E/Z and CIP

Cahn-Ingold-Prelog (CIP) Priority Rules or Convention

Used to describe configuration of stereoisomers

1) alkene E/Z designation
2) tetrahedral stereocenters R/S designation
3) axial chirality R/S designation


I) Alkene E/Z Designation

We must prioritize and compare R1 and R2, and then separately R3 and R4.  Based on priorities we can then assign E/Z.  The configuration is considered Z, if the highest priority substituents are on the same side (Zon Zame Zide  - On Same Side is Z).

a) Priority is first assigned based on the atomic number (or atomic mass for isotopes) of the first atom connected to each sp² carbon.

Z configuration, since the highest priority are "zon zhe zame zide"

b) If priority can't be established from the first atom, proceed to next atom and so on until a difference is realized.

On the left side the priorities are tied until we reach the 3rd carbon atom.  The 3rd carbon on top/left is connected to another C and 2 H's, while the 3rd C on bottom/left is connected to 3 H's.

On the right side the first difference is realized at the 2nd C atom.  On the bottom/right the 2nd atom is O (atomic #8) versus on the top/right where the 2nd atom is C (atomic # 6).  The bottom right winds.

Therefore Z configuration

c) Double and triple bonds are considered as if they are bonded to two or three single atoms.

Here are a few examples.

Example

E configuration