Pericyclic Reaction

A pericyclic reaction is a concerted reaction in which the number of rings in the transition state is greater than the total number of rings in the reactant molecules.

eg:  Cope Rearrangement



mechanism:



# covalent bonds broken = 3
# covalent bonds formed = 3
# covalent bonds broken + # covalent bonds formed = 3 + 3 = 6
# steps = 1
Thus, the reaction is a concerted reaction.  The transition state of the reaction can be drawn roughly as follows.



# rings in the reactant molecule = 0
# rings in the transition state = 1
# rings in the transition state > # rings in the reactant molecule
Thus, the reaction is a pericyclic reaction.  For more examples of pericyclic reactions, see Cope elimination,  Claisen rearrangement, Diels-Alder reaction

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