Organic Chemistry
4th Edition
Paula Yurkanis Bruice
Chapter 5
Stereochemistry
The Arrangement of
Atoms in Space;
The Stereochemistry of
Addition Reactions
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall
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
The R,S system of nomenclature
Rank the groups (atoms) bonded to the chirality center
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
1
4
2
3
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
CH3CH2
CH2CH2CH3
(R)-3-chlorohexane
H
2. If the lowest priority is on a horizontal bond, the naming is
opposite to the direction of the arrow
CH3
H
OH
CH2CH3
(S)-2-butanol
3. The arrow can go from group 1 to 2, passing group 4, but not
group 3
CO2H
H
CH3
OH
(S)-lactic acid
A Fischer projection can only be rotated 180° in the plane
of the paper to yield the same molecule
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 (a)
a  =
Τ
λ
[a ]T = specific rotation
a
lxc
T is the temp in °C
 is the wavelength
a is the measured rotation in degrees
l is the path length in decimeters
c is the concentration in grams per mL
Each optically active compound has a characteristic specific
rotation
A racemic mixture, which contains an equal amount of
the two enantiomers, is optically inactive
observed specific rotation
optical purity =
specific rotation of the pure enantiomer
enantiomeric excess =
excess of a single enantiomer
entire mixture
Isomers with more than one chiral carbon: a maximum of
2n stereoisomers can be obtained
* *
CH3CHCHCH3
Cl OH
Diastereomers are stereoisomers that are not enantiomers
Identification of Asymmetric Carbons in
Cyclic Compounds
H
these two groups *
are different
H
Br
*
CH3
H
H
H
H
CH3
cis-1-bromo-3-methylcyclohexane
H
CH3
Br
Br
CH3
H
CH3
Br
Br
H
H
trans-1-bromo-3-methylcyclohexane
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
plane of
symmetry
plane of
symmetry
The R,S nomenclature of isomers with more than one
asymmetric carbon
Reactions of compounds that contain an asymmetric
carbon.
CH2CH2CH2OH
CH2CH2CH2Cl
CH3CH2
CH3CH2
CH3
CH
CH3CH2CH2
H
OH-
H
CH2
H
CH3
CH3
CH2CH3
H2
Pd/C
CH3CH2CH2
H
CH3
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+ (S)-baseH+
+
+
(S)-acid
(R)-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
CH3
Br-
N+
CH3CH2CH2
H
CH2CH3
CH3CH2O
BrN+
H
CH2CH2CH3
CH3CH2
O
O
P
CH3
H
OCH 3
H P
OCH 2CH3
CH3O
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
CH3
CH3CH2C
CH3
CH2
+
2-methyl-1-butene
HBr
peroxide
CH3CH2CHCH2Br
*
1-bromo-2-methylbutane
Addition reactions that form an additional asymmetric
carbon
Addition reactions that form two asymmetric carbons
A carbocation reaction intermediate
CH3CH2
C
CH2CH3
+ HCl
C
H3C
CH3
CH3CH2
CH3
C
H3C
Cl
H3C
C
Cl
H
CH2CH3
CH2CH3
C C CH
3
H
CH3CH2 Cl
CH3CH2CH
CCH2CH3
CH3 CH3
CH3CH2
Cl C
H3C
CH3
C
H
CH2CH3
CH2CH3
H3C
C
H
CH3CH2
C
Cl
CH3
Two substituents added to the same side of the double bond: syn
Two substituents added to opposite sides of the double bond: anti
Addition reactions that form two asymmetric carbons
A radical reaction intermediate
Stereochemistry of Hydrogen Addition
(Syn Addition)
Hydrogenation of Cyclic Alkenes
H
+
H2
H
Pt/C
CH3 CH(CH3)2
H3C
CH(CH3)2
H3C
H
CH(CH3)2
H
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)