EZisomerism

EZisomerism is a system used to describe the configuration of substituents around a double bond in organic chemistry. It is particularly relevant for alkenes and other compounds containing double bonds, where free rotation is restricted. The system is an extension of the older cis-trans isomerism nomenclature, offering a more precise description when more than two different substituents are present.

To determine the E or Z configuration, priority rules are applied to the substituents on each atom of the double bond. These rules, known as the Cahn-Ingold-Prelog priority rules, assign a priority (higher or lower) to each substituent based on atomic number. If the two higher-priority substituents are on the same side of the double bond, the isomer is designated Z, derived from the German word "zusammen," meaning together. If the two higher-priority substituents are on opposite sides of the double bond, the isomer is designated E, derived from the German word "entgegen," meaning opposite.

The EZ nomenclature is crucial for understanding the stereochemistry of many organic molecules, including pharmaceuticals and natural products, as different isomers can exhibit distinct physical, chemical, and biological properties. For simple cases with only two different substituents on each carbon of the double bond, EZ isomerism is equivalent to cis-trans isomerism. However, when there are three or four different substituents, the EZ system provides an unambiguous way to differentiate between the possible geometric isomers.