What is E-Z isomerism?

In stereoisomers, the atoms are joined in the same order, but they have a different spatial arrangement.

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.

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.

One way to remember the designations is to think of `Z` as Zame Zide (same side).

Since `"CH"_3` has higher priority than `"H"`, it is pretty easy to determine that the butene isomers above are `E` and `Z`, respectively.

But what about the isomers of 2-methylbut-2-enoic acid (below)?

`"CH"_3` has higher priority than `"H"`, and `"COOH"` has higher priority than `"CH"_3`.

Angelic acid has the higher priority groups on the same side, so it is the `Z` isomer.

Tiglic acid has the higher priority groups on opposite sides, so it is the `E` isomer.