Stereochemistry
The study of the three
dimensional structure of
molecules.
Isomers—A review
Constitutional/Structural Isomers: differ in
their bonding sequence; their atoms are
connected differently.
 Stereoisomers: have the same bonding
sequence, but differ in the orientation of
their atoms in space. One special type of
stereoisomerism that you have seen is
geometric
isomerism
(e.g.
cis/trans
isomers).

Chirality
Chiral: a chiral object is one that has right and
left handed forms, or a nonsuperimposable
(nonidentical) mirror image. A chiral object has
a mirror image that is different from the
original object. An object that is not chiral is
described as “achiral”.
 An atom is chiral only if it has four different
groups attached to it.
 The mirror image of a chiral molecule is called
its enantiomer.

Do problems 5-1, 5-2 and 5-3 in the text.
Chiral Molecules vs. Chiral Centers
A molecule with one chiral center is a chiral
molecule.
 A molecule with more than one chiral center
is achiral if it has a plane of symmetry.
 Enantiomers
have
identical
physical
properties (e.g. bp, mp, density) except that
they rotate plane polarized light in equal but
opposite directions. They are said to possess
“optical activity” or to be “optically active”.
Achiral molecules are not optically active!

Do problem 5-4 in the text.
Chiral: does not contain
a plane of symmetry
Achiral: contains a
plane of symmetry
Br
H
Br
Br
H
Br
H
H
chiral
achiral
Optical Activity
Optical Activity is measured with a polarimeter.
 A compound in a polarimeter can rotate plane
polarized light to the right or left by a specific
number of degrees that must be determined
experimentally. Compounds that rotate plane
polarized light to the right (clockwise) are
called dextrorotatory (d), and compounds that
rotate plane polarized light to the left (counterclockwise) are called levorotatory (l). In IUPAC
notation, this is abbreviated by the signs (+)
and (-) respectively.

Nomenclature of Chiral Carbon
Atoms
The configuration about chiral carbons is
named using the Cahn-Ingold-Prelog
convention, which assigns to each chiral
carbon atom a letter (R) or (S).
Cahn-Ingold-Prelog Rules:

Assign a priority to each group bonded to the
chiral carbon. The higher the atomic number
of the atom, the higher its priority. With
different isotopes of the same element, the
heavier isotopes have higher priority. When
there is a tie, use the next atoms along the
chain as tiebreakers. Treat double and triple
bonds as if each were bonded to a separate
atom.
Cahn-Ingold-Prelog Rules

Using a three dimensional drawing or a model,
put the fourth priority group in the back and
view the molecule along the bond from the
chiral carbon to the fourth priority group.
Draw an arrow from the first priority group,
through the second, to the third. If the arrow
points clockwise, the chiral atom is called (R).
If the arrow points counterclockwise, the
chiral atom is called (S).
Do problem 5-5 in the text.
Biological Discrimination of
Enantiomers

Enantiomers can be distinguished through the
use of chiral probes. A polarimeter is one
example of a chiral probe. Enzymes are a type
of chiral probe that are found in living systems.
In general, just one out of a pair of
enantiomers produces the characteristic effect;
the other either has a no effect or has a totally
different (and sometimes toxic) effect.
Do problem 5-10 in the text.
Racemic Mixtures
A mixture that contains equal amounts of a
pair or enantiomers is called a racemic
mixture or a racemate, a (±) pair, or a (d,l)
pair. A racemic mixture is symbolized by
placing (±) or (d,l) in front of the name of
the compound.
 Racemic mixtures are optically inactive.
Since the enantiomers rotate plane
polarized light in equal but opposite
directions, the net result is an optical
rotation of zero.

Enantiomeric Excess
When a mixture of enantiomers is neither
enantiomerically pure (all one enantiomer)
nor racemic (equal amounts of two
enantiomers), the relative amounts of the
enantiomers in the mixture can be expressed
as the enantiomeric excess (optical purity).
e.e. = d - l x 100
d + l
(excess of one over the other) x100
=
(entire mixture)
Chiral Compounds Without Chiral Atoms
There are some molecules that do not contain chiral
carbons but are chiral.

Biphenyls: some ortho substituted biphenyls are
locked into one of two chiral, enantiomeric
staggered conformations.
Br
Br
I
I
Staggered conformation
(chiral)
Br
Br
I
I
Staggered conformation
(chiral)
Chiral compounds without chiral atoms
•Allenes: Compounds containing a C=C=C unit are
called allenes. In allene, the central C atom is sp
hybridized, but the two outer carbons are sp2. The whole
molecule does not lie in the same plane. An allene is
chiral if each end has two distinct
substituents.
H3C
CH3
C
H
C
H3C
C
CH3
C
H
C
H
Enantiomers of 2,3-pentadiene
Do problem 5-14 in the text.
C
H
Fischer Projections




Easiest to use when > 1 chiral center
Sugars almost always drawn as FP
The carbon chain is always drawn along the
vertical line of the Fischer projection
The rules for assignment of R,S configurations are
the same.
=
C
horizontal is always
coming out at you
=
C
C
consider each chiral
center separately
Do problems 5-15, 5-16, 5-17 and 5-18.
Diastereomers
Must have >1 chiral center
 Stereoisomers which are not enantiomers.
i.e. stereoisomers that are not mirror
images.
 Have different physical properties
 The maximum # of stereoisomers = 2n
where n is the number of chiral centers.
 Examples of diastereometric relationships
include cis/trans isomerism in rings and cis/
trans isomerism about double bonds.

Do problem 5-19 and 5-22 in the text.
Diastereomers
HO H
CH3
HO H
CH3
H Br
(2R, 3S)-3-bromo-2-butanol
CH3
CH3
Br
H
(2R, 3R)-3-bromo-2-butanol
DIASTEREOMERS
MIRROR IMAGES AT C3 BUT NOT AT C2
Meso Compounds
has >1 chiral center
 chiral centers are mirror images

» has mirror plane through center of molecule

contains chiral centers, but compound is
achiral
» doesn’t rotate light

no enantiomeric pair (mirror images
superimposable)
Resolution of Enantiomers
The separation of enantiomers is called
resolution.
 A chiral probe is necessary for the resolution
of enantiomers. Such a probe is called a
resolving agent.
 Enantiomers can be resolved chemically or
chromatographically.
