Types of Isomers

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Types of Isomers
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Enantiomers

• Enantiomers are nonsuperimposable mirror images.
• Molecules are nonsuperimposable if there is no
  orientation in which all atoms of both molecules
  can be superimposed.

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Ibuprofen
R form non-active side effects
S form active anti-inflammatory
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Thalidomide is now being used to treat plasma
cell cancer, leprosy, and has shown anti-HIV
activity
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Chirality

• A molecule is chiral if it has an enantiomer.
• Does NOT have mirror symmetry
• A molecule is achiral if it does not have an
  enantiomer.
• DOES have mirror symmetry

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Terminology in Stereochemistry

• Stereocenter
• An atom about which exchange of 2 groups produces
  a stereoisomer
• Chiral Center
• Type of Stereocenter
• A tetrahedral atom that is bonded to four
  different groups

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Fischer Projections

• The Fischer projection is a convenient way to
  depict complex molecules having more than one
  stereocenter.
• The intersection of a horizontal line and a
  vertical line indicates a carbon stereocenter.
• The substituents on the horizontal bonds are
  understood to point toward you (like a bowtie),
  whereas the substituents on the vertical bonds
  are understood to point away from you.

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Chirality and Conformational Isomers

• Rotation about single bonds can determine whether
  a molecule is chiral or achiral.
• If a molecule and its mirror image are rapidly
  interconverting conformational isomers, then the
  molecule is effectively achiral.
• The mirror image of 1,2-dibromoethane in one of
  its gauche conformations is its second gauche
  conformation.

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Haworth Projections

• Haworth projections are helpful in determining
  the chirality of all cyclics.
• Taking the chair conformation of cyclohexane to a
  flat hexagon allows us to quickly and accurately
  determine whether it is chiral or achiral.

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Frozen Conformational Isomers
biphenyls
has been resolved half-life for racemization is
78 min at 118oC
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Physical and Chemical Properties of Isomers

• Constitutional Isomers
• Due to different connectivities, these isomers
  must have different physical and chemical
  properties.
• Enantiomers
• Have the same connectivities and precisely the
  same polarities.
• Diastereomers
• Have the same connectivities but different 3-D
  arrangement, so they have different properties.

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Optical Activity

• Chiral molecules interact with plane-polarized
  light.
• When all photons from a light source have their
  electric fields oscillating in the same plane,
  then the light is plane polarized.

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The Polarizer

• Most light sources emit light that is
  unpolarized.
• A polarizer generates plane-polarized light by
  filtering out light whose electric field
  oscillates in any other plane.
• If plane-polarized light passes through a sample
  of a compound, the plane in which the light is
  polarized can change, depending upon whether the
  compound is chiral or achiral.

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Enantiomers rotate Plane Polarized Light

• One enantiomer rotates polarized light in one
  direction while the other enantiomer rotates it
  in the opposite direction.
• Enantiomers have identical physical and chemical
  properties except the direction at which they
  rotate polarized light.

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Louis Pasteur

• Louis Pasteur was the first to isolate a pair of
  enantiomers from each other.
• Pasteur noted that the crystals appeared to grow
  in one of two varietiesleft-handed crystals and
  right-handed crystalsthat are mirror images of
  each other
• Pasteur physically separated the two types of
  crystals using tweezers.

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Separating (Resolving) Enantiomers

• Option for Separating Enantiomers
• Pasteur Method
• Conversion to Diastereomers
• Temporarily convert the enantiomers into a pair
  of diastereomers (will now have different
  physical properties).
• Separate those diastereomers from each other by
  exploiting their different physical and chemical
  properties.
• Regenerate the enantiomers from the separated
  diastereomers.
• Enzymes (version of 2)
• Chiral Chromatography (version of 2)

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enantiomers to be separated
one enantiomer of a chiral amine
diastereomeric salts
one diastereomer
one pure enantiomer