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Organic Chemistry
4th 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. 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
(R)-3-chlorohexane
2. If the lowest priority is on a horizontal bond, the naming is
opposite to the direction of the arrow
(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
(S)-lactic acid
Chiral compounds are optically active; they rotate the
plane of polarized light.
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
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
Diastereomers are stereoisomers that are not enantiomers
Isomers with more than one chiral carbon: a maximum of
2n stereoisomers can be obtained
Identification of Asymmetric Carbons in Cyclic Compounds
Meso Compounds
Have two or more asymmetric carbons and a plane of
symmetry
They are achiral molecules
As long as any one conformer of a compound has a
plane of symmetry, the compound will be achiral
The R,S nomenclature of isomers with more than one
asymmetric carbon
Reactions of compounds that contain an asymmetric
carbon.
No reaction at the asymmetric carbon; both the reagent and the
product have the same relative configuration.
If a reaction breaks a bond at the asymmetric carbon, you need to
know the reaction mechanism in order to predict the relative
configuration of the product.
Resolution of a Racemic Mixture
(R)-acid (S)-acid
enantiomers
(S)-base
(R,S)-salt (S,S)-salt
diastereomers
(R,S)-salt
(S,S)-salt
HCl
HCl
(S)-baseH+
+
(R)-acid
(S)-baseH+
+
(S)-acid
Discrimination of Enantiomers by Biological Molecules
Terminologies Associated with Stereochemistry
pro-R-hydrogen
pro-S-hydrogen
Enantiotopic hydrogens have the same chemical
reactivity and cannot be distinguished by achiral agents,
but they are not chemically equivalent toward chiral
reagents
Diastereotopic hydrogens do not have the same reactivity
with achiral reagents
Atoms other than carbon can be asymmetric
Amine Inversion
A regioselective reaction: preferential formation of one
constitutional isomer
A stereoselective reaction: preferential formation of a
stereoisomer
A stereospecific reaction: each stereoisomeric reactant
produces a different stereoisomeric product or a different
set of products
All stereospecific reactions are stereoselective
Not all stereoselective reactions are stereospecific
Stereochemistry of Electrophilic Addition Reactions of Alkenes
What is the absolute configuration of the product?
Addition reactions that form one asymmetric carbon
Addition reactions that form an additional asymmetric
carbon
A carbocation reaction intermediate
Addition reactions that form two asymmetric carbons
Two substituents added to the same side of the double bond: syn
Two substituents added to opposite sides of the double bond: anti
A radical reaction intermediate
Addition reactions that form two asymmetric carbons
Stereochemistry of Hydrogen Addition (Syn Addition)
Hydrogenation of Cyclic Alkenes
Only cis isomers are obtained with alkenes containing
fewer than eight ring atoms
Both cis and trans isomers are possible for rings
containing eight or more ring atoms
Stereochemistry of Hydroboration–Oxidation
Addition reactions that form a bromonium ion
intermediate (anti addition)