Expt 29'5 Stereochemistry

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Expt 29.5 Stereochemistry
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Administrative Notes

• A Modeling lab
• Nothing required for your notebooks
• Prelab worksheet
• Download from the web
• Worth points
• Datasheet will be handed out in lab
• Due at the end of the lab period

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Isomers Different compounds with the same
molecular formula
Stereoisomers Isomers with the same order of
attachment of atoms, but a different orientation
of their atoms in space
Constitutional Isomers Isomers with a different
order of attachment of their atoms
Diastereomers Stereoisomers whose molecules are
not mirror images
Enantiomers Stereoisomers whose molecules are
nonsuperimposable mirror images
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Chirality

• Chirality comes from a Greek word meaning
  hand
• An object is chiral if it is nonsuperimposable on
  its mirror image in three dimensions
• Several commonplace things are chiral
• Our hands, cars, cell phones, laptop computers
• Some sea shells, propellers, machine screws
• Spiral notebook bindings
• Most biologically active compounds are chiral
• Enzymes, sugars, lipids, prescription drugs

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Chirality

• Chiral molecule
• a molecule that has nonsuperimposable mirror
  image
• a molecule with no plane of symmetry
• Chiral carbon
• A carbon with 4 different groups attached
• also called a stereo center or stereogenic center
• also called an asymmetric carbon

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Chiral carbons

• Four different groups
• nonsuperimposable (test with models!!!!)
• is a chiral molecule (assumes one chiral carbon)
• can exist as a pair of enantiomers (mirror images
  that are nonsuperimposable)

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Naming Enantiomers

• If enantiomers are indeed different compounds,
  they must have different names!!
• We need to be able to specify which stereoisomer
  we mean
• R and S system (Cahn-Ingold-Prelog system of
  nomenclature for stereoisomers)
• does not indicate how a stereoisomer rotates
  plane polarized light---that can only be done
  experimentally
• different from D and L designation used in
  biology

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Rules for Determining R and S

• Step 1. Assign priorities to each group attached
  to the chiral carbon according to the sequence
  rules
• higher atomic number
• (Br gt Cl gt F gt O gt N gt C gt H)
• if same atom, go to next atom until you encounter
  first point of difference
• t-butyl gt isopropyl gt
  ethyl gt methyl
• triple bonds gt double bonds gt single bonds

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Rules for Determining R and S

• Step 2 Using three-dimensional models, hold the
  model so that the lowest priority group is
  pointing away from you.
• Step 3 Determine the direction of procession of
  the other three groups, from highest to lowest
  (highest 1, lowest 3)
• clockwise R (Latin rectus--right)
• counterclockwise S (Latin sinister)

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R and S Practice
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Fischer Projections

• Hard to draw 3-dimensionally
• redraw this as a Fischer projection
• ?
• horizontal lines come out of plane
• top and bottom lines go behind plane

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Rules for using Fischer projections

• Cannot rotate the Fischer projection 90o
• Cannot flip over out of the plane of the paper
• Can rotate 180o
• To draw mirror image just exchange horizontal
  groups

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Datasheet Example

• Draw several depictions of (S)-2-bromobutane
• Start by building a model (be absolutely certain
  your configuration is correct)
• Wedge diagram
• Orient the molecule to match the partial picture
• Fill in the missing pieces by looking at the
  model
• Draw the mirror image
• Sawhorse, Newman, Stereo, Fischer projections
• Rotate the molecule or single bonds until it
  matches the partial diagram
• Fill in the missing pieces draw the mirror image

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Chem 333
(S)-2-bromobutane
(R)-2-bromobutane
Br
Wedge
H
Newman
Fischer