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 sp
2 orbitals instead of between one sp
2 orbital and one sp
3 orbital. Because sp
2 orbitals have more "s" character than sp
3 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 Ψ
1, 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.