Cpt 5' STEREOCHEMISTRY

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Cpt 5. STEREOCHEMISTRY

• Study of 3 Dimensional Structure of compounds
• Objective Determine and explain the
  stereochemistry and stereoisomerism involving
  alkanes, alkyl halides, and alcohols
• Thought teaser
• 1. Build two molecules of CHIClBr. Superimpose
  the resulting molecules atom to atom. Do they
  coincide? Place the molecules face to face. Do
  the atoms face one anonther?
• 2. Switch the Cl and Br bonds. Superimpose the
  resulting molecules atom to atom. Do they
  coincide? Place the molecules face to face. Do
  the atoms face one another?
• 3. Which molecules are identical, which ones are
  different? What is the difference due to?

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5.1. Stereoisomerism

• Stereoisomers Isomers that differ only by their
  configuration (the way their bonds are arranged
  in space).
• Ex
• HCI(Br)(Cl)
• Cis-Trans isomers

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5.2. Enantiomerism

• Enantiomers two stereoisomers that are mirror
  images of one another
• Ex HCI(Br)(Cl)
• All enantiomers are Chiral (Handed) not
  superimposable with one another
• ex 2
• CH3CHBrCH2CH3
• CH3CBr2CH2CH3
• Chirality center asymetric center a C that is
  bound to 4 different atoms or groups of atoms
• Contributes to the chirality of the molecule

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5.3. Optical Activity (Specific Rotation)

• Resource Ctr Slide 15-17
• Components of normal light oriented in all
  directions
• Components of Plane-Polarized lightdirected only
  in one plane (direction)
• Polarizer lens that lets through light that is
  oriented in only one direction and blocks light
  oriented in all other directions
• Chiral molecules change direction of polarized
  light

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Optical activity (Continued)

• Polarimeter instrument used to measure optical
  activity
• Optical activity ability to change direction of
  polarized light
• Optical Rotation angle by which a chiral
  compound turns the direction of polarized light.
• Dextrorotatory compounds that rotate polarized
  light to the right
• Levorotatorycompounds that rotate polarized
  light to the left

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Optical activity (Continued 2)

• Specific Rotation of degrees by which
  polarized light is rotated by a sample of
  substance at 1 g/ml concentration inside a 1 dm
  long optical rotation measuring tube 
• aD a / (l x C)
• a specific rotation
• a observed rotation
• l length of measuring tube(dm) 
• d density for pure liquids or concentration for
  solutions      (g/ml)
• ex 5-9, pg 183

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Optical activity (Continued 3)

• Enantiomers have opposite optical activities.
• Ex 2-butanol
• 13.5 o for the D enantiomer
• - 13.5 o             L
• Most chemical and physical properties of
  enantiomers are identical. Ex mp, bp, Rf,

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5.4. Absolute Configuration R S

• Definition Systematic way to assign the
  stereochemistry of molecules.
• Thought teaser Which one of the 2-butanol
  enantiomers is D?
• Cahn-Ingold-Prelog Configuration Determination
  Procedure
• Step 1 set chiral center with lightest atom (H)
  or group of atoms oriented away from you
• Step 2 number the remaining atoms (or groups of
  atoms) from the heaviest to the lightest
• If numbering clockwise configuration R
• If numbering anticlockwise configuration S

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R S Configuration(Illustrations)
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R S Configuration(Illustrations 2)
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R S Configuration(Exercises)
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R and S Configuration (Continued)

• Special cases
• Double and triple bonds
• atoms involved in double bonds are duplicated
• atoms involved in triple bonds are triplicated

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Absolute configuration (Special Cases
Illustration)
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 Fisher projections

•  Fisher projections simple representation of
  chiral compounds
• Horizontal lines used for bonds directed
  toward us
• Vertical lines used for bonds directed away
  from
• Molecule chain must be set in eclipsing
  conformation.
• Determine the stereochemistry of Fisher
  projections one atom at a time.

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Fisher projections (Examples)
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5.5. Diastereomers Meso Compounds

• Diastereomers chiral isomers which are not
  mirror images of one another.
• ex 2R,3R,4R,5S-2,3,4,5-hexanetetraol, and 
  2R,3R,4R,5R-2,3,4,5-hexanetetraol.
• Meso compounds nonchiral molecules that have
  chiral centers. Reason for nonchirality symmetry
  of the molecule. Apparent meso isomers are
  identical molecules.
• ex 2R,3R,4S,5S-2,3,4,5-hexanetetraol.

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Diastereomers Meso Compounds (Continued)
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Diastereomers Meso Compounds (Exercises)
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5.6. Chirality of Substituted Cyclohexanes

• Thought teaser Build the models of
  1-ethyl-4-methylcyclohexane and
  1-ethyl-3-methylcyclohexane. Which molecule is
  symmetrical?
• Principles the following molecules are achiral
• Symmetrical molecules
• Molecules that rapidly convert to their
  enantiomers
• Molecules in all other cases are chiral
• Ex
• 1,2-disubstituted cyclohexanes
• 1,3-disubstituted cyclohexanes

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5.7. Optical Purity (OP) Enantiomeric Excess
(EE).

• OP and EE measure the purity of non racemic
  mixtures of enantiomers.
• OP ratio of rotation of mixture to rotation of
  pure enantiomer
• OP a(observed)/a (pure) x 100
• EE Wt(d) - Wt(l)/Wt(d) Wt(l) x 100
• Ex(35-12, pg 187)

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Optical Purity (Example)

• Problem When R-2-bromobutane is heated with
  water, a mixture of 2x as much S-2-butanol as
  R-2-butanol is formed. Calculate the EE and the
  specific rotation of expected for the product.
• Info provided Proportion of S-2-butanol (a
  13.5o) 2x as much as R-2-butanol.
• Info requested EE and the specific rotation of
  expected for the product.
• Solution EE (2-1)/(21) X 100 33.33
• a 13.5o x 33.33/100 4.5o

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Optical Purity (Example 2)

• Problem The specific rotation of pure
  (-)-2-butanol is -13.5o. What of a mixture of
  the two enantiomeric forms is the (-)-form if the
  specific rotation of this mixture is 7.0o?
• Info provided
• a of (-)-2-butanol -13.5o
• a of a mixture of (-) and ()-2-butanol
  7.0o
• Info requested of ()-2-butanol

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Optical Purity (Solution)

• OP (a(observed) / a(pure)) x 100 of the
  majority enantiomer
• of the minority enantiomer 100 - of the
  majority enantiomer

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Optical Purity (Exqmple 3)

• Problem (-)-Lactic acid has a specific rotation
  of -3.8o. What is the specific rotation of a
  solution containing 7.5 g of (-)-lactic acid and
  2.5 g of ()-lactic acid?
• Info provided
• a of (-)-lactic acid -3.8o
• Wt of (-) lactic acid 7.5 g
• Wt of () lactic acid 2.5 g
• Info requested a of the mixture of (-) and ()
  lactic acids.

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Optical Purity (Solution3)

• a(observed) OP x a(pure) / 100
• OP EE 100 x (d l) / (dl)