Ch05 Estereoquimica
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Transcript of Ch05 Estereoquimica
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Organic Chemistry
4th Edition
Paula Yurkanis Bruice
Irene Lee
Case Western Reserve UniversityCleveland, OH
2004, Prentice Hall
Chapter 5
Stereochemistry
The Arrangement of
Atoms in Space;
The Stereochemistry of
Addition Reactions
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Isomers
Nonidentical compounds having the same molecular
formula
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Cis-Trans Isomers
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Achiral compounds have superimposable mirror images
Chiral compounds have nonsuperimposable mirror
images
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Enantiomers
nonsuperimposable mirror-image molecules
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Drawing Enantiomers
Perspective formula
Fischer projection
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A stereocenter (stereogenic center) is an atom at which
the interchange of two groups produces a stereoisomer
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Naming Enantiomers
Rank the groups (atoms) bonded to the chirality center
The R,Ssystem of nomenclature
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Orient the lowest priority (4) away from you
Clockwise = Rconfiguration
Counterclockwise = Sconfiguration
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Naming from the Perspective Formula
1
23
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,
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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
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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 lowestpriority
Cl
H
CH2CH2CH3CH3CH2 (R)-3-chlorohexane
2. If the lowest priority is on a horizontal bond, the naming is
opposite to the direction of the arrow
CH3
CH2CH3
OHH (S)-2-butanol
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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
CO2H
OH
CH3H
(S)-lactic acid
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Chiral compounds are optically active; they rotate the
plane of polarized light.
Clockwise (+) Counterclockwise (-)
Different from R,Sconfiguration
Achiral compounds do not rotate the plane of polarized
light. They are optically inactive.
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A polarizer measures the degree of optical rotation of a
compound The observed rotation (a)
lxc
=
aa
Tis the temp in C
is the wavelength
a is the measured rotation in degrees
lis the path length in decimeters
cis the concentration in grams per mL
[ ] rotationspecificT=
a
Each optically active compound has a characteristic specific
rotation
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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
I ith th hi l b i f
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Diastereomers are stereoisomers that are not enantiomers
Isomers with more than one chiral carbon: a maximum of
2n stereoisomers can be obtained
CH3CHCHCH3
Cl OH
* *
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Identification of Asymmetric Carbons in
Cyclic Compounds
H
CH3
H
Br*
*these two groups
are different
H
CH3
H
Br
H
Br
CH3
H
H
CH3
H
Br
CH3
H
H
Br
cis-1-bromo-3-methylcyclohexane trans-1-bromo-3-methylcyclohexane
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Meso Compounds
Have two or more asymmetric carbons and a plane of
symmetry
They are achiral molecules
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As long as any one conformer of a compound has a
plane of symmetry, the compound will be achiral
plane ofsymmetry
plane of
symmetry
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The R,Snomenclature of isomers with more than one
asymmetric carbon
R ti f d th t t i t i
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Reactions of compounds that contain an asymmetric
carbon.
CH2CH2CH2Cl
CH3CH2 CH3
H
CH2CH2CH2OH
CH3CH2 CH3
HOH-
No reaction at the asymmetric carbon; both the reagent and theproduct have the same relative configuration.
CH
CH3CH2CH2 CH3
H
CH2 CH2CH3
CH3CH2CH2 CH3
H
H2
Pd/C
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.
R l i f R i Mi
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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
Di i i ti f E ti b
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Discrimination of Enantiomers by
Biological Molecules
T i l i A i t d ith
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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
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Diastereotopic hydrogens do not have the same reactivity
with achiral reagents
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Atoms other than carbon can be asymmetric
N+CH3CH2CH2 CH2CH3
H
CH3
N+
CH2CH2CH3CH3CH2H
CH3
Br- Br-
O
POCH2CH3
H
O
PCH3CH2O OCH3
HCH3O
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Amine Inversion
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A regioselective reaction: preferential formation of one
constitutional isomer
A stereoselective reaction: preferential formation of a
stereoisomer
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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
St h i t f El t hili
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Stereochemistry of Electrophilic
Addition Reactions of Alkenes
What is the absolute configuration of the product?
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Addition reactions that form one asymmetric carbon
CH CH
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CH3CH2C CH2 HBr CH3CH2CHCH2Br
2-methyl-1-butene
CH3
+peroxide
CH3
*
1-bromo-2-methylbutane
Addition reactions that form an additional asymmetric
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Addition reactions that form an additional asymmetric
carbon
Addition reactions that form two asymmetric carbons
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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
C C
CH3CH2 CH2CH3
CH3H3C
+ HCl CH3CH2CH
CH3
CCH2CH3
Cl
CH3
C C
CH3CH2
H3C
CH2CH3
CH3
H
C C
C C
CH3CH2
H3C
Cl
CH2CH3
CH3
H
Cl
CH3CH2
CH2CH3
HCl
H3C
CH3 C C
CH3CH2
CH2CH3
HCH3
H3C
Cl
Addition reactions that form two asymmetric carbons
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A radical reaction intermediate
Addition reactions that form two asymmetric carbons
Stereochemistry of Hydrogen Addition
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Stereochemistry of Hydrogen Addition
(Syn Addition)
H d ti f C li Alk
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Hydrogenation of Cyclic Alkenes
CH(CH3)2
H3C
+ H2Pt/C
H
CH3
H
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
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Stereochemistry of HydroborationOxidation
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Addition reactions that form a bromonium ion
intermediate (anti addition)
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