polienos

Polienos, also known as polyenes, are hydrocarbons that contain two or more carbon–carbon double bonds (C=C) along their carbon skeleton. They can be acyclic (open chains) or cyclic, and their double bonds may be arranged in different patterns. When the double bonds alternate with single bonds along a chain, the system is conjugated; when double bonds are consecutive, the system is cumulene. The simplest examples are butadiene (two double bonds) and hexatriene (three double bonds). Longer conjugated chains occur in natural pigments such as beta-carotene and lycopene, and form the basis of many synthetic dyes and organic semiconductors.

Structure and types: Conjugated polyenes feature delocalized π electrons across the chain, which lowers the energy gap between the highest occupied and lowest unoccupied molecular orbitals and often gives vivid colors. They can exhibit cis–trans (E/Z) isomerism around each double bond, influencing stability, color, and reactivity. Polyenes are generally more reactive toward electrophiles at the C=C sites and participate in polymerization and various cycloaddition or rearrangement reactions.

Synthesis and occurrence: Industrially, polyenes are commonly prepared by elimination reactions from vicinal dihalides or by coupling methods that assemble conjugated chains. In nature, polyenes appear as pigments and signaling molecules; carotenoids like beta-carotene and lycopene play roles in photosynthesis and photoprotection. In materials science, conjugated polyene chains underpin conducting polymers such as polyacetylene and related oligomers used in organic electronics and solar cells.

See also: carotenoids, polyacetylene, conjugated polymer, dye.