E–Z notation

E–Z notation is a system used in organic chemistry to denote the stereochemistry of double bonds in organic compounds. This notation helps in specifying the configuration of double bonds relative to the substituents attached to the carbon atoms involved in the bond. The E–Z system is an extension of the earlier Cahn–Ingold–Prelog priority rules (CIP rules), which are used to assign priorities to substituents attached to a double bond or chiral center.

Overview

The terms "E" and "Z" are derived from the German words "Entgegen" and "Zusammen", respectively, which translate to "opposite" and "together". In the E–Z notation, a double bond is classified as "E" if the highest priority substituents on each carbon are on opposite sides of the double bond. Conversely, a double bond is classified as "Z" if the highest priority substituents are on the same side of the double bond.

To determine the priority of substituents, the Cahn–Ingold–Prelog priority rules are applied. These rules consider the atomic number of the atoms directly attached to the double-bonded carbons. If there is a tie, the rules consider the atomic number of the atoms or groups attached to the directly attached atoms, and so on, until a difference is found.

Application

E–Z notation is widely used in the naming of alkenes, cycloalkenes, and other organic molecules containing double bonds. This system is particularly useful for molecules that are not adequately described by the older cis-trans isomerism notation, which is limited to cases where the substituents are identical or when the molecule can be divided into two halves in a symmetrical manner.

Examples

Consider the molecule but-2-ene, which can exist in two forms:

• If the methyl groups (−CH₃) are on the same side of the double bond, the molecule is named (Z)-but-2-ene.
• If the methyl groups are on opposite sides of the double bond, the molecule is named (E)-but-2-ene.

Advantages

The E–Z notation provides a clear and unambiguous way to describe the stereochemistry of double bonds in complex molecules. It is universally recognized and can be applied to a wide range of organic compounds, making it an essential tool in organic chemistry.

Limitations

While the E–Z notation is highly useful, it requires knowledge of the Cahn–Ingold–Prelog priority rules, which can be complex to apply in cases of highly substituted double bonds. Additionally, the notation does not provide information about the absolute configuration (R or S) of chiral centers in the molecule.

See Also

• Stereochemistry
• Isomerism
• Cis-trans isomerism
• Cahn–Ingold–Prelog priority rules
• Alkene