Geometric isomers are formed when the rotation of the Carbon-Carbon bond is restricted either due to a double bond or the carbon-carbon single bond is a part of a chain.
There are two conventions to assign the configuration of a geometric isomer
- Cis and Trans
- E/Z (Including Cahn Ingold Prelog rules)
Cis and Trans
This notation is used when both carbons have the same substituent. If the substituents are on the same side, then the isomer is cis else it is trans.
The physical properties of cis and trans isomers vary.
In an acyclic molecule,usually the stability of a trans isomer is higher than a cis isomer because of steric hindrance between the substituents in the cis isomers.
When the carbons have different substituents, cis/trans nomenclature cannot be used. Here, E/Z is used.
E/Z Convention
First we need to understand how to assign priority to the substituents to denote the E/Z configuration. The priority is denoted by following Cahn Ingold Prelog rules.
- Higher atomic number- higher priority
This means if the carbon has two substituents- Cl and F. Cl has a higher atomic number than F, so, Cl gets higher priority.
- In case isotopes of the are present, higher priority is given to the isotope which possesses higher atomic weight.
If the carbon is attached to C¹² and C¹³, higher priority is given to the isotope having higher atomic mass. So C¹³ gets higher priority.
- If the atoms are identical, the first point of difference is found and the chains are compared.
Here, the alkene carbon on the left has two substituents consisting of atoms
- C, H
- C, C, Cl
Since Cl has a higher atomic number, this group gets a higher priority.
The alkene carbon on the right has two substituents consisting of the atoms,
-C,O
-C,C
Oxygen has a higher atomic number, so this group gets a higher priority
- Multiple bonds are considered multiples of the same atom.
Each carbon of the triple bond is considered to be attached to two atoms of carbon.
In E/Z nomenclature,
E stands for Entgegen which, in German, means opposite.When groups of higher priority are on the opposite sides of the double bond the bond is assigned the configuration E.
Z stands for Zusammen which, in German, means together.When groups of higher priority are on the same side of the double bond the bond is assigned the configuration Z.
For the alkene carbon on the left, Br gets a higher priority over Cl due to its higher atomic number.
For the alkene carbon on the right, C gets a higher priority over H due to the atomic number.
In the first compound, the higher priority groups are on the same side, so, Z configuration. Whereas, in the second compound, higher priority groups are on the opposite , so, E configuration.
E/Z notation is preferred over cis/trans.
