Intro constitutional isomers: conformational isomers: stereoisomers : (note – many slides from Soderberg text)

intro

constitutional isomers:

conformational isomers:stereoisomers:

(note – many slides from Soderberg text)

3.3

Stereoisomers

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

3.3

enantiomers: mirror images

not superimposable

3.3

two enantiomers of thalidomide

thalidomide is chiral

3.3

more examples of enantiomers

all are chiralif 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

stereocenters? asymmetric centers? chiral?

3.4

The Cahn-Ingold-Prelog system

3.4

3.4

3.4

(effective stereoisomer)

what if H is drawn pointing back?

3.5

commercial thalidomide sold as racemic mixture

3.5

proteins recognize stereochemistry!

other examples of enantiomers with different biological activity

but . . . enantiomers have identical physical properties! (except optical rotation, next)

3.6

determining stereochemistry: optical activity

l in dmc in g/mL

enantiomers have equal but opposite specific rotations

racemic mixtures: optically inactive

enantiomeric excess

eg. if you have 75% R and 25% S, ee = 50%

3.7A

molecules with more than one stereocenter

(mirror images)

3.7Anotice: diastereomers are not mirror images

diastereomers have different physical propertiesdifferent optical rotationat least one, but not all asymmetric centers different

enantiomers: all asymmetric centers different

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?

3.7A

Naming chiral compounds

3.7B

meso compounds

rings – look for mirror plane!

3.8

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)

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

water adds with stereospecificity(enzyme reactions are stereospecific)

enzymatic reactions are stereospecific

3.9

substrate stereoselectivity: