecules below have the same connectivity, they are NOT identical Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.1 Isomers Identify the following pairs as either constitutional isomers, stereoisomers, or identical. EXPLAIN Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.1 Isomers With rings and with C=C double bonds, cis-trans notation is used to distinguish between stereoisomers Cis – identical groups are positioned on the SAME side of a ring Trans – identical groups are positioned on OPPOSITE sides of a ring Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.1 Isomers Cis – identical groups are positioned on the SAME side of a C=C double bond Trans – identical groups are positioned on OPPOSITE sides of a C=C double bond Practice with SkillBuilder 5.1 trans because of the H's Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.1 Isomers Identify the following as either cis, trans, or neither. EXPLAIN Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.2 Stereoisomers Beyond cis-trans isomers, there are many other important stereoisomers To identify such stereoisomers, we must be able to identify chiral molecules A chiral object is NOT identical to its mirror image You are a chiral object. Look in a mirror and raise your right hand. Your mirror image raises his or her left hand. You can test whether two objects are identical by seeing if they are superimposable. Can you be superimposed upon your mirror image? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.2 Stereoisomers Some other chiral objects include gloves. HOW? Think of some other examples of chiral everyday objects Chirality is important in molecules. Because two chiral molecules are mirror images, they will have many identical properties, but because they are not identical, their pharmacology may be very different Visualizing mirror images of molecules and manipulating them in 3D space to see if they are superimposable can be VERY challenging It is highly recommended that you use handheld models as visual aids until you get more experience Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.2 Stereoisomers Chirality most often results when a carbon atom is bonded to 4 unique groups of atoms. Make a handheld model to prove to yourself that they are NOT superimposable Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.2 Stereoisomers When an atom such as carbon forms a tetrahedral center with 4 different groups attached to it, it is called a chirality center (aka stereocenter or stereogenic center) Analyze the attachments for each chirality center below Practice with SkillBuilder 5.2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.2 Stereoisomers Identify all of the chirality centers (if any) in the following molecules Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.2 Enantiomers Some stereoisomers can also be classified as enantiomers Enantiomers are TWO molecules that are MIRROR IMAGES but are NONidentical and NONsuperimposable For the pair below, draw in the missing enantiomer Make handheld models for both structures to verify that they are NONidentical mirror images Practice with SkillBuilder 5.3 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e 5.2 Enantiomers What is the relationship between the two molecules below? Given that the molecules are so similar, how is it possible that our noses can distinguish between them? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e