E-Z notation

E–Z notation is a system used in organic chemistry to describe the stereochemistry of double bonds in alkenes and other similar compounds. It is an extension of the Cahn-Ingold-Prelog priority rules and provides a clear and unambiguous method for naming the stereoisomers of alkenes.

Introduction

The E–Z notation is particularly useful for alkenes with more than two substituents, where the traditional cis-trans notation becomes inadequate. The notation is based on the priority of the substituents attached to the carbon atoms of the double bond.

Cahn-Ingold-Prelog Priority Rules

The E–Z system uses the Cahn-Ingold-Prelog priority rules to assign priorities to the substituents attached to the double-bonded carbons. The substituents are ranked based on atomic number, with higher atomic numbers receiving higher priority. If the atomic numbers are the same, the next atoms in the substituents are compared, and so on.

Determining E or Z

Once the priorities of the substituents are determined, the configuration of the double bond is assigned as either E or Z:

• E (from the German entgegen, meaning "opposite") is assigned when the highest priority substituents on each carbon are on opposite sides of the double bond.
• Z (from the German zusammen, meaning "together") is assigned when the highest priority substituents on each carbon are on the same side of the double bond.

Example

Consider the alkene with the following substituents:

• Carbon 1: Methyl group (CH₃) and ethyl group (C₂H₅)
• Carbon 2: Chlorine (Cl) and hydrogen (H)

Using the Cahn-Ingold-Prelog rules, chlorine has the highest priority on carbon 2, and the ethyl group has the highest priority on carbon 1. If these groups are on opposite sides of the double bond, the configuration is E. If they are on the same side, the configuration is Z.

Applications

E–Z notation is widely used in organic synthesis, medicinal chemistry, and pharmacology to describe the stereochemistry of compounds. It is crucial for understanding the biological activity of stereoisomers, as different configurations can lead to different pharmacological effects.

Also see

• Cis-trans isomerism
• Stereochemistry
• Cahn-Ingold-Prelog priority rules
• Alkene
• Organic chemistry