cis/trans nomenclature is effective only when the alkene has two different groups on each carbon atom of the double bond and each carbon has one of the same group.
We saw that cis and trans fails in rings when the two carbons lacked a common substituent. It also fails for alkenes under these circumstances. (detailed explained inside the lecture).
Thankfully, we can apply the ranking system developed by Cahn, Ingold, and Prelog for chiral centers for this purpose. The protocol is as follows:
Each carbon in the pi bond is attached to two substituents. For each carbon, these two substituents are ranked (1 or 2) according to the atomic numbers of the atom directly attached to the carbon. (e.g. Cl greater F )
If both substituents ranked 1 are on the same side of the pi bond, the bond is given the descriptor Z (short for German Zusammen, which means “together”).
If both substituents ranked 1 are on the opposite side of the pi bond, the bond is given the descriptor E (short for German Entgegen, which means “opposite”).
So Z resembles “cis” and E resembles “trans”. (Note: they are not necessarily the same and do not always correlate:)The E/Zsystem is comprehensive for all alkenes capable of geometric isomerism, including the cis/trans alkene examples above. We often use cis/trans for convenience, but E/Z is the “official”, IUPAC approved a way to name alkene stereoisomers].
One easy way to remember Z is to say “Zee Zame Zide” in a German accent. My way of doing it was pretending that the Z stands for “zis”. Whatever works for you.
An Important Bottom Line:
cis-trans- is OK for describing simple alkene stereoisomers, but only works in certain cases. Furthermore, it only gives relative configurations. The E/Z system is comprehensive and describes the absolute configuration of the molecule.
See below for an example of an E alkene which is “cis” and a Z alkene which is “trans”.
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