Organic Chemistry/Introduction to reactions/Pericyclic reactions

Pericyclic reactions are 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 the π bond electrons are involved, and all bonds are changed in a single cyclic step without any intermediates being formed.

Using the Diels-Alder reaction 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 a concerted reaction.

The types of pericyclic reactions are:

Cycloadditions (including the Diels-Alder reaction)
Electrocyclic reactions
Sigmatropic rearrangements