9' Stereochemistry

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9. Stereochemistry
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Topics to discuss

• Enantiomers
• Chirality
• Optical activity
• Sequence Rules
• Diastereomers
• Meso compounds
• Molecules with multiple chiral centers
• Physical properties
• Racemic mixtures and resolution
• A review of isomerism
• Stereochemistry of HBr addition to alkenes
• Stereochemistry of Br2 addition to alkenes
• Stereochemistry of HBr addition to CHIRAL alkenes
• Chiral atoms other than carbon
• Chirality in nature
• prochirality

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Stereochemistry

• Q What is stereochemistry?
• Q Why do we care?

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Isomerism

• Constitutional isomers
• Stereoisomers

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Enantiomers

• Q What is an enantiomer?
• Q What is a chiral molecule?
• Q What is an example of an enantiomer?
• Q What are some examples of achiral objects?

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Examples of Enantiomers

• This is where molecular model kits help!

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Mirror-image Forms of Lactic Acid

• When H andOH substituents match up, COOH and CH3
  dont
• when COOH and CH3 coincide, H and OH dont

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The Reason for Handedness Chirality

• Q What is chirality?
• Q What do we look for?

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Plane of Symmetry

• The plane has the same thing on both sides for
  the flask
• There is no mirror plane for a hand

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Chirality Centers

• Q How do we identify a chiral carbon?

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Chirality Centers in Chiral Molecules

• Q How do we know if a group is different?

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Optical Activity

• Q What is plane polarized light?
• Q What is optical activity?

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Optical Activity

• Q How is optical activity measured?
• Q What does levorotatory and dextrorotatory
  mean?

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Polarimeters

• Q How does a polarimeter work?

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Specific Rotation

• Q What is specific rotation?
• Q How do we determine specific rotation?
• Q Are there standard conditions to use for
  comparison?

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Specific Rotation and Molecules
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Sequence Rules Assigning Absolute
Configuration

• Q How do we distinguish between enantiomers?
• Q How do we assign absolute configuration?

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CIP Sequence Rules

• 1. Look at the atoms directly bonded to the
  chiral carbon and assign priorities (1-4) in
  order of decreasing atomic number. The highest
  atomic number gets the highest priority.
• 2. If two atoms bonded to the same chiral carbon
  have the same atomic number, look at the atomic
  number of the atoms bonded to these atoms. One
  atom of higher atomic number determines a higher
  priority. Continue to look at the next atoms
  until a point of difference is reached.
• 3. To assign a priority to an atom that is part
  of a multiple bond, consider the multiply bonded
  atom as an equivalent number of singly bonded
  atoms.

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CIP Sequence Rules

• Q Ive assigned priority, now what?

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Drawing Fischer Projections

• Q What is a Fischer Projection?

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Diastereomers

• Q What if there is more than one chiral center?
• Q What if they are not mirror images?
• Q What is the difference between an enantiomer
  and a diastereomer?

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Diastereomers
Diastereomers
Diastereomers
Enantiomers
Enantiomers
Diastereomers
Diastereomers
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Another Example
Look closely at these two There is a plane of
symmetry. The top half of the molecules is
identical to the bottom half! The plane of
symmetry makes them achiral, although they do
have chiral centers.
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Meso Compounds

• Q What is a meso compound?

Symmetry Plane
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Molecules with More Than Two Chirality Centers

• Q Can a molecule have more than two chirality
  centers?
• Q How do I know
• how many stereoisomers
• are possible?

Cholesterol has 8 chirality centers
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Physical Properties of Stereoisomers

• Enantiomers
• Diastereomers

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Racemic Mixtures

• Q What is a racemic mixture?
• Q So what? Whats the big deal if we get a
  racemic mixture?

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Resolution of Racemic Mixtures

• Q What is resolution of racemic mixtures?
• Q How do we resolve the enantiomers?

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Review of Isomerism

• The flowchart summarizes the types of isomers we
  have seen

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Stereochemistry of Reactions Addition of HBr to
Alkenes
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Achiral Intermediate Gives Racemic Product

• Q How does this happen?

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Stereochemistry of Reactions Addition of Br2 to
Alkenes

• Stereospecific
• Bromonium ion leads to

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Addition of Bromine to Trans 2-Butene

• Gives meso product

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Chirality at Atoms Other Than Carbon

• Q What atoms, other than carbon, can be chiral?

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Chirality in Nature

• Stereoisomers are readily distinguished by
• Properties of drugs depend on
• Think of biological recognition as
• See Figure 9-19

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Prochirality

• Q What is prochirality?

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Summary

• Enantiomers
• Chirality
• Optical activity
• Sequence Rules
• Diastereomers
• Meso compounds
• Molecules with multiple chiral centers
• Physical properties
• Racemic mixtures and resolution
• A review of isomerism
• Stereochemistry of HBr addition to alkenes
• Stereochemistry of Br2 addition to alkenes
• Stereochemistry of HBr addition to CHIRAL alkenes
• Chiral atoms other than carbon
• Chirality in nature
• prochirality