Chapter 9
INTRODUCTION TO STEREOCHEMISTRY
Isomers are compounds with the same molecular formula but not identical structures
Constitutional isomers are isomers which have the same molecular formula but differ
in the way their atoms are connected
Constitutional Isomers
Constitutional isomers are isomers which have the same molecular formula but differ
in the way their atoms are connected
Drawing Constitutional or Structural Isomers of Alkanes
Example 8.4
Write all the constitutional isomers having the
molecular formula C6H14
start by
connecting
the carbons
in a line
C C C C
CC CC C
C CC
C C
C C
C
C C
C
determine
the C
skeleton of
the other
isomers
C C
C
Example 8.4
Write all the constitutional isomers having the
molecular formula C6H14
H
fill in the H
to give each
C 4 bonds
H
H
H
H
H C
C C
C
C
C H
C C
H
H
H
H
H
C
H C
H C
H
C
C H
C C
H
H
H
H
H
H C
C
C
C H
C C
H
H
H
C H H
C H
H H H H H
H C
H
H H H
H
H
H
H C
H C
H CC C
C
H C
H C
H
H
H
H
H
H
C C
C H
H
H
H
H
H
H
H
C CC C C CC C
C C
H
H
H C H H C H
H
H
H
Example 8.4
Write all the constitutional isomers having the
molecular formula C6H14
convert each
to a carbon
skeleton
formula –
each bend
and the ends
represent C
atoms
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H H H H H
H
C
H
H
H
C
C
C
H H H
H
H
H
H
H
C
C
H
H
C
C
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
C
H
H
H
H
H
H
H
C
C
C
H
C
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
H
H
H C H H C H
H
H
H
Stereoisomers have the same molecular formula, maintain the same connectivity, but
differ in the way their atoms are arranged in space
Conformational isomers (or conformers or rotational isomers or rotamers) are
stereoisomers produced by rotation about single bonds, and are often rapidly
interconverting at room temperature
Conformations of Alkanes and Cycloalkanes
Conformations of Ethane
ethane
Staggered
conformation of
ethane
Newman
Projection
Sawhorse
Representation
Eclipsed
conformation of
ethane
Newman
Projection
Sawhorse
Representation
ethane
Staggered
conformation of
ethane
Newman
Projection
Sawhorse
Representation
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Eclipsed
conformation of
ethane
Newman
Projection
Sawhorse
Representation
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Conformations of Alkanes and Cycloalkanes
Conformations of Butane
butane
Staggered
conformation of
butane
Eclipsed
conformation of
butane
Conformations of Alkanes and Cycloalkanes
Staggered
conformation of
butane
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Eclipsed
conformation of
butane
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Configurational Isomers are stereoisomers that do not readily interconvert at room
temperature and can (in principle at least) be separated.
Geometric isomers are configurational isomers that differ in the spatial position
around a bond with restricted rotation (e.g. a double bond):
Geometric (Cis and Trans) Isomers
Geometric (Cis and Trans) Isomers result from restriction rotation
Compounds with double bonds
cis isomer – have same substituents on the same side of the double bond (= Z
with more complex molecules having high priority groups on the same side)
trans isomer – have the same substituents on the opposite side of the double
bond (= E with more complex molecules having high priority groups on opposite
sides)
Compounds with bonds in a ring:
cis isomer – have the same substituents on the same side of the ring
trans isomer - have the same substituents on the opposite side of the ring
Cis-trans (Geometric) isomerism in Alkenes
cis-1,2-dichloroethene
trans-1,2-dichloroethene
(Z)-1,2-dichloroethene
(E)-1,2-dichloroethene
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Cis-trans (Geometric) isomerism in Alkenes
*If one of the two carbon atoms of the double bond has two identical substituents,
there are no cis-trans isomers for that molecule
1,1-dichloroethene
Identifying cis and trans isomers of Alkenes
Example 11.3
Two isomers of 2-butene are shown below. Which is the
cis isomer and which is the trans isomer
cis-2-butene
trans-2-butene
Naming cis and trans compounds
Example 11.4
Name the following geometric isomers.
