Stereochemistry

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Stereochemistry
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Review of Stereoisomers
cis-2-butene
trans-2-butene
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cis-1,2-difluorocyclohexane
trans-1,2-difluorocyclohexane
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What are the following?
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After rotating the one on the right.
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You can see that the two structures do not
overlap...
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The two structures are enantiomers of each other

• Non-superimposable mirror images.

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Why do enantiomers exist?

• Linear
• trigonal planar
• tetrahedral centers

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Enantiomers exist when two structures have the
maximum number of differing groups attached to
the center atom (I.e. carbon has 4 different
groups).Opposite enantiomers have the groups
attached different ways.Each is a mirror image
of the other, and neither are superimposable on
each other.
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Definitions

• Chiral Carbon a carbon with four different
  groups attached to it.
• Chiral object an object or compound that
  contains at least one chiral center

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How can you tell the difference?

• Using the Cahn-Ingold-Prelog rules, the groups
  attached to the center carbon are ranked.
• These groups are determined to be attached in a
  clockwise fashion. R or a
  counterclockwise fashion. S

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Dealing with Stereochemistry

• How to assign R and S configurations from a
  structure.
• How to assign R and S configurations from a
  Fisher Projection.
• Having Greater than One Chiral Center
• Diastereomers
• Meso Compounds

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Bromine red Chlorine blue Fluorine green Hydrogen
white
Br
Rotate the molecule so that the lowest group is
in the back...

Cl
F
H
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Br
F
Cl

Connecting bromine to chlorine to fluorine takes
you counterclockwise, so this structure would be
S
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Using Spartan to view enatiomers...

• Click here to go to spartan.

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Drawings
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Fisher Projections

• Originally designed to help aid in the
  determination of sugar molecules
• It flattens out the chiral carbon (so you dont
  have to use the dimensional analysis).

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Br
Rotate the molecule so that the horizontal
groups are coming out toward you like a hug...
F
Cl
H

The Fisher Projection from the view shown...
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Fisher Projections
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Are these Fisher Projections of the same compound?

• Determine the Configuration of each one
• If one is R and one is S they are enantiomers.
• If both are the same, they are the same
  compound.

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With more than one stereocenter, there are other
relationships

• Enantiomers are non-superimposible mirror images
  of each other. No matter how many stereogenic
  centers (chiral carbons), all are opposite in
  enantiomers.
• Diasteriomers are stereoisomers that are not
  mirror images of each other. (They have opposite
  configurations at some centers, but have the same
  configurations at others).

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Examples

• Easy to tell....

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Meso Compounds...

• ------------------------------------------
• Contain a Plane of Symmetry through the middle of
  the compound

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Enantiomers or Diasteriomers?
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Group Problem Session
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Newman Projections view a carbon-carbon bond
from the side. The carbon in the background
winds up straight behind the carbon in the
foreground.
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Geometry of Propane
Best Geometry is Staggered