Why does E-Z isomerism occur?

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Answer 1 · 2015-06-11T19:22:27

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

In $E - Z$ $E-Z$ isomers you must have:

restricted rotation, often involving a $C=C$ $"C=C"$ 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 $E - Z$ $E-Z$ 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 $Z$ $Z$ isomer.

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

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