Stereochemistry: Chapter 5

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Stereochemistry Chapter 5

• The Arrangement of Atoms in Space The
  Stereochemistry of Addition Reactions

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Contents of Chapter 5

• Conformational and Configurational Isomers
• Chirality Centers, Enantiomers
• Optical Rotation, Optical Purity
• Isomers with More Than One Chirality Center
• Separation of Enantiomers
• Reactions of Chiral Compounds
• Absolute Configurations
• 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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Fischer Tricks

• If low priority group on vertical line (into
  plane) draw normal rotating arrow.
• If low priority group on horizontal line draw
  rotating arrow and reverse the R or S result.
• Rule of thumb If group 4 is Vertical, Very true.
  If group 4 is Horizontal, Horribly wrong.

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Fischer Tricks

• 90º rotation reverses all chiralities.
• 180º rotation maintains all chiralities.

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Multi-Carbon Fischer Trick

• Can rotate 3 groups either CW or CCW without
  changing configuration
• If 3 grps have same rel CW or CCW sequence Cs
  same

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Newman Projections

• For front C swap low priority group to back C
• If swap was necessary curved arrow now gives
  reversed chirality otherwise correct

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Newman Projections

• For back C swap low priority group to front C
• If swap was necessary curved arrow now gives
  correct chirality otherwise reversed

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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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Topology of Stereoisomers With Two Chiral Centers
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R,S System for Isomers with More Than One
Chirality Center

• Compound is (2S,3R)-3-bromo-2-butanol

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R,S System for isomers with more than one
Chirality Center

• Compound is (2S,3R)-3-bromo-2-butanol

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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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Enantiotopic, Diastereotopic, and Homotopic
Hydrogens
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Enantiotopic, Diastereotopic, and Homotopic
Hydrogens
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Regioselective, Stereoselective, and
Stereospecific Reactions

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

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Regioselective, Stereoselective, and
Stereospecific Reactions

• A stereoselective reaction can produce multiple
  stereoisomers theoretically, but more of some
  produced than others.

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Regioselective, Stereoselective, and
Stereospecific Reactions

• A stereospecific reaction produces different
  stereoisomer products from different stereoisomer
  reactants.

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Syn Addition

• When the two substituents add to the same side

• Addition of H2 is a syn addition

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Anti Addition

• When the two substituents add to opposite sides

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Stereochemistry of Addition
CIS-SYN-ERYTHRO RULE

• Unmodified only works for addition of X2
• For addition of XY use to calculate X eclipses Y
  or X opposite to Y in Fischer projection.

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Stereochemistry of Addition
Cis-Syn-Erythro Example
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Stereochemistry of Addition
Pro-Fischer Analysis
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Stereochemistry of Addition
Pro-Fischer Analysis Example