Transalkenes

Transalkenes, commonly called trans alkenes, are alkenes in which the substituents attached to the two carbon atoms of the carbon–carbon double bond lie on opposite sides. In simple cases such as trans-2-butene or trans-1,2-dichloroethene, the geometry is described by the older term trans; in modern nomenclature the stereochemistry is designated by the E/Z system, and for disubstituted alkenes trans corresponds to the E isomer when the higher-priority groups are opposite.

The trans arrangement results in a more extended, often more thermodynamically stable geometry compared with the cis isomer, because it reduces steric interactions between substituents on adjacent carbons. The energy difference between cis and trans forms varies with substituents and can influence the relative abundance of isomers at equilibrium.

Synthesis and interconversion methods include selective hydrogenation of alkynes to give trans-alkenes under appropriate catalytic conditions, and transformations that convert cis alkenes to trans under heat, light, or catalytic isomerization. Trans alkenes also arise by various stereospecific reactions in organic synthesis, and can be obtained from ring-opening or rearrangement processes that impose anti geometry.

Properties of transalkenes depend on substituents rather than geometry alone. They are typically less flexible than their cis counterparts and may exhibit different boiling points, melting points, and solubilities. Like other alkenes, transalkenes participate in standard alkene chemistry, including addition reactions, though reaction rates and stereochemical outcomes can be influenced by the trans geometry.

See also: cis alkenes, E/Z nomenclature, alkenes.