Chapter 5 STEREOCHEMISTRY

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Chapter 5STEREOCHEMISTRY
Stereochemistry
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CHIRALITY AND ENATIOMERS

1. Chiral Object an object that IS NOT
   superimposable with its mirror image.
2. Achiral Object an object that IS superimposable
   (i.e. identical) with its mirror image.
3. Enantiomers stereoisomers, which are
   non-superimposable mirror images.

Stereochemistry
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CHIRAL CARBON ATOMS

1. Chiral Carbon Atoms (also known as asymmetric
   carbon atoms) carbon atoms bonded to FOUR
   DIFFERENT groups. Chiral atoms are generally
   called chiral centers.

mirror

• Some rules
• If a molecule has NO chiral center, it is
  usually, but not necessarily achiral.
• If a molecule has just ONE chiral center, it is
  definitely chiral.
• If a molecule has MORE THAN ONE chiral center, it
  may or may not be chiral.

Stereochemistry
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THE MIRROR IMAGES OF CARBON CENTERS WITH THREE
DIFFERENT SUBSTITUENTS ARE SUPERIMPOSABLE
120o
IDENTICAL WITH THE ORIGINAL
mirror
Stereochemistry
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THE MIRROR IMAGES OF CARBON CENTERS WITH FOUR
DIFFERENT SUBSTITUENTS (CHIRAL CARBON ATOMS) ARE
NOT SUPERIMPOSABLE
120o



NOT IDENTICAL WITH THE ORIGINAL
mirror
Stereochemistry
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CARBON CENTERS WITH FOUR DIFFERENT SUBSTITTUENTS
(CHIRAL CARBON ATOMS) DO NOT HAVE INTERNAL MIRROR
PLANES OF SYMMETRY
Stereochemistry
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CHIRAL COMPOUNDS ALSO DO NOT HAVE INTERNAL MIRROR
PLANES OF SYMMETRY
MIRROR IMAGE IDENTICAL WITH ORIGINAL
MIRROR IMAGE IS NOT IDENTICAL WITH ORIGINAL
Stereochemistry
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THE CAHN INGOLD PRELOG CONVENTION

• Assign a priority and a number to each group,
  bonded to the chiral carbon atom. Highest
  priority group gets number 1, second in priority
  number 2, third in priority number 3, lowest
  in priority number 4.
• First compare the atoms immediately bonded to the
  chiral center. The atom with highest atomic
  weight is highest in priority and therefore 1,
  the atom with lowest atomic weight is 4.
• If two or more atoms of the immediate environment
  are identical, compare the next set of atoms
  along the chains, applying the criterion from
  (a).
• REMEMBER One higher-priority atom is more
  important
• than any number of
  lower-priority atoms.

Stereochemistry
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THE CAHN INGOLD PRELOG CONVENTION

1. Treat double- and triple-bonded atoms as an
   equivalent number of single-bonded atoms, i.e.
   imagine that the p-bonds are broken and the atoms
   at both ends of the bond are duplicated (if it is
   a double bond) or triplicated (if it is a triple
   bond).

Stereochemistry
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THE CAHN INGOLD PRELOG CONVENTION
2. Orient the molecule in a fashion such that
the lowest in priority group (number 4) is in
back and view the molecule along the bond from
the chiral center to group 4. Draw a curved
arrow, starting from 1, through 2, to 3. If
the arrow points in a clockwise direction
R-enantiomer, if the arrow points in a
counter-clockwise direction S-enantiomer.
Stereochemistry
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THALIDOMIDE DISASTROUS BIOLOGICAL ACTIVITY OF
THE WRONG ENANTIOMER
Stereochemistry