Chap-5A - Chapter 5 Stereochemistry 5-1 Introduction This...

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Chapter 5 Stereochemistry
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Chapter 5 2 5-1 Introduction • This is study of the 3 dimensional arrangement in space of molecules. • In organic chemistry, subtle differences in spatial arrangements can give rise to prominent effects. E.g. the isomers of butenoic acid: • The cis isomer (maleic acid) is toxic, whereas the trans isomer (fumaric acid) is an essential metabolite for plants and animals.
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Chapter 5 3 Isomers
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Chapter 5 4 5-2 Chirality • Chirality means “handedness”. • Every object has a mirror image, but if a molecule’s mirror image is different from the molecule, it is said to be a chiral molecule. • Chiral objects include: hands, feet, gloves, screws, cork screws • Achiral objects have mirror images that are identical to the object.
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Chapter 5 5 5-2A Chirality and Enatiomerism in Organic Molecules • If a mirror image of a molecule can be placed on top of the original, and the 3 dimensional arrangement of every atom is the same, then the two molecules are superimposable , and the molecule is achiral . • If a molecule has a non superimposable mirror image, it is chiral . • Consider stereoisomers of 1,2-dichlorocyclopentane:
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Chapter 5 6 • The cis isomer has a superimposable mirror image – achiral. • The trans isomer is non-superimposable on its mirror image – chiral. • Such pairs of non superimposable mirror images are called enantiomers . • Every chiral compound has enantiomers. • Achiral compounds do not have enantiomers.
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Chapter 5 7 5-2B Asymmetric (Chiral) Carbon Atoms, Chirality Centers, and Stereocenters • Tetrahedral carbons with 4 different attached groups are chiral, and is usually denoted with an asterisk (*). • Its mirror image will be a different compound (enantiomer).
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Chapter 5 8 • The absolute law is whether or not a molecule’s mirror image is identical to the original, but a few quick rules are: 1) if the molecule has no chiral carbon, it is usually achiral 2) if the molecule has just one chiral carbon, it is usually chiral 3) if it has 2 or more chiral carbons, it may or may not be chiral. (There are exceptions…)
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Chapter 5 9 5-2C Mirror Planes of Symmetry • If two groups are the same, carbon is achiral. Plane of symmetry : an imaginary plane passing through an object dividing it such that one half is the mirror image of the other half • A molecule with an internal mirror plane cannot be chiral.* Caution! If there is no plane of symmetry, molecule may be chiral or achiral. See if mirror image can be superimposed.
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Chapter 5 10
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Chapter 5 11 • Plane of symmetry (contd.) HO OH mirror plane
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Chapter 5 12 5-3 ( R ), ( S ) Nomenclature of Asymmetric Carbon Atoms • Different molecules (enantiomers) must have different names. • Usually only one enantiomer will be biologically active.
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This note was uploaded on 04/08/2008 for the course CHE 201 taught by Professor Bong during the Spring '08 term at SUNY Buffalo.

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Chap-5A - Chapter 5 Stereochemistry 5-1 Introduction This...

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