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|
- Extended Pi systems may cyclize in a Pericyclic manner
to gain a sigma bond and loose a Pi bond.
- By the reverse of the same mechanism, cyclobutenes will
revert to butadiene systems but can be created by using
ultraviolet light on butadienes.
- Hexatrienes and Octatetraenes favorably form
cyclohexadienes and cyclooctatrienes, respectively, with
heat or light.
- Because the phases of the ends of the HOMO (the
molecular equivalent of a valence shell) of the Pi system
must match to undergo this intra-molecular reaction, the
groups at the end of the Pi system rotate con-rotatory for
4n Pi electron systems and dis-rotatory for 4n + 2 Pi
electron systems with heat (some times spontaneously without
light).
- Because light converts the LUMO into the HOMO for
extended Pi systems, the groups at the end of the Pi system
rotate dis-rotatory for 4n Pi electron systems and con-rotatory
for 4n + 2 Pi electron systems with light.
| Pi electrons\Conditions |
Heat (not light) |
Light |
| 4n, n = 1, 2, ... (e.g. butadiene) |
conrotatory |
disrotatory |
| 4n+2, n = 0, 1, ... |
disrotatory |
conrotatory |
|
|
Electrocyclic Reactions