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Geometric Isomerism in Alkenes

Why does geometric isomerism exist in Alkenes?

  • The carbons in the double bond are sp2 hybridized, so they form one sigma bond and one pi bond.

  • Sigma bonds have freedom of rotation, but pi bonds do not.

  • Therefore, the groups attached to the double bond are locked in place.

  • If there are different groups attached to the double bond, this makes for 2 possible arrangements.

Cis- and Trans- naming

  • If the molecule has an identical pair of atoms on the same side of the double bond, it is a **cis-alkene.

  • If the molecule has an identical pair of atoms on opposite sides of the double bond, it is a **trans-alkene.

  • The problem with using the cis-trans system is that you need at least 1 identical pair of atoms.

  • So we sometimes use another system of naming

E-Z Naming

  • First you have to rank the substituents based on their atomic number. Elements with the higher atomic number get higher priority.

    • E.g. a chlorine atom is ranked higher than an ethyl group which is ranked higher than a hydrogen

  • If the higher ranked atoms/groups are on the same side, it is a (Z)-alkene

  • If the higher ranked atoms/groups are on opposite sides, it is a (E)-alkene

Geometric Isomerism in Alkenes

Why does geometric isomerism exist in Alkenes?

  • The carbons in the double bond are sp2 hybridized, so they form one sigma bond and one pi bond.

  • Sigma bonds have freedom of rotation, but pi bonds do not.

  • Therefore, the groups attached to the double bond are locked in place.

  • If there are different groups attached to the double bond, this makes for 2 possible arrangements.

Cis- and Trans- naming

  • If the molecule has an identical pair of atoms on the same side of the double bond, it is a **cis-alkene.

  • If the molecule has an identical pair of atoms on opposite sides of the double bond, it is a **trans-alkene.

  • The problem with using the cis-trans system is that you need at least 1 identical pair of atoms.

  • So we sometimes use another system of naming

E-Z Naming

  • First you have to rank the substituents based on their atomic number. Elements with the higher atomic number get higher priority.

    • E.g. a chlorine atom is ranked higher than an ethyl group which is ranked higher than a hydrogen

  • If the higher ranked atoms/groups are on the same side, it is a (Z)-alkene

  • If the higher ranked atoms/groups are on opposite sides, it is a (E)-alkene

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