Question

Why does the E-Z notation replace the limited cis-trans notation?

Answer 1

The `E-Z` notation replaces the old cis-trans notation because it is applicable to many more examples of stereoisomerism that involve restricted notation.

Explanation:

The cis-trans notation works only when you have the same two groups on each end of the double bond, as in but-2-ene.

Here, it is obvious which is the cis isomer and which is trans.

But what about 3-methylpent-2-ene?

Some chemists say this is trans, because the `"CH"_3` groups are on opposite sides of the double bond.

Others say this is cis, because they follow the longest chain through the double bond, and both of its parts are on the same side.

The `E-Z` system is unambiguous. This is the `Z` isomer!

And what about molecules like those below?

The cis-trans notation doesn't apply here at all.

But you can easily determine that the one on the left is `Z` and the one on the right is `E`.