Chapter 3 Stereoisomerism and Chirality

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Chapter 3Stereoisomerism and Chirality

• Lecture 11
• Chem 30A

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Enantiomers Diastereomers

• 2-Methylcyclopentanol

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Enantiomers Diastereomers

• 1,2-Cyclopentanediol

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Enantiomers Diastereomers

• cis-3-Methylcyclohexanol (pair of enantiomers)

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Enantiomers Diastereomers

• trans-3-Methylcyclohexanol (pair of enantiomers)

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Isomers
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Properties of Stereoisomers

• Enantiomers have identical physical and chemical
  properties in achiral environments.
• Diastereomers are different compounds and have
  different physical and chemical properties.
• meso tartaric acid, for example, has different
  physical and chemical properties from the R,R and
  S,S enantiomers (see Table 3.1).

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Properties of Stereoisomers

• Properties of the stereoisomers of tartaric acid

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Plane-Polarized Light

• Ordinary light Light oscillating in all planes
  perpendicular to its direction of propagation.
• Plane-polarized light Light oscillating only in
  parallel planes.
• Optically active Refers to a compound that
  rotates the plane of plane-polarized light.

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Plane-Polarized Light

• Plane-polarized light is the vector sum of left
  and right circularly polarized light.
• Circularly polarized light interacts one way with
  an R chiral center, and the opposite way with its
  enantiomer.
• The result of interaction of plane-polarized
  light with a chiral compound is rotation of the
  plane of polarization.

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Plane-Polarized Light

• Polarimeter A device for measuring the extent of
  rotation of plane-polarized light.

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Optical Activity

• Observed rotation The number of degrees, ?,
  through which a compound rotates the plane of
  polarized light.
• Dextrorotatory () Refers to a compound that
  rotates the plane of polarized light to the
  right.
• Levorotatory (-) Refers to a compound that
  rotates the plane of polarized light to the left.
• Specific rotation Observed rotation for a sample
  in a tube 1.0 dm in length and at a concentration
  of 1.0 g/mL for a pure liquid, concentration is
  expressed in g/mL (density).

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Optical Purity

• Optical purity A way of describing the
  composition of a mixture of enantiomers.
• Enantiomeric excess The difference between the
  percentage of two enantiomers in a mixture.
• optical purity is numerically equal to
  enantiomeric excess, but is experimentally
  determined.

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Enantiomeric Excess

• Example a commercial synthesis of naproxen, a
  nonsteroidal anti-inflammatory drug (NSAID),
  gives the S enantiomer in 97 ee.
• Calculate the percentages of the R and S
  enantiomers in this mixture.

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Chirality in the Biological World

• Except for inorganic salts and a few low
  molecular weight organic substances, the majority
  of molecules of living systems are chiral.
• Although these molecules can exist as a number of
  stereoisomers, generally only one is produced and
  used in a given biological system.
• Its a chiral world!

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Chirality in the Biological World

• Consider chymotrypsin, a protein-digesting enzyme
  in the digestive system of animals.
• chymotrypsin contains 251 chiral centers.
• the maximum number of stereoisomers possible is
  2251
• there are only 238 stars in our galaxy!

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Chirality in the Biological World

• Enzymes are like hands in a handshake.
• The substrate fits into a binding site on the
  enzyme surface.
• A left-handed molecule, like hands in gloves,
  will only fit into a left-handed binding site and
• a right-handed molecule will only fit into a
  right-handed binding site.
• Because of the differences in their interactions
  with other chiral molecules in living systems,
  enantiomers have different physiological
  properties.

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Chirality in the Biological World

• A schematic diagram of an enzyme surface capable
  of binding with (R)-glyceraldehyde but not with
  (S)-glyceraldehyde.

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