Chapter09 - Stereochemistry 9. Stereochemistry Based on...

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1 9. Stereochemistry Based on McMurry’s Organic Chemistry , 6 th edition 2 Stereochemistry ± Some objects are not the same as their mirror images (technically, they have no plane of symmetry) ± A right-hand glove is different than a left-hand glove (See Figure 9.1) ± The property is commonly called “handedness” ± Organic molecules (including many drugs) have handedness that results from substitution patterns on sp 3 hybridized carbon 3 Enantiomers – Mirror Images ± Molecules exist as three-dimensional objects ± Some molecules are the same as their mirror image ± Some molecules are different than their mirror image ± These are stereoisomers called enantiomers 4 9.1 Enantiomers and the Tetrahedral Carbon ± Enantiomers are molecules that are not the same as their mirror image ± They are the “same” if the positions of the atoms can coincide on a one-to-one basis (we test if they are superimposable , which is imaginary) ± This is illustrated by enantiomers of lactic acid
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2 5 Examples of Enantiomers ± Molecules that have one carbon with 4 different substituents have a nonsuperimposable mirror image – enantiomer ± Build molecular models to see this 6 Mirror-image Forms of Lactic Acid ± When H and OH substituents match up, COOH and CH 3 don’t ± when COOH and CH 3 coincide, H and OH don’t 7 9.2 The Reason for Handedness: Chirality ± Molecules that are not superimposable with their mirror images are chiral (have handedness) ± A plane of symmetry divides an entire molecule into two pieces that are exact mirror images ± A molecule with a plane of symmetry is the same as its mirror image and is said to be achiral (See Figure 9.4 for examples) 8 Chirality ± If an object has a plane of symmetry it is necessarily the same as its mirror image ± The lack of a plane of symmetry is called “handedness”, chirality ± Hands, gloves are prime examples of chiral object ± They have a “left” and a “right” version
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3 9 Plane of Symmetry ± The plane has the same thing on both sides for the flask ± There is no mirror plane for a hand 10 Chirality Centers ± A point in a molecule where four different groups (or atoms) are attached to carbon is called a chirality center ± There are two nonsuperimposable ways that 4 different different groups (or atoms) can be attached to one carbon atom ± If two groups are the same, then there is only one way ± A chiral molecule usually has at least one chirality center 11 Chirality Centers in Chiral Molecules ± Groups are considered “different” if there is any structural variation (if the groups could not be superimposed if detached, they are different) ± In cyclic molecules, we compare by following
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Chapter09 - Stereochemistry 9. Stereochemistry Based on...

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