124 Pericyclic reactions
Pericyclic reactionsare one of the three major classes of organic reactions, along with
polar/ionic reactions and radical reactions. Pericyclic reactions have been understood only
relatively recently compared to the other classes.
In a polar/ionic reaction, one reactant (the nucleophile) donates two electrons to another
(the electrophile) to form a bond. In a radical reaction, each reactant donates one electron.
In a pericyclic reaction, only theπbond electrons are involved, and all bonds are changed
in a single cyclic step without any intermediates being formed.
Figure 216
Using the Diels-Alder reaction^1 as an example, pushing arrows are used to show electron
flow, but they are drawn in a circle, and the direction of the arrows can arbitrarily be shown
as clockwise or anticlockwise. All the bonds are made and broken in a single step. This is
called aconcertedreaction.
The types of pericyclic reactions are:
- Cycloadditions (including the Diels-Alder reaction)
- Electrocyclic reactions
- Sigmatropic rearrangements
1 Chapter 125 on page 395