Experiment 9:
Stereochemistry:
Enantiomers and Diastereomers
Objectives




To learn how to identify stereocenters using
molecular models.
To practice assigning the absolute
configuration of stereoisomers using the R-S
convention.
To learn to identify the stereochemical
relationship between stereoisomers.
To learn how to use Fischer projections.
Chirality
ACHIRAL

If an object has a
plane of symmetry,
such as this flask, it is
identical to its mirror
image.
CHIRAL

The lack of a plane of
symmetry is called
“handedness”, or
chirality, such as this
hand. Its mirror images
are NOT identical.
Chirality

A point in a molecule where four
different groups (or atoms) are
attached to carbon is called a chirality
center
Enantiomers

When H and
OH substituents
match up, COOH and
CH3 don’t

when COOH and CH3
coincide, H and OH
don’t
Enantiomers vs. Diastereomers

Molecules with more
than one chirality
center have mirror
image stereoisomers
that are enantiomers
In addition they can
have stereoisomeric
forms that are not
mirror images, called
diastereomers
Diastereomers

Enantiomers
2R,3R
2R,3S
2S,3S
2S,3R
Enantiomers
Meso Forms



Tartaric acid has two chirality centers and two
diastereomeric forms
An achiral compound with chirality centers is
called a meso compound – it has a plane of
symmetry
The two structures on the right in the figure
are identical so the compound (2R, 3S) is
achiral
MESO
Fischer Projections
oThe conversion of a perspective drawing to a
Fischer projection requires rotating the molecule
so that the “top” and “bottom” groups are
oriented back, away from you as is shown in the
molecule below.
Press flat
W
X
Y Z
Z
Z
C
W C
Y
X
W C
Y
X
Fischer Projections

And another example…
Stereochemical Relationships
Diastereomers
Diastereomers
CH3
CH3
Cl
H
H
OH
H
Cl
Cl
H
H
Cl
HO
H
HO
H
H
OH
CH3
CH3
"A"
CH3
CH3
"B"
Enantiomers
CH3
"C"
CH3
"D"
Enantiomers
Diastereomers
Diastereomers
Enantiomers and Meso Forms
COOH
COOH
H
HO
OH
H
COOH
2R, 3R
HO
H
COOH
COOH
H
H
OH
HO
H
OH
H
OH
HO
H
COOH
2S, 3S
COOH
2R, 3S
COOH
2S, 3R
Look closely at these two… There is a plane of symmetry.
The top half of the molecules is identical to the bottom half!
The plane of symmetry makes them achiral,
although they do have chiral centers.
Identifying Stereochemical
Relationship



Identify stereocenters.
Assign absolute configuration to each
stereocenter.
Determine stereochemical relationship
based on absolute configurations.


ENANTIOMERS
 Different absolute configuration at ALL
stereocenters
DIASTEREOMERS
 Same absolute configuration at SOME
stereocenters, different at some.
Physical Properties of Stereoisomers

Enantiomers:



SAME: mp, d, solubility.
DIFFERENT: direction of rotation of plane-polarized light.
Diastereomers :

DIFFERENT: mp, d, solubility, AND direction of rotation of
plane-polarized light.
EXPERIMENTAL PROCEDURE

In today’s experiment, molecular models
will be used to:





Practice identification of chiral centers.
Assign priority to groups attached to chiral
centers based on the CIP rules.
Assign absolute configuration of chiral
centers.
Convert 3d structures to 2d Fischer
projections.
Identify stereochemical relationships between
isomers.