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chapter 5 - Start of chapter 5 lecture notes Isomers...

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Start of chapter 5 lecture notes
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Isomers Different compounds with the same molecular formula Constitutional (or structural) isomers : isomers that differ in the bonding sequence or functional groups Molecular formula = C 5 H 10 Stereoisomers : isomers that differ in the 3-dimensional arrangement of the bonds – these molecules will have the same bonding sequence for the atoms These can be: cis-/trans- isomers – diastereomers multiple chiral centers where some arrangements are the same – diastereomers mirror images that are not superimposable - enantiomers H H H H
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Stereoisomers that are diastereomers: isomers that differ in how atoms are orientated in space IMPORTANT: identical groups on one of the double-bonded carbons means there are NO cis-trans isomers Stereoisomers in comparison to structural isomers
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NO! C H C H CH 3 cis C H C H CH 3 trans YES! Can these molecules have cis- trans- isomers? H C CH 3 H C H C CH 3
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Stereochemistry Chiral – “handed” - an object and it’s mirror image are NOT identical Achiral – “not chiral” - an object and its mirror image are identical These 2 objects are not “super imposable” on one another Prochiral – not chiral but can become chiral if reacted for example a carbonyl C is reacted to become a C with 4 different groups bound To understand and determine the stereoisomer relationship between compounds we need to know how to assign a stereochemical configuration to a molecule Molecules can fall into 3 basic categories with respect to stereochemistry:
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A molecule cannot be chiral if it has a plane of symmetry (an internal mirror plane or internal symmetry) Ways to determine chirality: Look for an asymmetric C that’s a C atom bound to 4 different groups - it must be an sp 3 hybridized atom - it’s a Chiral center or chiral C & you can designate it with an * The following types of C atoms cannot be chiral centers: C=C C C -CH 2 - & -CH 3 Internal symmetry – the molecule is comprised of 2 identical halves
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Chirality Other atoms, besides C, can be chiral: P, S or N very bulky molecules or highly strained molecules adopt a conformation that’s chiral A molecule is chiral if it has at least 1 asymmetric C atom at least one chiral center For example 1,2-Dienes: C
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Enantiomers - stereoisomers that are not identical - the two molecules are the nonsuperimposable mirror image of one another Chiral molecules can exist in two forms These two forms are enantionmers of one another 2 rule : # of different ways you can draw a chiral molecule is based For a molecule with 4 chiral C centers There are 2 4 (or 16) different ways to draw that molecule the only thing changing between these 16 different molecules is the rotation around the chiral centers Number of stereoisomers in a chiral molecule
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