cisalkenes

cisalkenes are alkenes in which the two substituents attached to the carbon–carbon double bond reside on the same side of the double bond. This stereochemical arrangement is most familiar in disubstituted alkenes, where the isomer is commonly referred to as the cis isomer. In modern nomenclature the configuration is described by the CIP-based Z/E system, with Z (zusammen, “together”) indicating the same-side arrangement of the higher-priority substituents and E (entgegen, “opposite”) the opposite arrangement. For simple cases, cis corresponds to Z.

Cis alkenes differ in stability and physical properties from their trans counterparts. They are typically less thermodynamically stable because steric repulsion between substituents on the same side raises the energy of the double bond. This difference is reflected in heats of hydrogenation and sometimes in boiling and melting behavior; cis isomers often have higher dipole moments due to lack of symmetry.

Synthesis and interconversion: cis alkenes can be prepared by selective hydrogenation of alkynes with poisoned catalysts such as Lindlar catalyst, which adds hydrogen syn across the triple bond to stop at the cis product. Trans alkenes can be formed by hydrogenation with regular catalysts or by isomerization of cis alkenes under heat or metal catalysts.

Examples and occurrence: common cis alkenes include cis-2-butene and the cis-butenedioic acid isomer, maleic acid. cis alkenes appear in natural products, pharmaceuticals, and as intermediates in organic synthesis.

See also: E/Z nomenclature; alkene; hydrogenation; stereochemistry.