CHEMISTRY COMMUNITY

What's the difference between cis- and Z-, and between trans- and E-?

E and Z are part of the IUPAC naming guidelines. But they mean the same thing.

Are you sure they mean the same thing? Because in class I was told that cis and trans are for identical substituents and the longest chains, while E and Z are for highest priority/highest atomic #, but I just need clarification on what "longest chain" and "highest priority" mean.

From what I understand, the "longest chain" refers to the longest continuous chain of carbon-carbon bonds that you identify as the main chain, which will be determined by the number of carbons in that chain (1C is meth-, 2C is eth-, 3C is prop-, etc.) In terms of deciding whether a particular molecule is E or Z, you look at what is bonded to each carbon of the carbon-carbon double bond, and the atom with the higher atomic number is what is given the "higher priority". You compare the "higher priority" groups on each carbon, and if they are on the same side of the double bond, you label the molecule Z. Conversely, if the higher priority groups on each carbon are on opposite sides of the double bond, then you label the molecule E.

Isn't longest chain just referring to your longest carbon chain and the highest priority referring to the functional group attached to the molecule?