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