C'I' 3'6

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C.I. 3.6

• Optical Isomerism

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Types of isomerism
Isomerism
Structural isomerism
Stereoisomerism
Geometric isomerism
Optical isomerism
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Different compounds- different properties.
geometric isomerism
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Optical Isomerism

• Arises because of the different ways you can
  arrange four different groups around a carbon
  atom.
• Hang on!! Arent ?-amino acids a group of
  compounds that have four different groups around
  a carbon atom?

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four groups of electrons around the central atom

• four bonding pairs
• TETRAHEDRAL shape
• bond angle exactly 109?

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alanine
imaginary mirrors
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• All molecules have mirror images however they
  dont all exist as two isomers.
• What makes an isomer is the fact that the mirror
  image and the original molecule are
  non-superimposable!

The only way to make these two superimpose is to
break reform bonds.
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Left and right hands are an example of
non-superimposable mirror images.
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Enantiomers ?

• Molecules such as alanine that exist in these two
  forms optical isomers or enantiomers.
• We distinguish between the two enantiomers of a
  molecule by /-, D/L or more correctly R/S.
• A 50/50 mixture of the two enantiomers is called
  a racemic mixture or a racemate.

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Some more key words

• Molecules that are not superimposable on their
  mirror images are called chiral molecules.
• A carbon surrounded by 4 different groups is
  called a chiral centre.

chiral centre of alanine
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The CORNy rules for naming enantiomers.

• Optical isomers exist as L-enantiomers or
• D-enantiomers.

L-enantiomer
D-enantiomer
R
R
CO
CO
N
N
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The CORNy rules for naming enantiomers.

• Imagine looking down on the molecule with the
  single H atom point straight up towards you.
• Label the other three groups
• COOH CO
• R-group R
• NH2 N
• If CORN is arranged in a CLOCKWISE direction it
  is the L-amino acid.
• If CORN is arranged ANTI-CLOCKWISE it is a
  D-amino acid.

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How do enantiomers differ?

• Behave identically in ordinary test-tube chemical
  reactions.
• Have same physical properties.
• BUT!!
• Behave differently in presence of other chiral
  molecules.
• e.g. Taste-buds are chiral D-amino acids taste
  sweet, L-amino acids are tasteless or bitter.

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