2. Enantiomers
• Enantiomers are nonsuperimposable mirror images.
• Molecules are nonsuperimposable if there is no
orientation in which all atoms of both molecules can be
superimposed.
4. Thalidomide is now being used to treat plasma cell
cancer, leprosy, and has shown anti-HIV activity
5. 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
6. 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
7.
8. 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.
9. 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.
10. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16.
17. 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 varieties—left-
handed crystals and right-handed crystals—that are mirror images of each other
• Pasteur physically separated the two types of crystals using tweezers.
18. Option for Separating Enantiomers
1. Pasteur Method
2. Conversion to Diastereomers
1. Temporarily convert the enantiomers into a pair of diastereomers (will
now have different physical properties).
2. Separate those diastereomers from each other by exploiting their
different physical and chemical properties.
3. Regenerate the enantiomers from the separated diastereomers.
3. Enzymes (version of #2)
4. Chiral Chromatography (version of #2)
Separating (Resolving) Enantiomers
19. enantiomers to be separated
one enantiomer of a chiral amine
diastereomeric salts
one diastereomer
one pure enantiomer
