Pericyclic reactions are separated into two classes based on the number of electrons that change molecular orbitals in the transition state. Thus, we must account for either 4n or 4n+2 π electrons where n is an integer. We must also know which molecular orbital is involved in the reaction and recognize its symmetry properties. Mechanistically, pericyclic reactions are divided into three classes: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements.