Electrocyclic Reactions
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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

 

 

 

 

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Thursday, April 06, 2017 13:18