chapter 7 stereochemistry copyright © the mcgraw-hill companies, inc. permission required for...
TRANSCRIPT
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Chapter 7Chapter 7StereochemistryStereochemistry
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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The class of stereoisomers that is presented in Chapter 7 is different than the cis/trans (or E/G) geometric isomers.
These isomers are a result of a mirror image relationship between two compounds.
The term “chirality” applies to these isomers and is introduced here.
Stereochemistry
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7.17.1Molecular Chirality: Molecular Chirality:
EnantiomersEnantiomers
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A molecule is chiral if its two mirror image forms are not superimposable upon one another.
A molecule is achiral if its two mirror image forms are superimposable. An achiral molecule does not exhibit stereoisomerism.
Chirality
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BrCl
H
F
Bromochlorofluoromethane is chiral
It is not superimposable point for point on its mirror image.
Note the four different attachments on C.
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BrCl
H
F
Bromochlorofluoromethane is chiral
H
ClBr
F
To demonstrate nonsuperimposability, rotate this model 180° around a vertical axis.
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BrCl
H
F
Bromochlorofluoromethane is chiral
H
ClBr
F
The structure on the right has been rotated.
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Another look
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These two structures are enantiomers.
Nonsuperimposable mirror images are called enantiomers, they occur in pairs.
Enantiomers
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Classification of Isomers
stereoisomersconstitutionalisomers
diastereomersenantiomers(geometric)(geometric)
Includes E-Z isomersIncludes E-Z isomers
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Isomers with Chiral Centers
diastereomersenantiomers
meso compounds
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Chlorodifluoromethaneis achiral
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Chlorodifluoromethaneis achiral
The two structures at the top are mirror images, but because they can be superimposed on each other they are identical and are not enantiomers.
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7.27.2The Chirality CenterThe Chirality Center
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A compound containing a carbon atom with four different groups attached to it.This means that this carbon must be sp3 and can not be sp2 or sp.
A Chiral Compound
w
x y
z
C
The carbon is called a(n):chirality or chiral centerasymmetric centerstereocenterstereogenic center
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A molecule with a single chirality center is chiral. Enantiomeric structures are possible.
Bromochlorofluoromethane is an example.
Chirality and chirality centers
Cl F
Br
H
C
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2-Butanol is another example.
Chirality and chirality centers
CH3
OH
H
C CH2CH3
Note that the CH2 and CH3's are not chiral.
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Examples of molecules with 1 chirality center
CH3
C
CH2CH3
CH2CH2CH2CH3CH3CH2CH2
4-ethyl-4-methyloctane, a chiral alkane
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Examples of molecules with 1 chirality center
3,7-dimethyl-1,6-octadien-3-ol,Linalool, a naturally occurring chiral alcohol
OH
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Examples of molecules with 1 chirality center
1,2-Epoxypropane: one of the ring carbon atoms is is a chirality center.
O
H2C CHCH3
attached to this chirality center are:
—H
—CH3
—OCH2 or just —O
—CH2O or just —CH2
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Examples of molecules with 1 chirality center
Limonene also has a chirality center as part of the ring.
CH3
H C
CH3
CH2
attached to thechirality center are:
—H
—CH2CH2
—CH2CH=
—C=
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Examples of molecules with 1 chirality center
Chiral as a result of isotopic substitution
CH3CD
T
H
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A molecule with a single chirality center must be chiral (enantiomeric structures are possible).
But, a molecule with two or more chirality centers may have structures that are chiral and others that are not (Sections 7.11-7.14).
Distinguish: enantiomers, diastereomers and meso compounds.
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7.37.3Symmetry in Achiral Symmetry in Achiral
StructuresStructures
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Symmetry tests for achiral structures
Any molecule with a plane of symmetryor a center of symmetry must be achiral.
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A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane
has a plane of symmetry.
Plane of symmetry
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A plane of symmetry bisects a molecule into two mirror image halves.
1-Bromo-1-chloro-2-fluoroethene has a planeof symmetry.
Plane of symmetry
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A point in the center of themolecule is a center of symmetry if a line drawn from it to any element, when extended an equal distance in the opposite direction, encounters an identical element.
