stereochem - L TOPIC 4. STEREOCHEMISTRY (Chapter 5) CHEM...

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CHEM 2311 B Topic 4 slide 1 TOPIC 4. STEREOCHEMISTRY (Chapter 5) L CHEM 2311 B Topic 4 slide 2 OBJECTIVES 1. Discuss the three-dimensional structure of organic molecules 2. Recognize enantiomers, diastereomers, meso compounds 3. Provide R/S designations of stereocenters 4. Calculate optical rotations, enantiomeric excesses 5. Use molecular models to determine the 3D arrangement of atoms in chiral molecules
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CHEM 2311 B Topic 4 slide 3 RECOGNIZING ISOMERS Isomers Compounds with same molecular formula, but different structures Constitutional Isomers Different connectivity Stereoisomers Same connectivity, different 2 or 3 dimensional arrangement Enantiomers Non-superposable mirror images Diastereomers Stereoisomers which are not enantiomers, cis-trans isomers Geometric isomer and conformers S:5.1-2 CHEM 2311 B Topic 4 slide 4 Chapter 8 slide 4 CHIRALITY: ENANTIOMERS An object which has a non-superposable mirror image is chiral (the opposite of chiral is “ achiral ”). Chirality found extensively in biological systems, amino acids, proteins, etc. Prob 5.30,31,33 35a,b,f S:5.3 5.5-5.7 * Important small print: In spite of Solomon and Fryhle’s footnote regarding the dictionary definition of superposable and superim posable (and their preference for the former), most chemists refer to non- superim posability of mirror images as a requirement for chirality. This instructor will undoubtedly use “superim posability” in lecture and on exams.
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CHEM 2311 B Topic 4 slide 5 Symmetry: Mirror Planes and Centers of Inversion Another test for chirality is to assess whether the object itself has a mirror plane of symmetry or point of symmetry (point of inversion). Objects (molecules) with mirror planes or centers of inversion are achiral (NOT chiral) Objects (molecules) without a mirror plane or point of inversion are chiral. They are non-superposable on their mirror images. Cl Br Br Br Cl Br Cl Cl CHEM 2311 B Topic 4 slide 6 Enantiomers Molecules can be chiral . Pairs of molecules which are non-superposable mirror images of one another are called enantiomers . Enantiomers are examples of stereoisomers : molecules which differ only in the spatial arrangement of atoms. Molecules with a carbon atom bearing four different substituents can exist as a pair of enantiomers which differ in the arrangement (“configuration”) of these substituents. The carbon with four different substituents is called the stereogenic carbon , or stereocenter (and sometimes the chiral center ) A 1:1 mixture of enantiomers is called a racemic mixture (more later) You must be able to recognize when pairs of molecules are identical (superposable) or entaniomers (non-superposable mirror images) A D B C A D B C
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Problem: Which of the following are identical to the first structure? CHEM 2311 B Topic 4 slide 8 Problem: Which of the following are identical to the first structure?
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This note was uploaded on 12/17/2009 for the course CHEM 2311 taught by Professor Tyson during the Spring '07 term at Georgia Institute of Technology.

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stereochem - L TOPIC 4. STEREOCHEMISTRY (Chapter 5) CHEM...

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