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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.

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