Center of symmetry
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7.47.4Optical ActivityOptical Activity
Optical activity is a property of some Optical activity is a property of some compounds containing chiral centers compounds containing chiral centers (enantiomers and diastereomers). (enantiomers and diastereomers).
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A substance is optically active if it rotates the plane of polarized light.
In order for a substance to exhibit opticalactivity, it must contain a chiral carbon or carbons.
Two enantiomers have equal and opposite specific rotation values.
Optical Activity
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Light
Light has wave properties, i.e. shows a periodic increase and decrease in amplitude of the electromagnetic wave.
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Light
Optical activity is usually measured using light having a wavelength of 589 nm (monochromatic).
This is the wavelength of the yellow light from a sodium lamp and is called the D line of sodium.
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Polarized Light
Ordinary (nonpolarized) light consists of many beams vibrating in different planes.
Plane-polarized light consists of only those beams that vibrate in the same plane.
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Nicol prismNicol prism
Polarization of light is accomplished using a Nicol prism or a polaroid lens.
Light that passes
through is in the
plane of the crystals
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Rotation of Plane-polarized Light by a chiral compound
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Observed rotation () depends on the number of molecules encountered and is proportional to:
path length (l), and concentration (c).
Therefore, specific rotation [] is defined as:
Specific Rotation
100
cl
c = concentration = g/100 mLl = length in decimetersobserved rotation
[] =
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A mixture containing equal amounts of a pair of enantiomers is called a racemic mixture.
A racemic mixture is optically inactive.( = 0)
A compound that is optically inactive can beeither an achiral substance, a racemic mixture or a meso compound.
Racemic Mixture
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An optically pure substance consists exclusively of a single enantiomer. The excess of one enantiomer over another in a mixture is given by:
Enantiomeric excess = (R-S)/(R+S) by weight or% one enantiomer – % other enantiomer
Optical purity = []o / []p (observed/pure)
Optical purity = enantiomeric excess
Optical Purity
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7.57.5Absolute and Relative Absolute and Relative
ConfigurationConfiguration
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Relative configuration compares the arrangement of atoms in space of one compound with those of another.
Until the 1950s, all configurations were relative.
Absolute configuration is the precise arrangement of atoms in space.
Note: Absolute configuration and Specific rotation are not readily predicted from one another.
Configuration
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No bonds are made or broken at the chirality center
in this experiment. In this case, when (+)-3-buten-2-ol
and (+)-2-butanol have the same sign of rotation, the
arrangement of atoms in space is analogous. The two
have the same relative configuration.
CH3CHCH2CH3
OH
Pd
[] + 33.2° [] + 13.5°
Relative Configuration
CH3CHCH
OH
CH2
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HHO
H OH H2, Pd
HHOH2, Pd
H OH
Two Possibilities
But in the absence of additional information, we can't tell which structure corresponds to(+)-3-buten-2-ol, and which one to (–)-3-buten-2-ol.
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HHO
H OH H2, Pd
HHOH2, Pd
H OH
Two Possibilities
Nor can we tell which structure corresponds to(+)-2-butanol, and which one to (–)-2-butanol.
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HHO
H OH H2, Pd
HHOH2, Pd
H OH
[] +33.2°[] +13.5°
[] –13.5° [] –33.2°
Optical Rotations
enantiomers enantiomers
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Relative configuration is independent of specific optical
rotation.
No bonds are made or broken at the chirality center in
the reaction shown, so the relative positions of the
atoms are the same, yet the sign of rotation changes.
CH3CH2CHCH2Br
CH3
HBr
[] -5.8° [] + 4.0°
Relative Configuration
CH3CH2CHCH2OH
CH3
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7.67.6The Cohn-Ingold-Prelog The Cohn-Ingold-Prelog R-S Notational SystemR-S Notational System
Designating Absolute Configuration Designating Absolute Configuration
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1. Rules for ranking substituents at a chirality center are needed.
