Why does E-Z isomerism occur?

1 Answer
Jun 11, 2015

#E-Z# isomerism occurs because there is restricted rotation about double bonds.

Explanation:

In #E-Z# isomers you must have:

  • restricted rotation, often involving a #"C=C"# double bond
  • two different groups on one end of the bond and two different groups on the other end.

For example, an alkene such as but-2-ene has two different groups on each alkene carbon.

It can exist as #E-Z# isomers that differ in the positions of the substituents on the double-bonded atoms.

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The substituents can be given "priorities", with atoms with higher atomic numbers given higher priorities (the Cahn-Ingold-Prelog rules).

If the highest priority groups for each carbon are on the same side of the molecule, we have the #Z# isomer.

If the highest priority groups for each carbon are on opposite sides of the molecule, we have the #E# isomer.

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