Transisomerization

Transisomerization, also called cis–trans isomerization or E/Z isomerization, is the chemical process by which a molecule with a carbon–carbon double bond or another restricted rotation interconverts between a cis (same side) configuration and a trans (opposite side) configuration. The two forms differ in the relative positions of substituents around the double bond, often yielding different physical, chemical, and biological properties.

Mechanisms of transisomerization vary with the system. Rotation around a carbon–carbon double bond is not permitted under normal conditions, so interconversion typically requires energy input. Thermal isomerization proceeds via higher-energy excited states or alternative pathways, sometimes aided by catalysts that enable rearrangements such as hydrogen shift or metallo-catalyzed isomerization. Photochemical isomerization uses light to excite the molecule, allowing rotation around the double bond; photosensitizers can transfer energy to the substrate to promote the switch. In some cases, radical or coordination chemistry with metals can facilitate the process.

Thermodynamics and kinetics play key roles. For many unhindered alkenes, the trans isomer is more stable than the cis form due to reduced steric hindrance, shifting the equilibrium toward trans at standard conditions. The rate of isomerization depends on substituents, solvent, temperature, and whether a catalyst or light is employed. Controlled isomerization is often achieved by selecting appropriate irradiation wavelengths or catalysts to favor the desired configuration.

Applications and significance include the control of material properties in polymers and liquid crystals, tuning the activity and selectivity of pharmaceuticals, and studying reaction mechanisms in organic synthesis. Cis–trans isomerization is also a consideration in the stability of photoactive compounds exposed to ambient light or heat.