2. A convention for orienting a molecule so that order of appearance of substituents can be compared with rank.
The system used was devised by R. S. Cahn, Sir Christopher Ingold, and V. Prelog. This CIP ranking system is the same as was used for E-Z isomers.
Two Requirements for a Systemfor Specifying Absolute Configuration
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43
2
1
Example
4 3
2
1
Order of decreasing rank:4 > 3 > 2 > 1
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1. Rank the substituents at the chirality center according to same rules used in E-Z notation.
2. Orient the molecule so that lowest-ranked substituent points away from you.
3. If the order of decreasing precedence traces a clockwise path, the absolute configuration is R. If the path is counterclockwise, the configuration is S.
The Cahn-Ingold-Prelog Rules(Table 7.1)
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43
2
1
Example
4 3
2
1
clockwiseclockwise
RR
counterclockwisecounterclockwise
SS
Order of decreasing rank:
4 > 3 > 2 > 1
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C OH
H3C
HCH3CH2
Enantiomers of 2-butanol CHO
CH3
HCH2CH3
(S)-2-Butanol (R)-2-Butanol
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Very important! Two different compounds with the same sign of rotation need not have the same
configuration.
Verify this statement by doing Problem 7.9 on page 289. All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?
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HH3C
H
H
Chirality Center in a Ring
RR
—CH2C=C > —CH2CH2 > —CH3 > —H
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7.77.7Fischer ProjectionsFischer Projections
Purpose of Fischer projections is to show Purpose of Fischer projections is to show configuration at chirality center without configuration at chirality center without necessity of drawing wedges and dashes or necessity of drawing wedges and dashes or using models. using models.
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Rules for Fischer Projections
Arrange the molecule so that at the chirality center, horizontal bonds point toward you and vertical bonds point away from you.This is important because the Fisher Projection is planar.
Br Cl
F
H
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Rules for Fischer Projections
Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.
Br Cl
F
H
Br Cl
F
H
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7.87.8Properties of EnantiomersProperties of Enantiomers
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Same: melting point, boiling point, density, index of refraction, etc (all physical properties).
Different: properties that depend on shape of molecule
(biological-physiological properties) can bedifferent.
Physical Properties of Enantiomers
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O O
CH3 CH3
H3C H3CCH2 CH2
Odor (–)-Carvonespearmint oil
(+)-Carvonecaraway seed oil
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Ibuprofen is chiral, but normally sold asa racemic mixture. The S enantiomer is the one responsible for its analgesic and anti-inflammatory properties.
Chiral Drugs
CH2CH(CH3)2
HH3C
C
O
C
HO
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7.97.9The Chirality AxisThe Chirality Axis
Compounds with no Chiral Center Compounds with no Chiral Center
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The Chirality Axis
Some molecules are chiral but do not contain a chirality center. Some of these contain a chirality axis, an axis about which groups are arranged so that the spatial arrangement is not superimposable on its mirror image.
Examples include substituted biphenyls and allenes:
A
BY
X
C C C
A
B Y
X
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In the appropriately substituted biphenyls, rotation around the bond joining the rings is restricted and the enantiomers can be isolated:
A
BY
X B
AY
X
Conformational isomers that are stable, isolable compounds are called atropisomers.
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Substituted 1,1’-binaphthyl derivatives exhibit atropisomerism due to hindered rotation about the single bond that connects the two naphthalene rings.
An example is (S)-(-)-BINAP shown below and discussed further in Chapter 14.
P(C6H5)2
P(C6H5)2
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7.107.10Reactions that Create a Reactions that Create a
Chirality Center Chirality Center
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Many Reactions Convert Achiral Reactants to Chiral Products
It is important to recognize, however, that if all of the components of the starting state (reactants, catalysts, solvents, etc.) are achiral, any chiral product will be formed as a racemic mixture.
This generalization can be more simply stated as "Optically inactive starting materials can't give optically active products." (Remember: In order for a substance to be optically active, it must be chiral and one enantiomer must be present in greater amounts than the other.)
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Example
CH3CH CH2
CH3COOH
O
H3C
O
CH2C
H
Chiral, but racemicAchiral
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Epoxidation from this direction gives R epoxide.
