2.-Stereoisomerism

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Quick Quiz
1.
Why do alkanes not show stereoisomerism?
– Sigma bonds are able to rotate.
2.
What is stereoisomerism?
– Same structural formula but a different arrangement in space.
3.
Why are pi bonds important for E/Z isomerism?
– They restrict rotation.
4.
If a stereoisomer has two identical groups on the same plane of the molecule, is
this an E or Z isomer?
– Z isomer
Stereoisomerism
4.1.3.2
Objective:
Identify stereoisomerism in compounds.
Outcomes:
Must: Define the term stereoisomerism.
Should: Describe how stereoisomerism occurs.
Could: Explain if a structure is showing E or Z isomerism.
Specification Links:
Module: 4. Core Organic Chemistry.
Unit:
1. Basic Concepts & Hydrocarbons.
Sub-Unit: 3. Alkenes.
Specification Points Covered
4.1.3 Properties of Alkenes
c. i. Explain the terms:
– Stereoisomers (compounds with the same structural formula but with a
different arrangement in space).
– E/Z isomerism (an example of stereoisomerism, in terms of restricted rotation
about a double bond and the requirement for two different groups to be
attached to each carbon of the C=C group).
d. Determine possible E/Z stereoisomers of an organic molecule, given
its structural formula.
Stereoisomerism
Stereoisomerism occurs in compounds with the same
structural formula but a different arrangement in space.
The π-bond in alkenes restricts the rotation of the molecule
potentially causing E/Z isomerism to occur.
•
A form of stereoisomerism.
σ-bonds can rotate so stereoisomerism can’t occur in alkanes.
E/Z Isomerism
E/Z isomerism can only occur if:
•
•
There is a C=C double bond.
Each carbon is bonded to a different atom/group.
The two forms of isomers are referred to as:
•
E (entgegen) – atoms/groups on the opposite sides.
•
Z (zusammen) – atoms/groups on the same side.
Stereoisomers
Compounds with the same structural formula but with a
different arrangement of atoms in space
E/Z Isomerism
A type of stereoisomerism resulting from a C=C group where
each carbon atom has two different groups attached to it.
E isomer= two highest priority groups on opposite sides of
double bond
Z isomer= two highest priority groups on the same side of the
double bond
Cause of E/Z Isomerism
The lack of rotation around the c=c bond
Identifying E/Z Isomerism
Which of the following have E/Z isomerism. Draw both forms.
H
a. H
a. Propene
b.
C
C H
b.But-1-ene
H
H
H
H
H
H C
C
C
C
C
H
c. 2-methylprop-1-ene
H
H
H
H
H C
C
d.Hex-3-ene
H
H
H
H
c. H C
H
e. 4-metylpent-2-ene
H
H
d.
C
C H
C
C
H
H
H
f. Pent-2-ene
H
C
C
C
H
H C
C
H
H
g. Hept-3-ene
H C
H
H
H
H
h.2,3-dimethylpent-2-ene
e. HH C H H
f. H
H
C
i. 2,3-dimethylcyclopent-1,3,-diene H C C C
H
C
H
H
H
C
C H
j. 3-ethylhept-2-ene
H
H
C
H
C
H
H
H
C
H
H
H
C
H
H
H
H
C
C
H
H
Answers
The following show E/Z isomerism:
– Hex-3-ene
– 4-metylpent-2-ene
– Pent-2-ene
– Hept-3-ene
– 3-ethylhept-2-ene
H
C
H
H
C
C
H
H
H
H
C
C
C
C
H
H
H
H
H
H
H
C
H
H
C
H
H
C
H
C
H
C
H
H
H
C
C
H
H
H
Two Types of Bond
1) Sigma Bonds (σ-bonds)
A sigma bond is formed by the overlap of two s or p orbitals
as shown:
2) Pi Bonds (π-bonds)
A pi bond is formed by the ‘sideways’ overlap of two p orbitals as shown:
1.
Butane, C4H10, reacts with chlorine to produce a chloroalkane with molecular formula C4H9Cl.
The reaction is initiated by the formation of chlorine radicals from chlorine.
(i)
What is meant by the term radical?
.........................................................................................................................
[1]
(ii)
State the conditions necessary to bring about the formation of the chlorine free radicals from Cl2.
.........................................................................................................................
[1]
(iii)
State the type of bond fission involved in the formation of the chlorine radicals.
.........................................................................................................................
[1]
(iv)
The chlorine radicals react with butane in several steps to produce C4H9Cl.
Write equations for the two propagation steps.
.........................................................................................................................
.........................................................................................................................
[2]
[Total 5 marks]
1.
(i) species with an unpaired electron (1)
1
(ii) uv (light)/high temperature/min of 400° C/sunlight (1)
1
(iii) homolytic (fission) (1)
1
(iv) C4H10 + Cl• (1)  C4H9• + HCl (1)
C4H9• + Cl2 (1)  C4H9Cl + Cl• (1)
2
[5]
Stereoisomerism
Geometric isomerism (E/Z isomerism): This
occurs for compounds where the bond
cannot be rotated fully... Alkenes!
The double bond in an Alkene is locked in a
position, so these two molecules are not the
same thing
Stereoisomerism
Geometric isomerism (E/Z isomerism):
If the two highest priority substituents are
on the same side, then it is the Z- isomer
Z-But-2-ene
• The Z comes from the German word
‘Zusammen’ which means together
• The old naming rules used cis-
Stereoisomerism
Geometric isomerism (E/Z isomerism):
If the two highest priority substituents are
on the opposite side, then it is the e- isomer
E-But-2-ene
• The E comes from the German word
‘Entgegen’ which means opposite
• The old naming rules used trans-
Same side = Z
Opposite = E
Stereoisomerism
Geometric isomerism (E/Z isomerism):
There cannot be E/Z isomerism if the two
substituents on one of the carbons is the
same
e.g.
 2x H’s
Cahn – Ingold-Prelog
If four groups around C=C we
have to prioritise the functional
groups
1. Assign priority to those closest
to C=C
2. The highest atomic number on
each C is given 1st priority
3. If this is the same, look at the
next and work out the priority
there
4. Finalise the arrangement
a. Opposite = E
b. Same side = Z
1-bromo-2-fluoro-ethene
Carbon 1:
Carbon 2:
Carbon 1 highest AN
Carbon 2 highest AN
Are the priority groups on
the same side or
opposite?
E
1-bromo-1-chloro-2methylbut-1-ene
Z
Stereoisomerism
Example: Cahn-Ingold-Prelog priority rules
1. Chlorine vs Carbon – Chlorine wins
On the opposite side, so it’s the E- isomer
Stereoisomerism
Example: Cahn-Ingold-Prelog priority rules
1. Both substituents are carbon atoms
Move down the chain
2. Hydrogen vs Carbon – ethyl chain wins
E isomer
Environmental Concerns
Disposal of Waste Polymers
Landfill
Using polymer waste for fuel
Feedstock recycling
– PVC – always been difficult
Biodegradable and Photodegradable Polymers
Biodegradable polymers
Made from natural materials
Reduce reliance on crude oil
Naturally decompose
Photodegradable Polymers
Designed to contain bonds that are weakened by
absorbing light (producing reactive radicals!)
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