zisomeres

Zisomeres, often written as Z isomers, are a class of geometric isomers observed in alkenes and related systems. They occur when the substituents of higher priority on each carbon of a double bond are on the same side of the molecule, according to the Cahn–Ingold–Prelog (CIP) priority rules. The converse arrangement, where the high-priority groups lie on opposite sides, is the E isomer. In simple cases, Z isomers are sometimes described using the older cis designation, while E corresponds to trans.

Common examples include cis-2-butene, which is the Z isomer, and trans-2-butene, which is the E isomer. Other familiar Z/E pairs include maleic acid (Z) versus fumaric acid (E) and stilbene derivatives. The Z configuration can influence physical properties such as dipole moment, boiling point, and reactivity compared with the corresponding E isomer.

Properties of zisomeres often reflect steric and electronic effects from substituents. E isomers are frequently more thermodynamically stable due to reduced steric clash, though this is not universal. Z isomers can exhibit larger dipole moments and, in some cases, distinct biological or optical properties.

Characterization commonly uses nuclear magnetic resonance (NMR) spectroscopy, where vicinal vinyl coupling constants (J) differ between cis and trans arrangements, and ultraviolet-visible (UV-Vis) spectroscopy, which can show shifts in absorption. Interconversion between Z and E isomers typically requires breaking the double-bond pi bond or photochemical isomerization; in many systems, light or heat can drive the process, enabling applications as molecular switches or in responsive materials.

See also: E isomer, cis/trans isomerism, geometric isomerism, CIP priority rules.