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Stereochemistry
Required background:
Shapes of molecular structures
Isomers, conformers
Essential for:
1. Understanding of SN1, SN2, E1, E2, and other reaction mechanisms
2. Stereochemistry of carbohydrates, proteins, and nucleic acids
Outline
1. Chirality, optical activity
2. Fisher projections. R-,S-nomenclature
3. Stereoisomers
1. Chirality, optical activity
2. Fisher projections. R-,S-nomenclature
3. Stereoisomers
Chirality – is a property of an object to be different from its mirror
image. In other words, the object and its mirror image are noncongruent.
Example: A human hand is chiral, since the right hand and the left hand
are different.
Enantiomers – are noncongruent mirror images of molecules
Chiral objects can not contain certain elements of symmetry,
for instance, plane of symmetry (most common), center of symmetry
or some other elements (rare cases beyond our course).
Optical activity – is the ability of chiral compounds to rotate the
plane of polarization of light.
1. Chirality, optical activity
2. Fisher projections. R-,S-nomenclature
3. Stereoisomers
Which structural elements bring chirality to a molecule?
a. Chiral center (sp3 – carbon with four different substituents)
b. Other elements (beyond our course)
The language for stereochemistry of molecules with
chiral centers – Fisher Projections
Rules of handling Fisher Projections
a. Rotation by 180o in the plane of paper gives the identical compound
H
OH
HOOC
CH3
H3C
COOH
OH
H
(R)-
(R)-
b. Rotation by 90o in the plane of paper gives the opposite enantiomer
H
HOOC
COOH
CH3
HO
H
OH
CH3
(R)-
(S)-
Rules of handling Fisher Projections
c. Switching of any two substituents gives the opposite enantiomer
H
HOOC
H
CH3
H3C
COOH
OH
OH
(R)-
(S)-
d. Rotation of any three substituents gives the identical compound
H
HOOC
H
CH3
HO
COOH
OH
CH3
(R)-
(R)-
1. Chirality, optical activity
2. Fisher projections. R-,S-nomenclature
3. Stereoisomers
Isomers
Constitutional isomers
(different connectivity)
Enantiomers
(mirror images)
Stereoisomers
(same connectivity)
Diastereomers
(not mirror images)
Enantiomers have exactly same physical and chemical properties,
except the sign of optical rotation and behavior toward chiral compounds.
Why? Let’s consider compounds with two chiral centers
Stereoisomers of tartaric acid
COOH
COOH
(R)H
H
(S)HO
OH
(S)-
HO
OH
COOH
(R)H
(R)-
H
COOH
o
COOH
H
HO
(S)OH
(R)-
H
COOH
180 rotation
Epimers.
Configuration of only one center
is opposite.
meso - Tartaric acid
COOH
HO
H
H
(S)-
OH
COOH
Enantiomers.
Configurations of all centers
are opposite.
(+)- Tartaric acid
(-)- Tartaric acid