Chapter 5 Notes - 10/8/2010 Isomers Chapter 5...

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10/8/2010 1 Chapter 5 Stereochemistry: Chiral Molecules 1 Isomers 2 Chirality Is “handedness” – non-superPOSABLE mirror images A Chiral molecule is non-superposable on its mirror image An Achiral molecule IS superposable on its mirror image 3 CHIRALITY IS A MOLECULAR PROPERTY! 4
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10/8/2010 2 Stereoisomers Are NOT constitutional isomers, have the same connectivity of the atoms but a different arrangement in space 2 categories: Enantiomers: stereoisomers whose molecules are nonsuperposable mirror images of each other (a chiral molecule and its mirror image) Diastereomers: stereoisomers whose molecules are NOT mirror images of each other 5 Chiral Molecules-Learn the Language A chiral molecule: not superposable on its mirror image A chiral molecule and its mirror image are ENANTIOMERS of each other A chiral molecule and its mirror image are a PAIR of ENANTIOMERS 6 Representing Molecules in 3D (REALLY important!) Try with your model kit! 7 Now it gets confusing. .. How to identify a chiral molecule? The term “chiral center” is used to describe a tetrahedral atom with four different groups attached to it (often identified with *) This is confusing. ... If a molecule contains ONLY ONE chiral center, then the MOLECULE is chiral But a molecule can have chiral centers and be ACHIRAL. .. And a chiral molecule DOES NOT need a chiral center to be chiral 8
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10/8/2010 3 A better term (more broad) Stereogenic center : an atom at which the interchange of two groups produces a stereoisomer ALL chiral centers are stereogenic centers, but not all stereogenic centers are chiral centers! 9 Biological Significance 1963 Thalidomide – a mixture of two enantiomers 1 enantiomer – good for treating morning sickness in pregnant women Other enantiomer – causes horrible birth defects 10 Quick Test for Chirality Is there an Internal Mirror Plane in the molecule? Plane of symmetry is an imaginary plane that bisects a molecule such that the two halves of the molecule are mirror images of each other If there is, the molecule is achiral If there IS NOT, check and see if the molecule is superposable on its mirror image (a) (b) 11 Nomenclature of Enantiomers: The R, S- System Also called the Cahn-Ingold-Prelog system Applies to CHIRAL CENTERS The four groups attached to the chiral center are assigned priorities from highest ( a ) to lowest ( d ), as follows: 1. Atoms directly attached to the chiral center are compared, and those with higher atomic number are given higher priority (same as for E, Z in alkenes ) 2. If priority cannot be assigned based on directly attached atoms, the next layer of atoms is examined 3. This is continued until a point of difference occurs 12
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10/8/2010 4 The R, S- System (cont) 4. Once priorities are assigned, we rotate the molecule (with a molecular kit, or in your head) so that the lowest priority group (d) is pointing away from us
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Chapter 5 Notes - 10/8/2010 Isomers Chapter 5...

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