Definitions of Concerted Reactions

1. Electrocyclic reactions:

   The cyclization of a q p-electron system to a (q -2) + 2 s-electron system or the ring opening of a (q -2) + 2 s-electron system to a q p-electron system

   1. Ring Closing (4n + 2 electrons):

      

      

   2. Ring Closing (4n electrons):

      

      

   3. Ring Opening (4n + 2 electrons):

      

      

   4. Ring Opening (4n electrons):

      

      

      Practice Problems:

      Classify each of the following as disrotatory or conrotatory.

      

      

      

       

       

2. Cycloaddition reactions:

   The addition of an x p-electron system plus a y p-electron system plus ... to produce a new ring, or the breaking of a ring into an x p-electron system plus a y p-electron system plus .... They are designated as [x + y + ...] cycloaddition (or cycloreversal) reactions.

   

   

   

    

    

3. Sigmatropic Reactions

   The migration of a s bond next to a conjugated p system from one part of the p system to another part. The system is numbered by starting at the atoms attached to the migrating s bond.

   

   The reaction is designated as an [i,j] sigmatropic shift when the bond migrates from between atoms [1,1] to between atoms [i,j].