Isomers

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Isomers

• Larry J Scheffler
• Lincoln High School
• IB Chemistry 3-4

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Structural Isomers

• Structural isomers have the same molecular
  formulas but they differ in their structural
  formulas.

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Stereoisomers

• Stereo isomers have the same structural formulas
  but they differ in their spatial arrangements.
• There are two types of stereoisomerism
• Geometrical isomerism
• Optical isomerism

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Geometrical isomers

• Geometrical isomers occur in organic molecules
  where rotation around a bond is restricted
• This occurs most often around CC
• The most common cases are around asymmetric
  non-cyclic alkenes

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Geometrical isomers

• Carbon to carbon double bonds are made up of a s
  and a p bond.
• The p bond is formed from the combination of two
  p orbitals, one from each of the two carbon atoms
• The two p orbitals must be in the same plane to
  combine

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Geometric Isomers in alkenes

• A cis isomer is one in which the substituents are
  on the same side of the CC

• A trans isomer is one in which the substituents
  are on the opposite sides of the CC

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Geometric Isomers in Cycloalkanes

• Ring structures like CC restrict rotation and
  therefore can result in cis and trans isomers

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Properties of Geometrical Isomers

• The chemical properties of geometrical isomers
  tend to be similar but their physical properties
  are different

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Properties of Geometrical Isomers

• The trans isomer has a much higher melting point.
  Unlike the cis isomer there is little
  intra-molecular hydrogen bonding

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Optical Isomers

• Optical isomerism is present in all compounds
  that contain at least one asymmetric (chiral)
  carbon atom
• An asymmetric carbon atom has four different
  atoms or groups attached

In this case there are two different ways to
arrange the four groups around the chiral carbon
atom (shown in red)
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Optical Isomers

• While these structures may look identical, in
  three dimensions they are mirror images of each
  other. Such molecules are called enantiomers.

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Distinguishing Enantiomers

• Optical isomers can be distinguished by the way
  they interact with plane polarized light

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Polarized light

• Polarized light is light that has been passed
  through a polarizing prism or filter. As a
  result the light vibrates in a single plane.

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Distinguishing Enantiomers

• Optical isomers can be distinguished by the way
  they interact with plane polarized light. One
  enantiomer will rotate polarized light to the
  right, the other to the left.

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Properties of Optical Isomers

• Apart from their optical activity enantiomers
  generally have similar physical and chemical
  properties.
• The chemical properties may be significantly
  different when the enantiomers interact with
  other optically active compounds.

Thalidomide has two optical isomers. One is a
tranquilizer, the other is a Powerful teratogen.