Difference Between E and Z Isomers

The key difference between E and Z isomers is that E isomers have the substituents with higher priority in the opposite sides whereas the Z isomers have the substituents with higher priority on the same side.

The E-Z nomenclature is a notation system to name different isomers having the same chemical formula, but different spatial arrangements. Furthermore, the E and Z isomers are alkenes. These isomers get their name based on the position of the substituents attached to the double bond of the alkene.

CONTENTS

1. Overview and Key Difference
2. What are E Isomers
3. What are Z Isomers
4. Side by Side Comparison – E vs Z Isomers in Tabular Form
5. Summary

What are E Isomers?

The E isomers are alkenes having the substituents with higher priority on the opposite sides of the double bond. The letter “E” comes from entgegen in German, which means “opposite”. The basis of E-Z notation is a set of rules known as priority rules. They are the Cahn-Ingold-Prelog (CIP) rules. These are a set of rules to name the organic molecules, to specify them unequally.

The steps of naming a molecule using CIP rules are as follows;

1. Identify chiral centers or double bonds present in the molecule.
2. Determine the priorities of the substituents attached to the chiral center or the double bond.
3. Use either R/S system or E/Z system to name the compound.

The Priorities of the Substituents

• First consider the atoms directly bonded to the chiral center or the double bond – higher the atomic number, higher the priority
• If there are equal atoms present, then there is a tie. Then check the substituent groups to find a point of difference in atomic number.
• If still there is a tie, consider the atoms bonded to each of the atoms in the main chain and check whether there is a point of difference.

In the above image, the E isomer has high priority substituents on the opposite sides of the double bond whereas the Z isomer has those substituents on the same side.

When determining the priority of the substituents, first consider the atoms directly bonded to the double bond; in the above example, there are three Carbon atoms (C) and one hydrogen atom (H). Therefore, there is a tie because one of the two vinyl carbon atoms (carbon atoms in the double bond) have directly bonded carbon atoms. Then, to determine the high priority group, consider the atom that comes after these directly bonded carbon atoms. Since the substituent group attached to this vinyl carbon are a methyl group (-CH₃) and an ethyl group (-CH₂CH₃), the priority is given to the ethyl group. That is because the atom comes after the directly bonded carbon atom (directly bonded to the vinyl carbon) is a hydrogen atom in methyl group and a carbon in the ethyl group.

What are Z Isomers?

Z isomers are alkenes having the substituents with higher priority on the same side of the double bond. The letter “Z” comes from zusammenin German, which means “together”.

In the above image, the substituents with high priority are on the same side of the double bond in the Z isomer whereas the E isomer has those substituents in the opposite sides. Furthermore, the CIP rules determine the priority of these substituents. For the above example, the atoms directly bonded to the double bonded carbon are Carbon atoms (C) of methyl groups and hydrogen atoms (H). Since carbon atom (14) has a high atomic number when compared with hydrogen (1), the high priority is to the methyl group (-CH₃).

What is the Difference Between E and Z Isomers?

Summary – E vs Z Isomers

E-Z notation or nomenclature is used to name isomers having the same molecular formula and spatial structure, giving each isomer a uniqueness. The difference between E and Z isomers is that the E isomers have the substituents with higher priority in the opposite sides whereas the Z isomers have the substituents with higher priority on the same side.

Reference:

1. “E-Z Notation.” Wikipedia, Wikimedia Foundation, 10 Apr. 2018. Available here  
2. Libretexts. “19.7: E,Z Notation.” Chemistry LibreTexts, Libretexts, 26 June 2017. Available here 
3. Libretexts. “3.6 Cahn-Ingold Prelog Rules.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here 

Image Courtesy:

1.’EZNotation’By Pete Davis – Own work, (Public Domain) via Commons Wikimedia 
2.’EZNotation’By Pete Davis – Own work, (Public Domain) via Commons Wikimedia