R
Epoxidation from this direction gives S epoxide.
SS
50%
50%
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Example
CH3CH CH2
Chiral, but racemic
Br2, H2O
CH3CHCH2Br
OH
Achiral
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Example
CH3CH CHCH3
Chiral, but racemic
HBrCH3CHCH2CH3
Br
Achiral
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Many Reactions Convert Chiral Reactants to Chiral Products
However, if the reactant is racemic, the product will also be racemic.
Remember: "Optically inactive starting materials can't give optically active products."
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Example
Chiral, but racemic
HBrCH3CHCH2CH3
OH
CH3CHCH2CH3
Br
Chiral, but racemic
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Many Biochemical Reactions Convertan Achiral Reactant to a SingleEnantiomer of a Chiral Product
Reactions in living systems may be catalyzed by enzymes, which are enantiomerically homogeneous.
The enzyme (catalyst) is part of the reacting system, so such reactions don't violate the generalization that "Optically inactive starting materials can't give optically active products."
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Example
fumarase
H2O
C C
HO2C H
CO2HH
C OH
HHO2C
HO2CCH2
Fumaric acid (S)-(–)-Malic acid
Achiral Single enantiomer
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7.117.11Chiral Molecules with Two Chiral Molecules with Two
Chirality Centers Chirality Centers
How many stereoisomers are How many stereoisomers are possible when a molecule possible when a molecule contains two chirality centers?contains two chirality centers?
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2,3-Dihydroxybutanoic Acid
What are all the possible R and S combinations of the two chirality centers in this molecule ?
4 Combinations = 4 Stereoisomers
O
CH3CHCHCOH
HO OH
23
Carbon-2 R R S SCarbon-3 R S R S
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2,3-Dihydroxybutanoic Acid
23
Carbon-2 R R S SCarbon-3 R S R S
What is the relationship between these stereoisomers ?enantiomers: 2R,3R and 2S,3S
2R,3S and 2S,3R
O
CH3CHCHCOH
HO OH
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HO
CO2H
CH3
H
OHHR
R
CO2H
CH3
H
HHO
OH
S
S
CO2H
H
CH3
HO
HHO
R
S
CO2H
CH3
H OH
OHHR
S
enantiomersenantiomers
enantiomersenantiomers
[] = -9.5° [] = +9.5°
[] = -17.8°[] = +17.8°
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2,3-Dihydroxybutanoic Acid
23
Carbon-2 R R S SCarbon-3 R S R S
But not all relationships are enantiomeric.Stereoisomers that are not enantiomers are diastereomers.
O
CH3CHCHCOH
HO OH
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HO
CO2H
CH3
H
OHHR
R
CO2H
CH3
H
HHO
OH
S
S
CO2H
H
CH3
HO
HHO
R
S
CO2H
CH3
H OH
OHHR
S
enantiomersenantiomers
enantiomersenantiomers
[] = -9.5° [] = +9.5°
[] = -17.8°[] = +17.8°
diastereomersdiastereomersdiastereomersdiastereomers
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CO2H
CH3
Fischer Projections
For Fischer projection: horizontal bonds point toward you; vertical bonds point away.
A staggered conformation does not have the correct orientation of bonds for Fischer projection.
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Fischer Projections
Transform molecule to eclipsed conformation in order to construct Fischer projection.
CO2H
CH3
OH
OH
H
H
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Erythro and Threo
Stereochemical prefixes used to specify relative configuration in molecules with two chirality centers
Easiest to apply using Fischer projections
Orientation: vertical carbon chain
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When the carbon chain is vertical and the same (or analogous) substituents are on the same side of the Fischer projection, this is the erythro form.
CO2H
CH3
OH
OH
H
H
–9.5° +9.5°
CO2H
CH3
H
H
HO
HO
Erythro
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When the carbon chain is vertical and the same (or analogous) substituents are on opposite sides of the Fischer projection, this is the threo form.
