Stereoisomerism

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Stereoisomerism

• Stereoisomers are isomers having same structural
  formula but differing in the way the atoms are
  arranged in space.

  Geometrical isomers Stereoisomers Optica
l isomers (Enantiomers)  
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Geometrical isomers

• It is due to the restricted rotation of covalent
  bond, which is caused by 1.  presence of a
  double bond or2. presence of a ring structure
• Examples
• But-2-ene

cis but-2-ene
trans but-2-ene
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Geometrical isomers

• Hexa-2,4-diene

• How many geometrical isomers can you draw ?

• cis, trans - isomer

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Geometrical isomers

• Are they the same ?

• trans,trans - isomer

• trans,cis - isomer

• cis,cis isomer

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Geometrical isomers

• Three isomers same as hexa-2,4-diene.

• How about hepta-1,5-diene ?

• Two isomers only

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One more criteria

• To have geometrical isomer
• W ? X AND
• Y ? Z
• You can try hepta-2,5-diene yourself.

• No geometrical isomer if
• W X OR
• Y Z

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Geometrical isomer

• Azo compound
• NN is called azo group which occurs in a
  lot of dye (called azo dye)

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Geometrical isomer ring compound

• There is also restricted rotation in a ring
  compound

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Geometrical isomer ring compound

• 1,2-dimethylhexane

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Geometrical isomers

• cyclohexene

Cis - isomer
Trans - isomer
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Physical properties of geometrical isomers

• Stability
• Bulky group packed closer together in cis-isomer
• ? less stable than trans-isomer
• Dipole moment
• Overall dipole moment is the total sum of the
  dipole moment, so dipole moment of cis-isomer gt
  trans-isomer.

Net dipole moment 0
Non-zero dipole moment
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Physical properties of geometrical isomers

• Boiling point
• Cis-isomer possesses net dipole moment,
• ? this cause dipole-dipole interaction between
  the cis-isomer
• ? higher boiling point

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Physical properties of geometrical isomers

• Melting point
• Trans-isomer have more regular and symmetrical
  structure
• ? packed more compactly in solid state
• ? intermolecular force between trans-isomer
  become greater
• ? higher melting point

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Trans isomer can pack closer