Cisalkeenien

Cisalkeenien, or cis alkenes, are alkenes in which the two substituents attached to the carbon atoms of the carbon–carbon double bond occupy the same side of the double bond. This geometry is a form of geometric isomerism arising from restricted rotation about the double bond. The opposite arrangement, where the substituents lie on opposite sides, is called trans alkenes.

In contemporary terminology the cis configuration corresponds to the Z-notation under the Cahn–Ingold–Prelog system, while the trans configuration corresponds to E. For simple alkenes, the terms cis and trans are still widely used, but for more complex molecules the E/Z system provides unambiguous designation.

Cis alkenes often differ in physical properties from their trans counterparts due to differences in dipole moment and molecular shape. The cis form typically has less symmetry and can experience greater steric strain from substituents on the same side, which can influence melting and boiling points and reactivity. Thermodynamically, trans is commonly more stable than cis for disubstituted alkenes, though exceptions occur, especially in cyclic systems where ring strain and substituent interactions play a role.

Synthesis and separation of cis alkenes rely on selective reactions and stereochemical controls, such as partial hydrogenation, isomerization under catalytic conditions, or separation by chromatography or distillation after formation. They occur in natural products and synthetic compounds and are important in fields ranging from organic synthesis to materials science.

Common examples include cis-2-butene and cis-1,2-dichloroethene; cis configurations also appear in agricultural, pharmaceutical, and polymer-related contexts.