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Isomers
Larry J Scheffler
Lincoln High School
IB Chemistry 3-4
1
Structural Isomers

Structural isomers have the same
molecular formulas but they differ in
their structural formulas.
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Stereoisomers
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Stereo isomers have the same
structural formulas but they differ in
their spatial arrangements.
There are two types of
stereoisomerism
1.
2.
Geometrical isomerism
Optical isomerism
3
Geometrical isomers


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Geometrical isomers occur in organic
molecules where rotation around a bond
is restricted
This occurs most often around C=C
The most common cases are around
asymmetric non-cyclic alkenes
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Geometrical isomers


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Carbon to carbon double bonds are made up of a s and a
p bond.
The p bond is formed from the combination of two p
orbitals, one from each of the two carbon atoms
The two p orbitals must be in the same plane to combine
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Geometric Isomers in
alkenes

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A cis isomer is one in which the substituents are
on the same side of the C=C
A trans isomer is one in which the substituents
are on the opposite sides of the C=C
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Geometric Isomers in
Cycloalkanes

Ring structures like C=C restrict
rotation and therefore can result in cis
and trans isomers
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Properties of Geometrical
Isomers
The chemical properties of geometrical isomers tend
to be similar but their physical properties are different
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Properties of Geometrical
Isomers
The trans isomer has a
much higher melting point.
Unlike the cis isomer there
is little intra-molecular
hydrogen bonding
9
Optical Isomers

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Optical isomerism is present in all
compounds that contain at least one
asymmetric (chiral) carbon atom
An asymmetric carbon atom has four
different atoms or groups attached
In this case there are
two different ways to
arrange the four groups
around the chiral
carbon atom (shown in
red)
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Optical Isomers
While these structures may
look identical, in three
dimensions they are mirror
images of each other. Such
molecules are called
enantiomers.
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Distinguishing Enantiomers
Optical isomers can be distinguished by the way
they interact with plane polarized light
Polarized light
Polarized light is light that has been passed through a
polarizing prism or filter. As a result the light vibrates in a
single plane.
Distinguishing Enantiomers
Optical isomers can be distinguished by the way they
interact with plane polarized light. One enantiomer will
rotate polarized light to the right, the other to the left.
Properties of Optical Isomers


Apart from their optical activity enantiomers generally
have similar physical and chemical properties.
The chemical properties may be significantly different
when the enantiomers interact with other optically
active compounds.
Thalidomide has two optical isomers.
One is a tranquilizer, the other is a
Powerful teratogen.