B. Properties of Enantiomers

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B. Properties of Enantiomers

• Same boiling point, melting point, density
• Same refractive index
• Different direction of rotation in polarimeter
• Different interactions with other chiral
  molecules
• Enzymes
• Taste buds, scent

B.1. Plane-Polarized Light
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Polarimetry

• Use monochromatic light, usually sodium D (l
  5996 nm)
• Movable polarizing filter to measure angle
• Clockwise dextrorotatory d or ()
  Counterclockwise levorotatory l or (-)
• Not related to (R) and (S)
  
  
  

B.2. Specific rotation
? (observed)
Observed rotation depends on the length of the
cell and concentration, temperature, and
wavelength of light.
c is concentration in g/mL l is length of path in
decimeters.
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Calculate ?D

• A 1.00-g sample is dissolved in 20.0 mL ethanol.
  5.00 mL of this solution is placed in a 20.0-cm
  polarimeter tube at 25?C. The observed rotation
  is 1.25? counterclockwise.

Length in decimeter
Concentration in g/mL.
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B.3. Absolute structure

• enantiomers give the same amount of rotation,
  but to the opposite directions.
• 2-butanol ?D25 13.52o, rotate light
  clockwise
• ?D25 -13.52o, rotate light
  counterclockwise
• There is no direct relationship between absolute
  structure of a compound and the direction of
  polarized light rotation.
• There is no relationship between (R) or (S) and
  () or (-).

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B.4. Racemic Mixtures

• Equal quantities of d- and l-enantiomers,
  Notation (d,l) or (?)
• E.g., (?)-2-butanol.
• The net optical rotation 0.
• The mixture may have different b.p. and m.p. from
  the enantiomers!
• A racemate can crystallize in several ways.
• Separate () and (-) (Pastor separation of
  tartaric acid)
• Racemic crystal gt only one type of crystal of
  equal amount of d and l forms.

B.5. Measure
Enantiomeric excess (ee) Amount of pure
enantiomer in excess of the racemic mixture.
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Calculate Composition
The specific rotation of (S)-2-iodobutane is
15.90?. Determine the composition of a mixture
of (R)- and (S)-2-iodobutane if the specific
rotation of the mixture is -3.18?.
Let x be the mole of (R), and (1-x) be the mole
of (S)
2x -1 0.2 2x 0.2 1 1.2 x 1.2/2 0.6
60 gt composition of the R
enantiomer (1-x) 1 0.6 0.4 40 gt
composition of the S enantiomer
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C. Assign (R) or (S), Nomenclature of
stereocenters

• Assign priority, according to the
  Cahn-Ingold-Prelog rules, to the 4 substituents
  on the stereocenter, in order of (1) gt (2) gt (3)
  gt (4).
• Arrange the molecule so that the lowest priority
  group, (4), is in the back, and then draw a
  Newman projection looking along the bond from the
  stereocenter C down to (4).
• Draw a circular arrow from highest (1) through
  (2) to the third priority group (3).
• Clockwise arrow, assign R to the stereocenter.
• Counterclockwise arrow, assign S to the
  stereocenter.

(1)
(2)
(3)
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Priority assignment Cahn-Ingold-Prelog Rules

• The higher atomic number has a higher priority
  than the lower one.
• e.g., Br gt Cl gt F gt O gt N gt C gt H
• The higher mass isotope has higher priority than
  the lower mass.
• e.g., T gt D gt H
• For the same atoms directly attached to the
  stereocenter, move along the two chains until a
  point of difference, and use the above priority
  rules to determine which group has higher
  priority.
• Double (triple) bonds are treated by assuming
  that each such bonded atoms is duplicated
  (triplicated).