STEREOCHEMISTRY
By Puan Azduwin Khasri
8th November 2012
ISOMERS
ISOMERS
• Isomers -Compound with the same molecular
formula but different structures
• Constitutional Isomer -differ in the way their
atoms are connected.
• Stereoisomer(Configurational isomer)-differ
in the way their atom are arranged in space
CONSTITUTIONAL ISOMERS
Example:
C2H6O
C4H9Cl
CONFIGURATIONAL ISOMER
Cis-Trans Isomers
• Cis–trans isomers (also called geometric
isomers) result from restricted rotation.
• Restricted rotation can be caused either by a
double bond or by a cyclic structure.
• Cis isomer has the substituent on the same
side of the double bond
• Trans isomer has the substituent on opposite
sides of the double bond.
Cis-Trans Isomers(Example)
Cisisomer
Transisomer
CONFIGURATIONAL ISOMER
An Asymmetric Center
• Also known as a chirality center.
• An atom bonded to four different atoms or groups.
Chiral vs achiral
CHIRAL
• Nonsuperimposable
mirror image.
• Its mirror image is not
the same as itself.
ACHIRAL
• Superimposable mirror
image.
• It looks the same in the
mirror
Enantiomers
YELLOW
GREEN
BLUE
Drawing Enantiomers
Perspective formula
Solid lines – bonds in the plane of the paper
Solid wedge- coming out of the paper toward you
Hatched wedge - going back into space behind the paper
 The solid wedge and hatched wedge must be adjacent
to one another.
Drawing Enantiomers
Fischer projection
• Horizontal line – The bonds project out of the plane toward the
viewer
• Vertical line- The bonds extend back from the plane away from
viewer
• The carbon chain usually drawn vertically
CLASS EXERCISE 1
• Draw enantiomers for each of the following
compounds using perspective formulas and
Fischer projections;
Naming Enantiomers
The R,S system of nomenclature
1.Rank the groups (atoms)
bonded to the asymmetric
center in order to priority
• The higher the atomic no.
the higher the priority
Naming Enantiomers
The R,S system of nomenclature
2.Orient the molecules
• Lowest priority (4) directed
away from you.
3.Draw imaginary arrow from
the group of highest
priority to the next highest
priority
• If the arrow point clockwise
(R-Configuration)
Counterclockwise
(S-Configuration)
Naming from the Perspective
Formula
1. Rank the groups bonded to the asymmetric
center with the lowest priority group in the
back.
2. If the group (or atom) with the lowest priority bonded by a
hatched wedge; Clockwise-R;Anticlockwise-S
3. If the group (or atom) with the lowest priority IS NOT
bonded by a hatched wedge; then switch two groups so
group 4 is bonded by a hatched wedge
Configuration from the Fischer Projection
1. Consider the 3-dimensional equivalent to the Fischer
Projection.
2. Rank the groups (or atoms) that are bonded to the asymmetric
center and then draw an arrow from the highest priority (1) to the
next highest priority (2).
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3. If the lowest priority is on a vertical bond; clockwise (RConfiguration),Counterclockwise (S-Configuration)
4. If the lowest priority is on a horizontal bond, the configuration is
opposite to the direction of the arrow.
ISOMERS WITH MORE THAN ONE
ASYMMETRIC CENTER
Max no stereoisomer = 2n
n= no. of asymmetric center
Diastereoisomers
Stereoisomers (1) and (2)
ENANTIOMERS
Stereoisomers (3) and (4)
Stereoisomers (1) and (3),(1) and (4),(2) and
(3), (2) and (4)
DIASTEREOISOMERS
Diastereoisomers are stereoisomers that are not
enantiomers
Identification of Asymmetric Carbons
in Cyclic Compounds
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Meso Compounds
Have two or more asymmetric centers and an
internal plane of symmetry (-plane)
Meso compounds are achiral molecules with asymmetric
centers
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If a compound with two asymmetric centers has the
same
four groups bonded to each of the centers, one of its
stereoisomers will be a meso compound
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THE END
THANK YOU