Stereochemistry: Chapter 6

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Stereochemistry Chapter 6

• The Arrangement of Atoms in Space

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Contents of Chapter 5

• Conformational and Configurational Isomers
• Chirality Centers, Enantiomers
• Optical Rotation
• Isomers with More Than One Chirality Center
• Separation of Enantiomers
• Stereochemistry of Reactions

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Kinds of Isomers
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Conformational Isomers
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Configurational Isomers Cis-Trans Diastereomers
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Isomers with One Chirality Center
A chirality center arises when four different
substituents are bonded to a carbon
Only two isomers are possible, an R isomer and an
S isomer.
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Chirality

• R and S enantiomers are mirror images of each
  other, just as your right hand is the mirror
  image of your left hand

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Chirality

• Any object that has a plane or point of symmetry
  is achiral (not chiral).

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Examples
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Mirror Trick
Whenever two structures can be positioned around
a symmetry plane if they arent identical theyre
enantiomers.
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Drawing Enantiomers
Fischer Projections
N and S are down into plane, E and W are up out
of plane of page or screen.
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Naming Enantiomers The R,S System of Nomenclature

• Rank groups by atomic number of the atom bonded
  to the chirality center. Use the same system
  that was used for the E and Z isomers of alkenes

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Naming Enantiomers The R,S System of Nomenclature

• Orient molecule so that group (or atom) of lowest
  priority is directed into plane.
• 3. Draw a curve from group of highest priority
  through the group of second priority to group of
  third priority

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Naming Enantiomers The R,S System of Nomenclature

• R (Latin rectus) right turn
• S (Latin sinister) left turn

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Swap Trick

• If for any reason you ever wish a group were in a
  different position simply swap it with another
  group.
• Swap simply reverses the chirality.

R for switched compound implies S for actual
compound
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Low Priority Group Up Trick

• If the low priority group in figure points up
  rather than down simply draw circular arrow and
  reverse chirality.

• R for reversed chirality implies S for actual
  chirality

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Rotation Trick

• If one group is in the same position in two
  different structures and the other three groups
  are ALL in different positions then the
  structures are identical.

• These 3 structures interconverted by rotation of
  bottom 3 groups around vertical bond attached to
  top group

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Compounds with More Than One Chirality Center

• Two Pairs of Similar Groups

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Compounds with More Than One Chirality Center

• Three Pairs of Similar Groups

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Meso Compounds
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Drill on Meso CompoundsProblem 27

• Does the following compound have a stereoisomer
  that is a meso compound?

Plane of symmetry
Meso form possible
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Relative and Absolute Configurations

• () amphetamine is known to have the R-
  configuration
• Therefore the () form has S configuration

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Separation of Enantiomers

• Racemic lactic acid can be reacted with a
  naturally occurring chiral base, such as morphine
  or strychnine
• The product is a pair of diastereomers
• Properties of diastereomers are sufficiently
  different from each other to allow separation
• After separation, each diastereomer can be
  reacted with hydrochloric acid, yielding an
  optically pure R or S acid

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Reaction Stereochemistry

• A regioselective reaction is one in which
  multiple constitutional isomers possible, but
  more of some formed than others.

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Syn Addition

• When the two substituents add to the same side

• Addition of H2 is a syn addition