Let's take a look at how cis/trans affects alkenes.

Are these two molecules the same?

Yes they are! Because this is a molecule, you can rotate this. So let's look at it. If you take the first molecule, and rotate it on the x-axis, then rotate it on the y-axis, you get the second molecule. They are the same. They are both 2-propene.

Are these two the same?

The answer is NO. No matter how many times you rotate the molecule, you will not end up with the same molecules. If you remember from our alkane tutorial, one of these is cis and one of these is trans. Which one is which?

The one on the LEFT is trans, because the methyl groups are on the opposite ends. The one on the right is a cis, because the methyl- groups are located on the same side. The molecule on the left is called trans-2-butene, while the right one is called cis-2-butene.

A general note: The double bond is rigid and is not able to freely rotate, unlike single bonds.

The Zusammen/Entgegen System

Cis/trans is a neat system for simple alkenes, but when you start getting more than one substituent, it gets confusing. So, the smart people over at the IUPAC made up another system, called the E-Z system.

Entgegen, which is the German word for opposite, is used when substituents are located on opposite sides of the double bond, much like the molecule on the right. Instead of having to write out the word, you can just write in E- before the molecule. Hence, the molecule on the right would be called E-1-bromo-2-chloroethene.

Zusammen means "together" in German. You use the Z- prefix when you have substituents on the same side, as seen in the molecule on the left. Hence, that molecule would be called Z-1-bromo-2-chloroethene.

E/Z isn't necessary used for substituents. When drawing -enes, you tend to get a H/X figure (mix the H/X and you get the idea). So parts of the main chain can seem to be a substituent, but they are not. If you have three groups off the double bond, you use the E-Z system to reference the higher priority groups. Let's look at some:

Name them. The one on the left is called Z-3-methyl-2-heptene. The one on the right is E-3-methyl-2-heptene. The groups that you were looking for were the higher priority groups. Since C₂H₅ and C₃H₇ take priority over the CH₃, you used those as your reference in the E/Z naming.