Illustrated Glossary of Organic Chemistry

Concerted reaction: A reaction in which all bond changes (new bonds formed and old bonds broken) occurs in a single mechanism step.

This Diels-Alder reaction is a concerted cycloaddition reaction. Two new sigma bonds and one pi bond are formed at the same time as three pi bonds are lost.


This S_N1 reaction is not concerted because three mechanism steps (carbon-leaving group ionization, nucleophile capture, and deprotonation) are required to convert tert-butyl bromide (the reactant) into tert-butanol (the product).