trans-3,4-dichloro-3-heptene
cis-3,4-dimethyl-3-octene
Identifying Geometric Isomers
Example 11.5
Determine whether each of the following molecules can
exist as cis-trans isomers:
(1) 1-pentene
(2) 3-ethyl-3-hexene
(3) 3-methyl-2-pentene
1-pentene
cis-3-methyl-2-pentene
3-ethyl-3-hexene
trans-3-methyl-2-pentene
Cis and Trans Fatty Acids
cis-9-octadecenoic acid
trans-9-octadecenoic acid
Cis and Trans Fatty Acids
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Cis-trans (Geometric) isomerism in Cycloalkanes
Cis-trans isomers are molecules having the same arrangement of atoms but differ in
the spatial orientation of their substituents.
cis-1,2-dichlorocyclohexane
trans-1,2-dichlorocyclohexane
Naming cis-trans Isomers of Substituted Cycloalkanes
Example 10.6
Determine whether the following susbstituted
cycloalkanes are cis or trans isomers.
trans-1,2-dimethylcyclopentane
cis-1,2-dimethylcyclopentane
Conformations of Alkanes and Cycloalkanes
Conformations of Cyclohexane
cyclohexane
Chair
conformation of
cyclohexane
Boat
conformation of
cyclohexane
Chair
conformation of
cyclohexane
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Boat
conformation of
cyclohexane
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Chair-Chair Interconversion
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Optical isomers are configurational isomers that differ in the 3D relationship of the
substituents about one or more atoms.
Diastereomers are optical isomers (stereoisomers) that are not enantiomers.
Enantiomers are optical isomers that are non-superimposable mirror images.
Chirality
Chiral objects are objects with left-handed and right-handed forms
Achiral objects - objects that have superimposable mirror images
Nonsuperimposable mirror images - a mirror image that is not the same as the image
itself - chiral objects have nonsuperimposable mirror images
Assymetric Center
Chirality is not reserved just for objects - molecules can be chiral
Chiral molecules - generally molecules containing an asymmetric center
Asymmetric (chiral) center - tetrahedral atom bonded to four different groups indicated with an asterisk (*)
Chiral Molecules with One Asymmetric Center
Molecules with one chiral center would have 2 enantiomers
*Chiral molecules would have 2n enantiomers (where n is the number of chiral centers)
Chiral vs Achiral
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How to Represent Enantiomers: Perspective Formulas
Perspective Formulas - shows two bonds of the asymmetric center as lines in the plane of
the paper, another bond as a solid wedge protruding forward out of the paper, and the
fourth bond as a hatched wedge extending behind the paper
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Fischer Projections
Fisher Projection - representation of an asymmetric center as the point of intersection of
two perpendicular lines
Horizontal lines represent bonds that project out of the plane of the paper
Vertical lines represent bonds that extend back from the plane of the paper away from the
viewer
Fischer Projections
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Naming Enantiomers -R,S System
STEP 1. Rank the groups/atoms bonded to the asymmetric center in order of priority.
- rank in terms of atomic mass. Higher atomic mass, higher priority.
- let’s have 1-chloro-1-ethanol as an example.
1.
2.
3.
4.
Chlorine = 36 amu
Oxygen = 16 amu
Carbon = 12 amu
Hydrogen = 1 amu
STEP 2. Rotate the molecule so that the lowest priority group is pointing away from the
reader.
Naming Enantiomers -R,S System
Step 3. Trace your finger around the three highest priority groups in order of the
priority.
- If the circle is moving counterclockwise the CIP designation is “S”. If the circle
is moving clockwise the CIP designation is “R”.
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Step 4. Name the enantiomer in terms of its R or S configuration.
- the name for this isomer is (S)-1-chloro-1-ethanol
Naming Enantiomers -R,S System
Assigning Priorities to Groups
1.
2.
3.
4.
Oxygen (from CH2OH) = 16
Carbon (from CH2CH3) = attached to carbon
Carbon (from CH3) = attached to hydrogen
Hydrogen = 1
Naming Enantiomers -R,S System
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