Stereochemistry

Alice Through the Looking Glass

Learning Goals

w After this chapter, you should be able to:

– Recognize chiral carbons in any compound

– Determine R & S designations for molecules

– Distinguish between and identify

• Enantiomers

• Diastereomers

• Meso compounds

– Use and draw Fischer projections

– Show how optically active and racemic mixtures come about in reactions.

Enantiomers

w Non Superimposable Mirror Images

To Identify Chiral Molecules

w No plane of symmetry in molecule

w Four different attachments to chiral center

w No meso configuration

– Meso compounds have a plane of symmetry excluding chiral center

Consequences of Chirality

w Two different enantiomers

w Each enantiomer is optically active

w Each enantiomer will rotate a plane of polarized light in different direction

Specific Rotation

[a]_D = ^a_{/(c x l)}

w Where [a]_D is specific rotation with the sodium D line at wavelength 589 nm

w a is angle of rotation in degrees

w c is concentration in g/mL and

w l is the path length in dm

Note: (+) and (-) used in nomenclature to distinguish between enantiomers

Sample Problem

w A pure enantiomer has an observed optical rotation of -0.82^o when measured in a one dm tube at a concentration of 0.3 g/10 mL. Calculate the specific rotation for this molecule.

Answer:[a]_D = -27.3^o g^-1 mL^-1 dm^-1

Cahn-Inbold-Prelog Sequence Rules for R and S Designation

w 1 Atom with highest atomic weight is #1

Lowest atomic weight is priority #4.

w 2) If rule 1 doesn’t decide, move down

substituent group until a decision can

be made.

w 3) Multiple-bonded atoms are equivalent

to the same number of single-bonded

atoms.

R,S Designation

w Put #4 priority into page and number remaining substituents 1 to 3

w R=Rectus=right=numbering clockwise

w S=Sinister=left=numbering counterclockwise

Identifying Chiral Carbons and Assigning R,S Designation

Identifying Chiral Carbons

Diastereomers

w More than one chiral center

w Opposite optical configuration about one of those centers

Enantiomers vs. Diastereomers

Identifying Chiral Carbons

Identifying Chiral Carbons Meso=Contains Chiral Centers but is Achiral

Identifying Chiral Carbons

Identifying Chiral Carbons

Fischer Projections

w In Fischer projections, the vertical carbons are always viewed as being positioned behind the plane of the paper (away from the viewer) while the horizontal carbons are imagined as coming out of the plane (toward the viewer).

Fischer Projections

w Make it easy to show mirror images

–

Glyceraldehyde

Fischer R,S Designations

Stereochemistry of Reactions

w Addition of HBr

–

Formation of carbocation gives racemic mix

– Carbocation is trigonal planar and can be attacked either in front or back of the plane of the cation giving both R and S stereochemistry.

Racemic Mixture of R&S

Stereochemistry of
Bromine Addition

w Bridged Bromonium ion intermediate

–

Forced syn addition

– Each intermediate can be attacked from the back side of the bridged carbons

– Four distinct products are formed here

Addition to Chiral Alkenes

w

Generally there is no change in chirality unless there is a rearrangement.

w Interesting R to S shift here for carbocation production. Priority changed.

Addition to Chiral Alkenes