CHAPTER FIVE i

CHAPTER FIVE i - HAPTER FIVE: Stereoisomers The...

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HAPTER FIVE: Stereoisomers The physiological effects of the image and mirror image of the leading bronchodilator albuterol are dramatically different. The R -image increases the bronchial airway diameter, whereas the S -mirror image cancels this effect and is a suspected inflammatory agent. Have you ever looked at yourself in the mirror in the morning and exclaimed: “That can’t be me!” Well, you were right. What you see, your mirror image, is not identical with you: You and your mirror image are nonsuperimposable . You can demonstrate this fact by trying to shake hands with your mirror counterpart: As you reach out with your right hand, your mirror image will offer you its left hand! We shall see that many molecules have this property—namely, that image and mirror image are nonsuperimposable and therefore not identical. How do we classify such structures? Do their functions differ, and if so, how? Because they have the same molecular formula, these molecules are isomers, but of a different kind from those encountered so far. The preceding chapters dealt with two kinds of isomerism: constitutional (also called structural) and stereo ( Figure 5-1 ). Constitutional isomerism describes compounds that have identical molecular formulas but differ in the order in which the individual atoms are connected ( Sections 1-9 and 2-4 ). Relations among isomers of various types. Constitutional Isomers
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Stereoisomerism describes isomers whose atoms are connected in the same order but differ in their spatial arrangement. Examples of stereoisomers include the relatively stable and isolable cis-trans isomers and the rapidly equilibrating (and usually not isolable) conformational ones ( Sections 2-5 through 2-7 and 4-1 ). Stereoisomers Exercise 5-2 Draw additional (conformational) stereoisomers of methylcyclohexane. ( Hint: Use molecular models in conjunction with Figure 4-8 .) This chapter introduces another type of stereoisomerism, mirror-image stereoisomerism. Molecules in this class are said to possess “handedness,” referring to the fact that your left hand is not superimposable on your right hand, yet one hand
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can be viewed as the mirror image of the other ( Figure 5-2A ). In this way, hands are different from objects that are superimposable with their mirror images, such as a hammer ( Figure 5-2B ). The property o f handedness in molecules is very important in nature, because most biologically relevant compounds are either “left-” or “right- handed.” As such, they react differently with each other, much as shaking your friend’s right hand is very different from shaking his or her left hand. A summary of isomeric relations is depicted in Figure 5-1 .
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This note was uploaded on 11/14/2010 for the course CHEM CHEM266 taught by Professor Forsey during the Fall '10 term at Waterloo.

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CHAPTER FIVE i - HAPTER FIVE: Stereoisomers The...

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