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Organic Chemistry
4
th
Edition
Paula Yurkanis Bruice
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall
Chapter 5
Stereochemistry
The Arrangement of
Atoms in Space;
The Stereochemistry of
Addition Reactions

Isomers
Nonidentical compounds having the same molecular
formula

Cis-Trans Isomers

Achiral compounds have superimposable mirror images
Chiral compounds have nonsuperimposable mirror
images

Enantiomers
nonsuperimposable mirror-image molecules

Drawing Enantiomers

Perspective formula
Fischer projection

A stereocenter (stereogenic center) is an atom at which
the interchange of two groups produces a stereoisomer

Naming Enantiomers
Rank the groups (atoms) bonded to the chirality center
The R,S system of nomenclature

Orient the lowest priority (4) away from you
Clockwise = R configuration
Counterclockwise = S configuration

Naming from the Perspective Formula
1
2
3
4
1. Rank the groups bonded to the asymmetric carbon
2. If the group (or atom) with the lowest priority is
bonded by hatched wedge,

3. If necessary, rotate the molecule so that the lowest
priority group (or atom) is bonded by a hatched wedge
4. You can draw group 1 to group 2, passing group 4,
but never 3

Naming from the Fischer Projection
1. Rank the groups (or atom) that are bonded to the asymmetric

carbon and draw an arrow with the highest priority to the lowest
priority
Cl
H
CH
2
CH
2
CH
3
CH
3
CH
2
(R)-3-chlorohexane
2. If the lowest priority is on a horizontal bond, the naming is
opposite to the direction of the arrow
CH
3
CH
2
CH
3
OHH
(S)-2-butanol

A Fischer projection can only be rotated 180
°
in the plane
of the paper to yield the same molecule

3. The arrow can go from group 1 to 2, passing group 4, but not
group 3
CO
2
H
OH
CH
3
H
(S)-lactic acid

Chiral compounds are optically active; they rotate the
plane of polarized light.
Clockwise (+) Counterclockwise (-)
Different from R,S configuration
Achiral compounds do not rotate the plane of polarized
light. They are optically inactive.

A polarizer measures the degree of optical rotation of a
compound
The observed rotation (α)
[ ]
lxc
=
Τ
λ
α
α
T is the temp in °C
λ is the wavelength

α is the measured rotation in degrees
l is the path length in decimeters
c is the concentration in grams per mL
[ ]
rotationspecific
T
=
λ
α
Each optically active compound has a characteristic specific
rotation

A racemic mixture, which contains an equal amount of
the two enantiomers, is optically inactive
optical purity =
observed specific rotation
specific rotation of the pure enantiomer
enantiomeric excess =
excess of a single enantiomer
entire mixture