ANGEL-transparancies-part-2

ANGEL-transparancies-part-2 - Chem 210 Stereochemistry...

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Chem 210 91 Stereochemistry Chiral : having handedness; a chiral molecule has a nonsuperimposable mirror image (chiral molecules cannot have a plane of symmetry) Stereogenic center: (chiral center, asymmetric center) an atom (usually carbon) bonded to four different substituents Enantiomers : molecules that are nonsuperimposable mirror images of each other Diastereomers : stereoisomers that are not mirror images (i.e. are not enantiomers) Optical activity : the rotation of plane-polarized light. An optically active sample contains chiral molecules. However, an optically inactive sample may still contain chiral molecule (e.g., a racemic mixture).
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Chem 210 92 One reason for chirality: stereogenic center In organic chemistry we will investigate only sp 3 - hybridized atoms (tetrahedral); other stereogenic centers (octahedral, for example) are possible.
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Chem 210 93 Optical rotation (optical activity) specific rotation: [ α ] D = α /(l x C) α = observed rotation (in degrees) l = pathlength (in dm) C = concentration (g/mL) [ α ] D is positive (+) (clockwise) dextrorotatory [ α ] D is negative (–) (counterclockwise) levorotatory Pure enantiomers have the same magnitude, but opposite signs of specific rotations; racemic mixtures (50/50 mixture of enantiomers) have no optical activity. Measured optical activity is an additive property of all molecules present in the sample.
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Chem 210 94 Optical rotation vs. absolute configuration [ α ] D = +3.82 [ α ] D = –3.82 These are values of the observed optical rotation for pure enantiomers. How does it relate to 3D structures: which is which? how to give them different names, even when the optical rotation cannot be measured? The specific 3D arrangement is called configuration. Absolute configuration based on a designed system of priorities is used to distinguish the isomers
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Chem 210 95 Absolute configuration 1. Assign priorities to atoms (groups) on chiral center 2. Orient the molecule with the lowest priority atom (group) pointing away from the viewer 3. If the remaining group’s priorities are arranged clockwise it is R absolute configuration; otherwise it is S .
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Chem 210 96 Sequence (priority) rules 1. Higher atomic number = higher priority 2. If the first atoms in the substituent are the same, the priority is decided by the following atoms in the substituent 3. Multiple-bonded atoms are considered equivalent to the same number of single bonded atoms. CCH C(C) C(C) C(C) C(C) C O H C O(C) O(C) H
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Chem 210 97 Absolute configuration: examples The absolute configuration does not have simple, direct relation to the optical rotation.
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Chem 210 98 Assigning absolute configuration: triad notation To develop good habits in assigning absolute configuration, it might be beneficial to use “triad” notation while analyzing priorities of substituents. H C(C,C,C) C( C,C,H) C(C,C,H).
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This note was uploaded on 04/10/2008 for the course CHEM 210 taught by Professor Maslak,przemyslaw during the Spring '08 term at Penn State.

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ANGEL-transparancies-part-2 - Chem 210 Stereochemistry...

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