Title: Lesson 9 Geometrical Isomerism
Learning Objectives:
– Understand the term stereoisomerism
– Understand and identify geometrical isomerism
– Understand the chemical significance of cis-trans isomerism
Molecular Modelling

Make a molecule of but-2-ene

Compare your model with your neighbours
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Stereoisomerism

Stereoisomers are compounds with the same structural formula but
different 3D arrangement of atoms

There are two types of stereoisomerism:


Geometrical isomerism
Optical isomerism
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Geometrical (cis-trans isomerism)
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Geometric isomerism involves the arrangement of groups around a double bond
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Or a single bond that can’t rotate freely such as in a cyclic compound
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The substituted groups are fixed in space relative to each other.

The position of the groups must be described with respect to a reference plane.
The blue lines show the
plane of reference for
aliphatic and cyclic
molecules…

It happens because double bonds are not free to rotate. (Free rotation would
allow the sideways overlapping p orbitals to move out of position, breaking the pi
bond)

Each C=C carbon must have two different groups attached to it.
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cis-trans isomers

When a molecule contains two or more different groups attached to the
double bond or ring, they can be arranged to give two different isomers.
• In the trans isomer, the substituents (the –CH3
groups) are on opposite sides of the double bond
• In the cis isomer, the substituents are on the same
side of the double bond
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Cis–trans isomerism
If an alkyl group or atom other than hydrogen is attached
to each carbon then the isomers can be named either
cis (‘on the same side’) or trans (‘on the opposite side’).
cis-but-2-ene
cis-1,2-dichloroethene
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trans-but-2-ene
trans-1,2-dichloroethene
© Boardworks Ltd 2009
cis or trans?
Note:You
write the cis
or trans as a
prefix in
italics…
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Limitations of cis–trans isomerism
In more complex organic compounds, in which multiple
hydrogens have been substituted by different groups,
isomers cannot be defined using the cis–trans notation.
For example, is it possible to identify which of these
halogenoalkanes is the cis isomer and which is the
trans isomer?
Instead, a different system is used for these type of
molecules: E–Z notation.
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© Boardworks Ltd 2009
E–Z isomerism
The E–Z notation is used to identify stereoisomers that
cannot be called cis or trans.
Isomers are identified as either E or Z depending on what
‘priority’ is given to the groups attached to the carbon
atoms in the double bond. The priority of these groups is
determined by a complex series of rules.
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
E represents the German word ‘entgegen’, and
corresponds to trans isomers. The highest priority
groups are on the opposite side of the double bond.

Z represents the German word ‘zusammen’, and
corresponds to cis isomers. The highest priority
groups are on the same side of the double bond.
© Boardworks Ltd 2009
E/Z isomerism
So, in this example:
Left side = Br (highest priority)
Right side = C2H5 (highest priority)
This makes a Z isomer…
Rules of priority
 Rule 1: Look at the atom bonded to carbon of the double bond. The atom with
the higher atomic number gets priority.
 Rule 2: If the atoms are the same, apply the rule to the next bonded atom. This
means longer hydrocarbon chains have higher priority…
Next, compare the positions of the highest priority groups on the two carbons of
the double bond…
 If the two highest priority groups are on the same side = ‘Z’ isomer
 If the two highest priority groups are on the opposite side = ‘E’ isomer
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Physical Properties

Cis and trans and E/Z isomers may have different physical properties depending on
the influence of the substituent groups on polarity and shape and symmetry of the
molecules.

BP, MP and solubility data are reported separately for each isomer…
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To do (individually, not pairs):

Build, draw and label the cis and trans isomers of:



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Pent-2-ene
2,3-dichloro-but-2-ene
1,2-dichlorocyclopropane
1,2-dichlorocyclobutane

Research the melting/boiling points of cis/trans but-2-ene, hex-3-ene and oct-4-ene.
What pattern do you notice between the cis and trans isomers? Why do you think
this happens?

Draw the cis and trans isomers of 1,2-dichloroethene. Predict, with a reason, which
you think will have the higher boiling points. Research online to determine whether
you were right and think of a reason if you were wrong.
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Chemical properties of geometric isomers

Can be different

For example:
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Why do you
think this
happens?

Hint: think about
the previous
couple of lessons
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Research

Research online the importance of geometrical isomerism

Find three examples of commercial compounds (medicines, plastics etc) where
geometrical isomerism plays a key role in their properties
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Key Points
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Cis isomers: the substituents are on the same side
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Trans isomers: the substituents are on opposite sides
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Cis alkenes have lower mp/bp than trans due to ‘rounder’ shape

Cis halogenoalkenes have higher mp/bp due to polarity

Chemical properties can be different where cis brings groups close enough to
react
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