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constitutional isomers:
stereoisomers: conformational isomers: (note – many slides from Soderberg text) intro
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Stereoisomers 3.3
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Definitions stereoisomers: different arrangement of atoms in space enantiomers: mirror images diastereomers: not mirror images (eg. cis/trans alkene) chiral: not superimposible on mirror image, no plane of symmetry asymmetric center: tetrahedral atoms with 4 different subs stereocenter: exchange two bonds, get different stereoisomer (eg. alkene)
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enantiomers: mirror images
not superimposable 3.3
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two enantiomers of thalidomide
thalidomide is chiral 3.3
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more examples of enantiomers
all are chiral if it has an asymmetric center, it is almost certainly chiral (exception – meso) 3.3
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achiral molecules (no stereocenters!)
wedges don’t necessarily mean a stereocenter, and vice-versa! 3.3
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other atoms can be stereocenters
Don’t worry about threo / erythro definition 3.3
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3.3
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stereocenters? asymmetric centers? chiral?
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The Cahn-Ingold-Prelog system
3.4
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3.4
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3.4
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what if H is drawn pointing back?
(effective stereoisomer) what if H is drawn pointing back? 3.4
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commercial thalidomide sold as racemic mixture
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proteins recognize stereochemistry!
3.5
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other examples of enantiomers with different biological activity
but enantiomers have identical physical properties! (except optical rotation, next)
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determining stereochemistry: optical activity
l in dm c in g/mL 3.6
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enantiomers have equal but opposite specific rotations
racemic mixtures: optically inactive
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enantiomeric excess eg. if you have 75% R and 25% S, ee = 50%
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molecules with more than one stereocenter
(mirror images) 3.7A
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notice: diastereomers are not mirror images
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enantiomers: all asymmetric centers different
diastereomers have different physical properties different optical rotation at least one, but not all asymmetric centers different
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n = # asymmetric centers + # asymmetric alkene groups
cis/trans alkenes are diastereomers – but not source of chirality! (show model) 2n stereoisomers n = # asymmetric centers + # asymmetric alkene groups eg. n = 3 8 total stereoisomers (including this one) R,R,E what is the enantiomer of this molecule?
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3.7A
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Naming chiral compounds
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meso compounds rings – look for mirror plane! 3.7B
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Fischer and Haworth projections
(looking down from above) 3.8
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ways of drawing open chain form of glucose:
3.8
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determining R/S on Fischers
convenient to compare sugars: 3.8
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Haworth projections (used for sugars in cyclic form) (not in Bruice)
3.8
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Determining the absolute configuration of (-)-glyceraldehyde
(+)-tartaric acid – configuration determined by x-ray crystallography bonds to asymmetric center not broken – (-) GA must be S!
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stereochemistry and organic reactions
(don’t worry about stereospecific vs. stereoselective)
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racemic mix of enantiomers
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2 new centers formed – 4 isomers formed
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anti addition
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enzymatic reactions are stereospecific
water adds with stereospecificity (enzyme reactions are stereospecific)
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substrate stereoselectivity:
3.9
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