Although most organic reactions take place by way of ionic or radical intermediates, a number of
useful reactions occur in one-step processes that do not form reactive intermediates.
• A pericyclic reaction is a concerted reaction that proceeds through a cyclic transition
state.
Pericyclic reactions require light or heat and are completely stereospeci! c; that is, a single
stereoisomer of the reactant forms a single stereoisomer of the product. We will consider two
categories of pericyclic reactions: electrocyclic reactions and cycloadditions.
An electrocyclic reaction is a reversible reaction that can involve ring closure or ring opening.