intro constitutional isomers: conformational isomers: stereoisomers : (note – many slides from...
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intro
constitutional isomers:
conformational isomers:stereoisomers:
(note – many slides from Soderberg text)
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3.3
Stereoisomers
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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)
Definitions
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3.3
enantiomers: mirror images
not superimposable
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3.3
two enantiomers of thalidomide
thalidomide is chiral
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3.3
more examples of enantiomers
all are chiralif it has an asymmetric center, it is almost certainly chiral (exception – meso)
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3.3
achiral molecules (no stereocenters!)
wedges don’t necessarily mean a stereocenter, and vice-versa!
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3.3
other atoms can be stereocenters
Don’t worry about threo / erythro definition
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3.3
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stereocenters? asymmetric centers? chiral?
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3.4
The Cahn-Ingold-Prelog system
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3.4
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3.4
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3.4
(effective stereoisomer)
what if H is drawn pointing back?
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3.5
commercial thalidomide sold as racemic mixture
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3.5
proteins recognize stereochemistry!
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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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3.6
determining stereochemistry: optical activity
l in dmc in g/mL
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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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3.7A
molecules with more than one stereocenter
(mirror images)
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3.7Anotice: diastereomers are not mirror images
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diastereomers have different physical propertiesdifferent optical rotationat least one, but not all asymmetric centers different
enantiomers: all asymmetric centers different
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2n stereoisomers
n = # asymmetric centers + # asymmetric alkene groups
OH
OH
eg. n = 38 total stereoisomers(including this one)
R,R,E
cis/trans alkenes are diastereomers – but not source of chirality! (show model)
what is the enantiomer of this molecule?
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3.7A
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Naming chiral compounds
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3.7B
meso compounds
rings – look for mirror plane!
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3.8
Fischer and Haworth projections
(looking down from above)
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3.8
ways of drawing open chain form of glucose:
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3.8
determining R/S on Fischers
convenient to compare sugars:
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3.8
Haworth projections(used for sugars in cyclic form)(not in Bruice)
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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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water adds with stereospecificity(enzyme reactions are stereospecific)
enzymatic reactions are stereospecific
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3.9
substrate stereoselectivity: