Summary Slide

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Summary Slide

• Stereochemistry
• For those students using Fundamentals of Organic
  Chem. this presentation refers to Chapter 6.

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Stereochemistry
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The Origins of Stereochemistry
Stereochemistry is the branch of chemistry
concerned with the three dimensional nature of
molecules. This branch of chemistry originated
as an offshoot of the research of the French
physicist Jean Baptiste Biot (1774-1862). Biot
was investigating the nature of "plane -
polarized light" when he accidentally discovered
optical activity. This discovery eventually led
to the development of stereochemistry.  
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Plane Polarized Light

• A beam of ordinary (unpolarized) light consists
  of waves that oscillate equally in all directions
  perpendicular to the line which defines the path
  of the light ray.
• Certain materials affect ordinary light in a
  special way. Polarized films interact with all
  the oscillating waves of ordinary light and
  filter out all planes of oscillation except one.
  The light which emerges from a Polaroid film
  consists of waves oscillating in one plane only
  and hence is referred to as "plane polarized
  light".  

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Manipulation of Light Using Polaroid Films

• In order to see an object, you need light.
• Polarized films only transmit light vibrating in
  one plane and filter out the rest. If two
  polarized films are set up in front of one
  another and one is rotated 90 degrees to the
  other, then the first will transmit light
  vibrating in a plane that is absorbed by the
  second and hence no light is transmitted and the
  object cannot be seen. If one of the two
  Polaroid films is rotated 90 degrees then the
  object can be seen again with maximum brightness

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Optical Activity

• In 1815, Biot discovered that when a beam of
  plane polarized light is passed through solutions
  of certain organic molecules, such as sugar or
  camphor, the plane of polarization is rotated.
  We call molecules that exhibit this property
  optically active.
• A. The amount of rotation can be measured with
  an instrument known as polarimeter. In a
  polarimeter, plane polarized light is passed
  through a tube containing a solution of some
  optically active molecules and rotation occurs.
  The extent of rotation is determined by rotating
  a second polarized film until the light passes
  through it. The observed rotation is symbolized
  by the Greek letter a. In addition to
  determining the extent of rotation, the direction
  is also given.

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The Direction of Rotation in a Polarimeter

• Some optically active molecules rotate plane
  polarized light to the left (counter clockwise)
  and are said to be levoratatory.
• Others rotate polarized light to the right
  (clockwise), and are said to be dextrorotatory
• By convention rotation to the left is given a (-)
  minus sign, and rotation to the right is given a
  () positive sign

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A Simple Polarimeter

• Measures extent of rotation of plane polarized
  light
• Operator lines up polarizing analyzer and
  measures angle between incoming and outgoing light

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The Amount of Rotation

• The amount of rotation obtained from a
  polarimeter is dependent upon the number of
  optically active molecules the beam encounters
  and the nature of the light source.
  Consequently, the amount of rotation is dependent
  upon
• length of sample tube
• concentration of optically active molecules in
  solution
• the wavelength of the light used  
• Because optical rotation is dependent upon these
  three variables,we must choose standard
  conditions so that comparisons can be made.

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Specific Rotation

• To have a basis for comparison, define specific
  rotation, ?D for an optically active compound
• aD observed rotation a
  . Path length l (dm) X
  concentration (g/ml)
• Specific rotation is that observed for 1 g/mL in
  solution in cell with a 10 cm path using light
  from sodium metal vapor (589 nanometers)

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Louis Pasteur and Optical Isomers

• . While recrystallizing sodium ammonium
  tartrate, Louis Pasteur noticed two differently
  shaped types of crystals. Separating the two
  types using tweezers, Louis Pasteur discovered
  that solutions of each type had specific
  rotations that were equal in degree, but opposite
  in sign. Louis Pasteur had discovered optical
  isomers. Optical isomers are also called
  enantiomers

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Isomers

• You will recall that isomers are molecules having
  the same molecular formula but different
  structural formulas.

n-
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Optical Isomers

• Optical isomers also fall under the general
  definition of isomerism, but the difference in
  their structural formulas is much more subtle! A
  pair of optical isomers have structural formulas
  that are related as "nonsuperimposable mirror
  images.They have the same relationship as do
  your two hands
• In fact, after separating the two types of
  crystals, Louis Pasteur noticed that their shapes
  had this same relationship 

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Mirror Images and Optical Isomers

• Every molecule has a mirror image. Only if the
  mirror image of a molecule is nonsuperimposable
  on the original do the two structures represent a
  pair of optical isomers. Only if the mirror image
  of a molecule is nonsuperimposable on the
  original do the two rotate plane polarized light
  to the same of degrees but in opposite
  directions. The criterion of sameness in
  chemistry is"superimposability".
• If two structures are superimposable, then they
  represent the same molecule

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Criteria for Optical Isomerism

• In order for a molecule to exist as a pair of
  optical isomers it must meet the following two
  criteria
• It must contain at least one carbon bonded to
  four different groups
• It must not contain a plane of symmetry

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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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Optical Isomers (Enantiomers) and Chirality

