7 Stereochem - Stereochemistry & Chiral Molecules...

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Stereochemistry & Chiral Molecules
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Stereochemistry stereochemistry is the three-dimensional structure of a molecule. As a consequence of stereochemistry, apparently minor differences in 3-D structure can result in vastly different properties. We can observe this by considering starch and cellulose, which are both composed of the same repeating unit.
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Stereochemistry of Carbohydrates
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isomers are different compounds with the same molecular formula. The two major classes of isomers are constitutional isomers and stereoisomers. Constitutional isomers have different IUPAC names, the same or different functional groups, different physical properties and different chemical properties. Stereoisomers differ only in the way the atoms are oriented in space. They have identical IUPAC names (except for a prefix like cis or trans and E or Z ). They always have the same functional group. Two Major Classes of Isomers
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Configurational Isomers Configurational isomers have a particular three- dimensional arrangement of atoms called a configuration. Configuration is the spatial array of atoms that distinguishes stereoisomers. Configurational isomers are stereoisomers that differ in configuration.
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Constitutional Isomers vs Stereoisomers
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Although everything has a mirror image, mirror images may or may not be superimposable. Some molecules are like hands. Left and right hands are mirror images, but they are not identical, or superimposable . Chirality or Handedness
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Other molecules are like socks. Two socks from a pair are mirror images that are superimposable. A sock and its mirror image are identical. A molecule or object that is superimposable on its mirror image is said to be achiral . Achiral Objects and Molecules
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Do these molecules conatain a Plane of Symmetry (Mirror Plane)? Achiral Molecules
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The molecule labeled A and its mirror image labeled B are not superimposable. No matter how you rotate A and B, all the atoms never align. Thus, CHBrClF is a chiral molecule, and A and B are different compounds. A and B are stereoisomers—specifically, they are enantiomers . A carbon atom with four different groups is a tetrahedral stereogenic center . Chiral Molecules
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In general, a molecule with no stereogenic centers will not be chiral. There are exceptions to this that will be considered in Chapter 17. With one stereogenic center, a molecule will always be chiral . With two or more stereogenic centers, a molecule may or may not be chiral, e.g. Meso compound (contains a plane of symmetry or a mirror plane ) Achiral molecules contain a plane of symmetry but chiral molecules do not. A plane of symmetry is a mirror plane that cuts the molecule in half, so that one half of the molecule is a reflection of the other half.
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7 Stereochem - Stereochemistry & Chiral Molecules...

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