Question

Why does E-Z isomerism occur?

Answer 1

`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.

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.