• Chirality a pair of optical isomers (or
  enantiomers) are related as are your two hands.
  They are nonsuperimposible mirror images of one
  another. Therefore, a pair of enantiomers have
  the structural property of "opposite handedness".
  The Greek word for hand is cheir" and from this
  we get the words "chiral" and "chirality". We
  may therefore call a pair of enantiomers "chiral"
  or say they posses"chirality". The opposite of
  chiral is achiral

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Facts About Optical Isomers

• . Physical Properties of Optical Isomers
  Although they are different substances, the
  structures of a pair of enantiomers are so
  similar that their physical properties are
  identical (bp, mp, density, etc.).
• Racemic mixtures a mixture that is a 5050 mix
  of each member of a pair of optical isomers.
  This mixture is optically inactive because the
  optical effect of each isomer cancels the other.
• If a molecule possesses n chiral centers, then
  the max number of optical isomers is 2n
• 1 chiral center 21 2 optical isomers 

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Reactions of Optical Isomers

• Chemical reactions for a pair of enantiomers are
  identical if the enantiomers are reacted with
  achiral (non- optically active) reagents
• right hand ski pole ski pole in hand
• left hand ski pole ski pole in hand
• Two different reaction products are observed for
  each member of an optically active pair if they
  are forced to react with a chiral reagent.
• right hand right glove R hand R glove
• left hand right glove Left hand in R. glove

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The Biochemistry of Chirality

• Many of the sensory receptors in the human body
  are chiral Therefore, the biochemical effect of
  each member of an enantiomeric pair is quite
  different.

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Bitter-Sweet Story of Asparagine

• Asparagine is an amino acid and furthermore two
  enantiomers of asparagine exist.

These two represent a pair of enantiomers. ie.
They are nonsuperimposable mirror images of one
another. Each of these two has a different
biochemistry. One tastes bitter, one is sweet
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Chirality Centers

• As mentioned earlier, the most common cause of
  chirality in a molecule is a carbon that is
  attached to four different groups. Such a carbon
  is referred to as a chiral center
• Detecting chiral centers in a complex molecule
  can be difficult because it is not always
  apparent that 4 different groups are bonded to a
  given carbon. The differences do not necessarily
  appear right next to the carbon centers .

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Chirality Centers in Cyclic Molecules

• Groups are considered different if there is any
  structural variation
• In cyclic molecules, we compare by following in
  each direction of the ring

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Identifying Specific Enantiomers

• The arrangement of groups around a chiral carbon
  is different for each member of an enantiomeric
  pair. We need a way of verbally identifying each
  member of an enantiomer pair
• We need a set of rules for specifying the exact
  configuration around a chiral carbon

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Sequence Rules for Specification of Configuration

• These rules allow us to specify the exact
  arrangement of atoms about each chiral carbon
  without having to draw a perspective structural
  formula

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Sequence Rules (IUPAC)

• Assign each group that is attached to the chiral
  carbon a priority according to the
  Cahn-Ingold-Prelog scheme highest 1 and
  lowest 4
• If, when the thumb of your left hand points in
  the direction of the lowest priority group(4),
  your fingers curl in the direction of descending
  priority (1-2-3), then your molecule has an S
  Configuration
• If your right hand is needed to accomplish the
  above, then the configuration is R

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Right Hand R Configuration
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Left Hand S Configuration
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Diastereomers

• If a molecule contains one chiral center, then 2
  stereoisomers exist for this molecule. The two
  stereoisomers represent a pair of enantiomers. 
• If a molecule contains more than one chiral
  center, then more than 2 stereoisomers exist for
  that molecule (actually the max number is 2n
  where n number of chiral centers). These
  stereoisomers usually exist as pairs of
  enantiomers. One member of each enantiomer pair
  is the mirror image of the other member and has
  the opposite coniguration at each chiral center.
  What is the relationship between two members of
  different enantiomeric pairs. These molecules
  are still stereoisomers of one another but they
  are not related as object and its mirror image.
  These molecules are called Diastereomers (2R,3R
  and 2R,3S or 2S,3S and 2R,3S)

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Meso Compounds

• Compounds having n chiral centers have a max
  number of 2n stereoisomers. These stereoisomers
  exist as pairs of enantiomers that have opposite
  configuration at each chiral center. If a pair
  of enantiomers is seen to have a plane of
  symmetry then both structures represent the same
  molecule (the two structures are superimposable).
  Any compound that contains both chiral centers
  and a plane of symmetry is called a Meso
  compound.
• Meso compounds have different physical
  properties than their enantiomers. Meso
  compounds are achiral .

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Physical Properties of Stereoisomers

• Enantiomeric molecules differ in the direction in
  which they rotate plane polarized but their other
  common physical properties are the same
• Daistereomers have a complete set of different
  common physical properties

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A Brief Review of Isomerism

• The flowchart summarizes the types of isomers we
  have seen

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Constitutional Isomers

• Different order of connections gives different
  carbon backbone and/or different functional groups

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Stereoisomers

• Same connections, different spatial arrangement
  of atoms
• Enantiomers (nonsuperimposable mirror images)
• Diastereomers (all other stereoisomers)
• Includes cis, trans and configurational