# Why does E-Z isomerism occur?

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 $\text{C=C}$ 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.