+17.8° –17.8°
OH
CO2H
CH3
H
H
HO
CO2H
CH3
OHH
HHO
Threo
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S SR R
Two Chirality Centers in a Ring
nonsuperimposable mirror images; enantiomers
trans-1-Bromo-1-chlorocyclopropane
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S RS R
Two Chirality Centers in a Ring
nonsuperimposable mirror images; enantiomers
cis-1-Bromo-1-chlorocyclopropane
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S RS R
Two Chirality Centers in a Ring
Stereoisomers that are not enantiomers are diastereomers (these are not mirror images).
cis-1-Bromo-1-chloro-cyclopropane trans-1-Bromo-1-chloro-
cyclopropane
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7.127.12Achiral Molecules with Two Achiral Molecules with Two
Chirality Centers Chirality Centers
It is possible for a molecule to It is possible for a molecule to have chirality centers yet be have chirality centers yet be achiral.achiral.
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2,3-Butanediol
Consider a molecule with two equivalently substituted chirality centers such as 2,3-butanediol.
CH3CHCHCH3
HO OH
32
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Three Stereoisomers of 2,3-Butanediol
2R,3R 2S,3S
chiral chiral
2R,3S
achiral
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Three Stereoisomers of 2,3-Butanediol
2R,3R 2S,3S 2R,3S
chiral chiral achiral
CH3
CH3
OHH
HHOH OH
CH3
CH3
HHO H
CH3
CH3
OH
OHH
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Three Stereoisomers of 2,3-Butanediol
2R,3R 2S,3S
chiral chiral
These two areenantiomers.
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Three Stereoisomers of 2,3-Butanediol
2R,3R 2S,3S
chiral chiral
These two areenantiomers.
CH3
CH3
OHH
HHOH OH
CH3
CH3
HHO
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Three Stereoisomers of 2,3-Butanediol
2R,3S
achiral
• The third structure is superposable on its
• mirror image.
Therefore, this structure and its mirror imageare the same.
It is called a meso form.
A meso form is an achiral molecule that has chirality centers.
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Three Stereoisomers of 2,3-Butanediol
2R,3Sachiral
HHO
CH3
CH3
HHO
Fischer projections of the meso form.
H
CH3
CH3
OH
OHH
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Three Stereoisomers of 2,3-Butanediol
2R,3S
achiral
Meso forms have a plane of symmetry and/or a center of symmetry.
Plane of symmetry is most common case.
Top half of molecule is mirror image of bottom half.
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Three Stereoisomers of 2,3-Butanediol
2R,3S
achiral
A line drawnthe center ofthe Fischer projection of ameso formbisects it intotwo mirror-image halves.
HHO
CH3
CH3
HHO H
CH3
CH3
OH
OHH
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S R RRRR
chiralmeso
There are three stereoisomers of 1,2-dichloro-cyclopropane; the achiral (meso) cis isomer and two enantiomers of the trans isomer.
Cyclic Compounds
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7.137.13Molecules with Multiple Molecules with Multiple
Chirality Centers Chirality Centers
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Maximum number of stereoisomers = 2n.
Where n = number of structural units capable of stereochemical variation.
Structural units include chirality centers and cis and/or trans double bonds.
Number is reduced to less than 2n if meso forms are possible.
How Many Stereoisomers?
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Example
4 chirality centers
16 stereoisomers
O
HOCH2CH—CH—CH—CHCH
OH OH OH OH
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HO OH
H
H
HO
H3C
H
HCH2CH2CO2H
CH3
H
CH3
11 chirality centers
211 = 2048 stereoisomers
One is "natural" cholic acid.
A second is the enantiomer of natural cholic acid.
2046 are diastereomers of cholic acid.
Cholic Acid (Figure 7.11)
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3-Penten-2-ol
HO H
E R
H OH
E S
HHO
Z R
H OH
S
How Many Stereoisomers? Z
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7.147.14Reactions that Produce Diastereomers Reactions that Produce Diastereomers
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In order to know understand stereochemistry of product, you need to know two things:
(1) Stereochemistry of alkene (cis or trans; Z or E)(2) Stereochemistry of mechanism (syn or anti)
Stereochemistry of Addition to Alkenes
C C + E—Y C CE Y
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R
S
Anti addition to trans-2-butene gives meso diastereomer.
