conjugated polyene

A conjugated polyene is any molecule in which double bonds alternate with single bonds. That is to say:

  H H H H H
  | | | | |
H-C=C-C=C-C-H
          |
          H

is conjugated, but

  H H H H H
  | | | | |
H-C=C-C-C=C-H
      |
      H

is not. Conjugated polyenes include many biological pigments, and cyclic conjugated polyenes include the famous benzene.

Conjugated polyenes are more stable than unconjugated polyenes. Hybridization theory explains that this is because in a conjugated polyene all of the single C-C bonds result from the overlap between 2 sp² orbitals instead of between one sp² orbital and one sp³ orbital. Because sp² orbitals have more "s" character than sp³ orbitals do they are closer to the C nucleus and form bonds that are shorter and stronger.

Molecular orbital theory explains conjugated polyene stability by saying that Ψ₁, the lowest energy bonding molecular orbital of such an molecule's electron system, has a favorable bonding interaction between even the ostensibly singly bonded carbons of the system. This leads to delocalization of the molecule's Π electrons over a Π framework connecting each carbon atom. In nonconjugated polyenes, Π electrons are localized, leading to a molecule with more energy and less stability.

Conjugated polyenes can be prepared via the base-induced elimination of H-X from an allylic halide, by thermal cracking of an alkane over a metal oxide catalyst, or by acid-catalyzed double dehydration of a diol.