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Isomerism
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Isomerism• Isomers are compounds that have the same molecular
formula but have different arrangements of atoms in space.
• Isomers have different physical and chemical properties (depending on the type of isomerism).
• E.g. Butane and 2-methyl propane
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Starter: make these molecules in the order given (5 minutes)
alkanes
alkenes
H CH3
| |H – C – C – H | | CH3 H
CH3 CH3
| |H – C – C – H | | H H
H CH3
| |H – C = C – H | | CH3 H
CH3 CH3
| |H – C = C – H | | H H
Do you notice whether it is easier to make 2 after 1 compared with 4 after 3? How is it different? Why is it different?
Challenge name these molecules
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Same molecular formula but different arrangement of their atoms.
CHAIN ISOMERISM
POSITIONAL ISOMERISM
FUNCTIONAL GROUP
ISOMERISM
GEOMETRIC ISOMERISM
OPTICAL ISOMERISM
Atoms are bonded in a
different order.
Atoms are bonded in the same order but arranged differently
in space.
Rotation prevented by double bond.
Non- superimposable mirror images.
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Stereoisomerism(Spatial isomerism)
SAME molecular formula, SAME functional groups, SAME bonds, but DIFFERENT arrangement in space.
(a)This often occurs with a double bond
Geometric Isomerism!(b) Or..(b) Or.. if a central atom is attached to 4 different groupsif a central atom is attached to 4 different groups
Optical Isomerism!
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Geometric isomerism.o This type of isomerism results from restricted
rotation somewhere in the molecule (usually C=C).
o HOWEVER.......
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Pi bonds are formed by a sideways overlap between p- orbitals, and if they are twisted then this
overlap is disrupted and the Pi bond is broken.
Therefore use this as a clue!
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trans-1,2-dichloroethene
cis-1,2-dichloroethene
“on this side”
“across”
Cis and trans isomers may have different properties e.g. In the way they react with biological molecules.
Cis- MP -80ºC, BP 60ºCTrans – MP -50ºC, BP 48ºC
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So what do we call this?
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E-Z isomerism
If 2 groups with the highest priority are on the same side they are a Z isomer.
Z –Zusammen = together E –Entgegen = opposite
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Rules
1. Separate the two sides of the double bond and treat them separately.
2. The atom with the highest atomic number is given the highest priority.
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Examples
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Comparing stereoisomerism and structural isomerism
Bob being a stereoisomer
Bob being a structuralisomer
Normal Bob
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TASK:
Illustrate the structures of the three different isomers of C5H12.
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The Same
or Not the SameSlides 2-17
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• E.g.
Optical isomers• Specific to compounds with four
different groups on the same carbon atom
• called Chiral carbons
• Four groups allows two different arrangements in space
Non-superimposable mirror imagesor
Enantiomers
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Optical isomers
• Chiral cmpds have the same properties as each other.
• Except for
• Optical activity
• Interaction with biological systems; i.e. enzyme action Bulb
Monochromatic light source
Polarising filter
Second ParallelPolarising filter
PlainSample insolution
detector
Plane Polarised Light
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PlainSample insolution
Optical isomers
• Chiral cmpds have the same properties as each other.
• Except for
• Optical activity
• Interaction with biological systems; i.e. enzyme action Bulb
Monochromatic light source
Polarising filter
SecondNon-parallelPolarising filter
detector
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Sample insolution
Optical isomers• The polarising filters are
perpendicular so no light should get through.
• However,
• With the optical isomer in place light is transmitted.
BulbMonochromatic light source
Polarising filter
SecondNon-parallelPolarising filter
detector
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Optical isomers• One enantiomer or optical
isomer rotates the beam of plane polarised light clockwise.
• i.e. it is dextrorotatory
• e.g. d-glucose
• e.g. (+) glucose
• The second polariser must be rotated clockwise to stop light being transmitted again.
BulbMonochromatic light source
Polarising filter
SecondPolarising filter
Sample insolution
detector
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Optical isomers• One enantiomer or optical
isomer rotates the beam of plane polarised light anticlockwise.
• i.e. it is laevorotatory
• e.g. l-glucose
• e.g. (-) glucose
• The second polariser must be rotated anticlockwise to stop light being transmitted again.
BulbMonochromatic light source
Polarising filter
SecondPolarising filter
Sample insolution
detector
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• Optically active substances often occur as a 50/50 mixture of the two enantiomers.
• This is known as a racemic mixture It has no effect on plane polarised light.
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C
H
CH3 C
H
OH OH
CO2H
CO2H
CO2H
OOHCH3 C
Cl
Cl
C
C2H5
CH3
Cl
C
H
CH3 C
F
OH OH
CH3
O
CH3
CO2H
OOH
Cl
C6H5 C
Cl
C6H5
C
C2H5
CH3
Cl
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Optical isomers
• Implications for pharmaceutical compounds.
• Preparation
• Dosage
• Side effects
• e.g lactic acid
• e.g. thalidomide.
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Some compounds exhibiting chirality have enantiomers with very different properties.
Eg. thalidomide
One enantiomer relieved morning sickness for mothers, the other hinder the development of babies and they were born with their limbs not formed properly.