Bromine Addition to trans-2-ButeneFig. 7.12
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Bromine Addition to cis-2-ButeneFig. 7.12
Anti addition to cis-2-butene gives racemic mixture of chiral diastereomer.
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RCO3H
R
R S
S
Syn addition to trans-2-butene gives racemic mixture of chiral diastereomer.
Epoxidation of trans-2-ButeneProblem 7.26
50% 50%
+
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R
S R
S
Epoxidation of cis-2-ButeneProblem 7.26
syn addition to cis-2-butene gives meso diastereomer
RCO3H
meso
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Of two stereoisomers of a particular starting material, each one gives differentstereoisomeric forms of the product.
Related to mechanism: terms such assyn addition and anti addition refer tostereospecificity.
Stereospecific Reaction
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.
transtrans-2-butene-2-butene
ciscis-2-butene-2-butene
transtrans-2-butene-2-butene
ciscis-2-butene-2-butene brominationbromination antianti 22RR,3,3RR + 2 + 2SS,3,3SS
brominationbromination
epoxidationepoxidation
epoxidationepoxidation
antianti
synsyn
synsyn
mesomeso
mesomeso
22RR,3,3RR + 2 + 2SS,3,3SS
Stereospecific reactions
Compound Reaction Attack Product(s)
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A single starting material can give two or morestereoisomeric products, but gives one of themin greater amounts than any other.
+
CH3
H
CH3
H
68% 32%
Stereoselective reaction
CH3
CH2
H CH3
H
CH3
H
H2
Pt
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7.157.15Resolution of Enantiomers Resolution of Enantiomers
separation of a racemic mixture into its two separation of a racemic mixture into its two enantiomeric formsenantiomeric forms
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Enantiomers, racemic
C(+)C(+)C(+)C(+) C(-)C(-)C(-)C(-)
2P(+)2P(+)
C(+)C(+)P(+)C(+)C(+)P(+) C(-)C(-)P(+)C(-)C(-)P(+)
Separate diastereomers
C(+)C(+)P(+)C(+)C(+)P(+)
C(-)C(-)P(+)C(-)C(-)P(+)
P(+)P(+)
P(+)P(+)
C(+)C(+)C(+)C(+)
C(-)C(-)C(-)C(-)
Strategy
pure
pure
Add pureenantiomer
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7.167.16Stereoregular Polymers Stereoregular Polymers
atacticatactic
isotacticisotactic
syndiotacticsyndiotactic
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Atactic Polypropylene
Random stereochemistry of sidechains (methyl groups here) attached to main chain = stereorandom polymer (atactic).Properties not very useful for fibers etc.Formed by free-radical polymerization.
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Isotactic Polypropylene
All sidechains (methyl groups here) onsame side of main chain = stereoregular polymer (isotactic).Useful properties.Prepared by coordination polymerization under Ziegler-Natta conditions.
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Syndiotactic Polypropylene
Sidechains (methyl groups here) alternate from one side to the other on main chain = stereoregular polymer (syndiotactic).Useful properties.Prepared by coordination polymerization under Ziegler-Natta conditions.
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7.177.17Chirality Centers Other than CarbonChirality Centers Other than Carbon
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Silicon
Silicon, like carbon, forms four bonds in its stable compounds and many chiral silicon compounds have been resolved.
Si Sid d
ab
c
ab
c
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Nitrogen in Amines
Pyramidal geometry at nitrogen can produce a chiral structure, but enantiomers equilibrate too rapidly to be resolved.
N N: :
ab
c
ab
c
very fast
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Phosphorus in Phosphines
Pyramidal geometry at phosphorus can produce a chiral structure; pyramidal inversion slower than for amines and compounds of the type shown have been resolved.
P P: :
ab
c
ab
c
slow
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Sulfur in Sulfoxides
Pyramidal geometry at sulfur can produce a chiral structure; pyramidal inversion is slow and compounds of the type shown have been resolved.
S S: :
ab
O_
ab
O_
slow
+ +
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End of Chapter 7End of Chapter 7StereochemistryStereochemistry