Chapter 5 notes

Chapter 5 notes - Chem 2261 Course Notes Stereochemistry...

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Chem 2261 Course Notes Stereochemistry (see Bruice, Chapter 5) 1 Introduction - Some Definitions Types of Isomers Constitutional isomers - Differ in connectivity (arrangement of bonds between atoms) and and and Stereoisomer - Same connectivity (bonds) Different spatial arrangement (configuration) and and Cl Cl Cl Cl cis/trans diastereomers and and Cl Cl Cl Cl mirror images enantiomers OH OH nonsuperimposable Types of Stereoisomers Stereoisomers can be further categorized according to the geometric relationship between the isomers. There are two types (illustrated above): Enantiomers - nonsuperimposable mirror images Diastereomers - stereoisomers that are not enantiomers Stereocenter - an atom where the interchange of two attached groups gives a stereoisomer (also called a stereogenic center ). Asymmetric Center - A tetrahedral atom bonded to four different groups. label the stereocenters in each biomolecule: OH O OH OH OH OH O O mannose vitamin K 1
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Chem 2261 Course Notes Stereochemistry (see Bruice, Chapter 5) 2 Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl identify the stereocenters in these molecules: A carbon bonded to four different groups (i.e., an asymmetric center) is always a stereocenter although, as we saw above, this is not a requirement for a stereocenter. For example: Et Me Br H interchange Et and Me Me Et Br H Et Me H Br Me Et H Br interchange H and Br ( R ) -isomer ( S ) -isomer ( S ) -isomer ( R ) -isomer Br * 2-bromobutane Br H Br H mirror the two enantiomers of 2-bromobutane A Carbon with Four Different Substituents is a Tetrahedral Stereocenter An asymmetric carbon is a type of stereocenter Interchanging a pair of groups gives a stereoisomer interchange Et and Me ( R ) -isomer ( S ) -isomer The labels ( R )- and ( S )- can be used to designate the two possible configurations of the asymmetric carbon (configuration refers to the spatial arrangement of the four substituents). How this is done will be discussed a bit later. Note that an asymmetric carbon atom is a type of stereocenter. Interchanging a pair of substituents on the asymmetric carbon of 2-bromobutane changes the original molecule to its enantiomer. Molecules (such as 2-bromobutane above) that can exist as enantiomers are said to be chiral . Objects other than molecules can also be chiral. For example your left or right hand, like a chiral molecule, is not superimposable on its mirror image (i.e., its enantiomer). We have the following definitions related to chirality: Chiral - Not superimposable on its mirror image (describes enantiomers). Achiral - Not chiral. An achiral molecule is identical to its mirror image. Plane of symmetry - A plane that cuts a object (molecule) into two halves, each of which is the mirror image of the other. Inversion center - A point at the exact center of an object (molecule), where identical groups (atoms) on either side are related by an inversion operation (180° rotation followed by a reflection).
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Chem 2261 Course Notes Stereochemistry (see Bruice, Chapter 5) 3 A chiral molecule does not contain a plane of symmetry or inversion center. Collectively, these are
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This note was uploaded on 04/21/2008 for the course CHEM 2261 taught by Professor Crowe during the Fall '08 term at LSU.

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Chapter 5 notes - Chem 2261 Course Notes Stereochemistry...

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