Ch 5 Lect' 2 Fischer Projections

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Ch 5 Lect. 2 Fischer Projections

• Simple Fischer Projections
• Fischer Projections are a method to draw 3D
  molecules in 2D
• There are multiple Fischer Projections for each
  molecule
• Rotating Fischer Projections may change the
  Stereochemistry (R/S)
• 90 degree rotation interconverts enantiomers
• 180 degree rotation retains enantiomer

S
S
S
S
S
R
S
R
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• Switching Substituents on a Fischer Projection
• Any single (odd ) exchange of 2 substituents
  gives the other enantiomer
• Any two (even ) of exchanges gives the same
  enantiomer
• Absolute Configuration from Fischer Projection
• For Dashed-Wedged line drawing you must visualize
  in 3D
• Fisher Projections becomes a 2D visualization
  (less error)
• Rules
• Draw any Fischer Projection of any chiral
  molecule
• Rank substituents as in R/S nomenclature
• Exchange once to put lowest priority substituent
  at the top
• Exchange any 2 substituents to retain original
  stereochemistry
• Clockwise a-b-c R Counterclockwise a-b-c S

R
S
R
S
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• Fischer Projections with Multiple Stereocenters
• 2-Bromo-3-chlorobutane (Figure 5-7)
• RR, RS, SR, and SS possibilities
• Fischer Projection has all substituents eclipsed,
  so to draw the Fischer from Dashed-Wedged or
  Newman, make eclipsed first
• To assign R/S to each stereocenter, treat just as
  a substituted methane

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• Relationships of 2-stereocenter Stereoisomers
• RR and SS are non-superimposable mirror images
  Enantiomers
• RS and SR are non-superimposable mirror images
  Enantiomers
• RR-RS, RR-SR, SS-RS, and SS-SR arent mirror
  images Diastereomers
• Two stereocenters with identical substitution
  2,3-dibromobutane
• RR/SS Enantiomeric pair
• RS/SR are mirror images, superimposable
  Identical Achiral
• Meso multiple stereocenters, superimposable
  with mirror image
• Have a mirror plane
• Look at eclipsed conformation

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• cis/trans Isomers
• Just like butane, Cyclobutane has RR, SS, RS, SR
  configurations
• RR, SS trans RS, SR cis
• cis and trans isomers are Diastereomers (for all
  1,2-disub-cycloalkanes)
• More than 2 stereocenters
• For n stereocenters, 2n stereoisomers possible
  (maybe less--meso)
• n 3 RRR RRS RSS SRS
• SSS SSR SRR RSR

Enantiomeric Pairs
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• Stereochemistry in Reactions
• Bromination of Butane
• Optically Inactive Reactants give
• Optically Inactive Products
• Achiral to Chiral gives Racemate
• Chlorination of (S)-2-bromobutane
• Chlorination at C1/ C4 give Achiral
• Optically Active?Optically Active
• Original Stereocenter untouched

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S
R
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• Chlorination at C2 (the stereocenter) gives a
  racemate
• Chlorination at C3
• Form another C, 4 diastereomers
• Transition state is chiral
• Stereoselective when a reaction
• gives major product when several
• stereoisomers are possible
• Sterics of TS control favored prod
• Hard to control in Organic Chem
• because always get some minors
• f) Enzymes do this very well

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S
R
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• Resolution of Enantiomers
• Separate the members of a racemate to get pure
  enantiomers
• Amino acids, carbohydrates, nucleic acids often
  are single enantiomers
• Drug molecules often only work in one
  enantiomeric form
• Methods
• Crystallization and manual separation (LUCK!)
• Chemical Separation enantiomers have same
  physical properties
• React Enantiomers to make them Diastereomers
  (different properties)
• Do chemical separation crystallization,
  distillation, etc
• React to get pure enantiomers back

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Resolution of Enantiomers