ESTEREOQUÍMICA
Isomers
stereoisomersconstitutionalisomers
diastereomersenantiomers
Molecular Chirality: Enantiomers
A molecule is chiral if its two mirror image forms are not superposable upon one another.
A molecule is achiral if its two mirror image forms are superposable.
Chirality
BrCl
H
F
Bromochlorofluoromethane is chiral
It cannot be superimposed point for point on its mirror image.
BrCl
H
F
Bromochlorofluoromethane is chiral
H
ClBr
FTo show nonsuperimposability, rotate this model 180° around a vertical axis.
BrCl
H
F
Bromochlorofluoromethane is chiral
H
ClBr
F
Another look
are enantiomers with respect to each other
and
nonsuperimposable mirror images are called enantiomers
Enantiomers
Chlorodifluoromethaneis achiral
Chlorodifluoromethaneis achiral
The two structures are mirror images, but are not enantiomers, because they can be superimposed on each other.
The Chirality Center
a carbon atom with fourdifferent groups attached to it
also called:chiral centerasymmetric centerstereocenterstereogenic center
The Chirality Center
w
x y
z
C
A molecule with a single chirality center is chiral.
Bromochlorofluoromethane is an example.
Chirality and chirality centers
Cl F
Br
H
C
A molecule with a single chirality center is chiral.
2-Butanol is another example.
Chirality and chirality centers
CH3
OH
H
C CH2CH3
Examples of molecules with 1 chirality center
CH3
C
CH2CH3
CH2CH2CH2CH3CH3CH2CH2
a chiral alkane
Examples of molecules with 1 chirality center
Linalool, a naturally occurring chiral alcohol
OH
Examples of molecules with 1 chirality center
1,2-Epoxypropane: a chirality centercan be part of a ring
O
H2C CHCH3
attached to the chirality center are:
—H
—CH3
—OCH2
—CH2O
Examples of molecules with 1 chirality center
Limonene: a chirality center can be part of a ring
CH3
H C
CH3
CH2
attached to thechirality center are:
—H
—CH2CH2
—CH2CH=
—C=
Examples of molecules with 1 chirality center
Chiral as a result of isotopic substitution
CH3CD
T
H
A molecule with a single chirality centermust be chiral.
But, a molecule with two or more chirality centers may be chiral
or it may not.
Properties of Chiral Molecules:
Optical Activity
A substance is optically active if it rotates the plane of polarized light.
In order for a substance to exhibit opticalactivity, it must be chiral and one enantiomer must be present in excess of the other.
Optical Activity
Light
has wave properties
periodic increase and decrease in amplitude of wave
Light
optical activity is usually measured using light having a wavelength of 589 nm
this is the wavelength of the yellow light from a sodium lamp and is called the D line of sodium
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
Nicol prism
Polarization of light
Rotation of plane-polarized light
Absoluteand
Relative Configuration
Relative configuration compares the arrangement of atoms in space of one compound with those of another.
Absolute configuration is the precise arrangement of atoms in space.
Configuration
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.
we can now determine the absolute configuration of almost any compound
Configuration
No bonds are made or broken at the chirality center
in this experiment. Therefore, 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
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.
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.
HHO
H OH H2, Pd
HHOH2, Pd
H OH
Absolute configurations
[] +33.2°[] +13.5°
[] –13.5° [] –33.2°
Not all compounds that have the same relative
configuration have the same sign of 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
The Cahn Ingold PrelogR-S Notational System
1. need rules for ranking substituents at chirality center in order of decreasing precedence
2. need convention for orienting molecule so that order of appearance of substituents can be compared with rank
The system that is used was devised by R. S. Cahn, Sir Christopher Ingold, and V. Prelog.
Two requirements for a systemfor specifying absolute configuration
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.
The Cahn-Ingold-Prelog Rules
43
2
1
Example
4 3
2
1
Order of decreasing rank:4 > 3 > 2 > 1
• 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
43
2
1
Example
4 3
2
1
Order of decreasing rank:4 3 2
clockwise
R
counterclockwise
S
C OH
H3C
HCH3CH2
Enantiomers of 2-butanol CHO
CH3
HCH2CH3
(S)-2-Butanol (R)-2-Butanol
Very important! Two different compounds with the same sign of rotation need not have the
same configuration.
Verify this statement by doing Problem. All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?
HH3C
H
H
Chirality center in a ring
R
—CH2C=C > —CH2CH2 > —CH3 > —H
Fischer Projections
Purpose of Fischer projections is to show configuration at chirality center without necessity of drawing wedges and dashes or using models.
Rules for Fischer projections
Arrange the molecule so that horizontal bonds at chirality center point toward you and vertical bonds point away from you.
Br Cl
F
H
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
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
Physical Properties of Enantiomers
Same: melting point, boiling point, density, etc
Different: properties that depend on shape of molecule
(biological-physiological properties) can bedifferent
Physical properties of enantiomers
O O
CH3 CH3
H3C H3CCH2 CH2
Odor (–)-Carvonespearmint oil
(+)-Carvonecaraway seed oil
Ibuprofen is chiral, but normally sold asa racemic mixture. The S enantiomer is the one responsible for its analgesic and antiinflammatory properties.
Chiral drugs
CH2CH(CH3)2
HH3C
C
O
C
HO
Chiral Moleculeswith
Two Chirality Centers
How many stereoisomers when a particular molecule contains two chirality centers?
2,3-Dihydroxybutanoic acid
What are all the possible R and S combinations of the two chirality centers in this molecule?
O
CH3CHCHCOH
HO OH
23
Carbon-2 R R S SCarbon-3 R S R S
2,3-Dihydroxybutanoic acid
4 Combinations = 4 Stereoisomers
O
CH3CHCHCOH
HO OH
23
Carbon-2 R R S SCarbon-3 R S R S
2,3-Dihydroxybutanoic acid
4 Combinations = 4 Stereoisomers
What is the relationship between these stereoisomers?
O
CH3CHCHCOH
HO OH
23
Carbon-2 R R S SCarbon-3 R S R S
2,3-Dihydroxybutanoic acid
O
CH3CHCHCOH
HO OH
23
Carbon-2 R R S SCarbon-3 R S R S
enantiomers: 2R,3R and 2S,3S2R,3S and 2S,3R
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°
2,3-Dihydroxybutanoic acid
O
CH3CHCHCOH
HO OH
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
Isomers
stereoisomersconstitutionalisomers
diastereomersenantiomers
HO
CO2H
CH3
H
OHHR
R
CO2H
CH3
H
HHO
OH
S
S
enantiomersenantiomers
CO2H
H
CH3
HO
HHO
R
S
CO2H
CH3
H OH
OHHR
S
enantiomersenantiomers
diastereomersdiastereomers
[] = -9.5° [] = +9.5°
[] = -17.8°[] = +17.8°
CO2H
CH3
Fischer Projections
recall for Fischer projection: horizontal bonds point toward you; vertical bonds point away
staggered conformation does not have correct orientation of bonds for Fischer projection
Fischer projections
transform molecule to eclipsed conformation in order to construct Fischer projection
Fischer projections
CO2H
CH3
OH
OH
H
H
S SR R
Two chirality centers in a ring
nonsuperposable mirror images; enantiomers
trans-1-Bromo-1-chlorocyclopropane
S RS R
Two chirality centers in a ring
nonsuperposable mirror images; enantiomers
cis-1-Bromo-1-chlorocyclopropane
S RS R
Two chirality centers in a ring
stereoisomers that are notenantiomers; diastereomers
cis-1-Bromo-1-chloro-cyclopropane
trans-1-Bromo-1-chloro-cyclopropane
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