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Organic Chemistry 9
Stereoisomers
IB Topic 20.6
Reference: Higher Level Chemistry, p. 408-415
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Isomers - Review
Isomers are molecules
with the same molecular formula
but different structural formulas.
molecular formula = C4H10
H
butane
2-methylpropane
These isomers are called structural isomers.
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Types of Isomers
isomers structural isomers stereoisomers different order of
attachment of atoms
e.g. linear vs branched molecules
e.g. different position of a double bond
or functional group
C4H8
C4H10
butane
H
2-methylpropane
but-1-ene
but-2-ene
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Types of Isomers
stereoisomers atoms are joined in the same order, but they have a different
arrangement in space (i.e. different “spatial” arrangements)
1. geometric isomers
2. optical isomers
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Geometric Isomers
Geometric isomers form when atoms
are restricted from rotating around a bond.
double C=C bonds
restrict free rotation
ring structures
restrict free rotation
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Geometric Isomers
Geometric isomers are also known
as cis-trans isomers.
cis isomer =
substituents on
the same side
trans isomer =
substituents on
opposite sides
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Geometric Isomers
Name these geometric isomers:
example 1
trans-1,2-dichloroethene
cis-1,2-dichloroethene
example 2
cis-but-2-ene
trans-but-2-ene
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Geometric Isomers
Name these geometric isomers of C3H4Cl2:
Note: An alkane ring includes the prefix “cyclo” in its name.
trans-1,2-dichlorocyclopropane
cis-1,2-dichlorocyclopropane
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Geometric Isomers
Draw and name the geometric isomers of CH3CH2CH2CHCHCH3.
TIP: Start by drawing the C=C bond first.
Draw and name one unbranched structural isomer of this compound.
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Geometric Isomers - Physical Properties
1. The boiling point of geometric isomers depends on
the polarity of the molecule.
∂-
∂net dipole
moment
∂+
NO net
dipole
moment
∂+
cis isomers are polar
trans isomers are non-polar
stronger dipole-dipole
intermolecular forces
weaker van der Waals
intermolecular forces
higher BP (60ºC)
lower BP (48ºC)
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Geometric Isomers - Physical Properties
2. The melting point of geometric isomers depends on
the shape of the molecule.
cis isomers
have lower MP
trans isomers
have higher MP
packing of cis isomers is looser
- weaker IMFs
packing of trans isomers is tighter
- stronger IMFs
melting point = 139 ˚C
melting point = 287 ˚C
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Geometric Isomers - Chemical Properties
Cis isomers are more reactive than trans isomers.
Functional groups are close
together in cis isomers and can react.
Functional groups are further
apart in trans isomers and
cannot react easily.
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Optical Isomers
Optical isomers are molecules that are mirror images,
but which cannot be superimposed on each other.
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Optical Isomers - Chiral Carbon Atoms
Optical isomers must have a carbon atom
known as a chiral carbon atom.
A chiral carbon atom is asymmetric, and has
four different atoms or groups bonded to it.
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Optical Isomers - Chiral Carbon Atoms
Chiral carbon atoms do NOT have a plane of symmetry.
non-chiral
carbon atom
chiral carbon
atom
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Optical Isomers - Chiral Carbon Atoms
Examples of Molecules with Chiral Carbon Atoms
glycine - none
alanine - carbon 2
ketose sugar
- carbon 3 and 4
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Optical Isomers - Chiral Carbon Atoms
Examples of Molecules with Chiral Carbon Atoms
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Optical Isomers
Optical
- Chiral
Isomers
Carbon Atoms
Mark the chiral atom in these molecules with an asterisk (*).
no chiral
carbon atom
*
*
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Optical Isomers - Chiral Carbon Atoms
Mark the chiral atom in these molecules with an asterisk (*).
no chiral
carbon atom
no chiral
carbon atom
*
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Optical Isomers - Enantiomers
The two non-superimposable forms of an optical isomer
are called enantiomers.
Note the prefixes used to distinguish between the 2 enantiomers.
(Other prefixes are + or -, and S or R.)
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Optical Isomers - Enantiomers
Enantiomers of lactic acid
Note the different uses of the enantiomers.
This relates to their biological activity.
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OpticalOptical
IsomersIsomers
- Enantiomers
Dash-wedge diagrams are used for 3D structures.
solid lines = in the same plane
dashed lines = behind the plane
wedge lines = in front of the plane
Enantiomers (optical isomers)
can be drawn like this:
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Optical Isomers - Enantiomers
Draw these molecules in 3D and show the enantiomers.
Put the chiral atom at the centre.
butan-2-ol
2-chloropropanoic acid
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Optical Isomers
Optical- Isomers
Racemic Mixtures
A mixture that contains
equal amounts (or equimolar amount)
of the two enantiomers of an optical isomer
is called a racemic mixture.
When an optical isomer is synthesized in the laboratory,
a racemic mixture is produced.
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Optical Isomers - Properties
Optical isomers (the enantiomers) have
identical physical and chemical properties,
with these two important exceptions:
1. optical activity (physical)
2. reactivity with other chiral molecules (chemical)
(e.g. many biological molecules are chiral)
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Optical Isomers - Optical Activity
Optical activity refers to how a substance interacts with light.
First, you need to know a little bit about light and “polarity”
Light from a source
travels as a wave.
The wave can be in an
infinite number of
planes and is called
“non-polarized” light.
The light that passes
through the polaroid is
called “plane polarized
light.
A “polaroid” is a filter
that only allows light
travelling in one plane
to pass through it.
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Optical Isomers - Optical Activity
When plane polarized light passes through an optical isomer, the isomer
interacts with it in a way that “rotates” the plane of the light.
plane polarized light
optical
isomers
light
rotates
The two different enantiomers rotate the light in different directions.
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Optical Isomers - Optical Activity
A polarimeter is an instrument that measures the rotation
of plane polarized light by a substance.
Plane polarized light is
rotated after passing
through a sample of an
optical isomer.
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Optical Isomers - Optical Activity
Observations of different mixtures of an optically active substance
(e.g. 2-aminopropanoic acid) on the analyzer of a polarimeter:
L-2-aminopropanoic acid
D-2-aminopropanoic acid
racemic mixture
plane polarized light
rotated counter-clockwise
plane polarized light
rotated clockwise
no rotation of planepolarized light
equal proportions of the
two isomers cancel out
the rotation
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Optical Isomers - Reactivity
Chemical properties of enantiomers are the same,
EXCEPT when they react with other chiral molecules
(like biological molecules).
This means the biological or biochemical reactivity
of enantiomers are DIFFERENT.
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Optical Isomers - Reactivity
Thalidomide was a drug given to pregnant women
to prevent morning sickness.
Where is the chiral carbon?
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Optical Isomers - Reactivty
One enantiomer of thalidomide is therapeutic,
acting as an anti-nausea agent.
The other enantiomer of thalidomide is teratogenic,
causing limb malformations during fetal development.
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