 Woodward–Hoffmann rules

The Woodward–Hoffmann rules are a set of principles used in organic chemistry to predict the outcome of pericyclic reactions, which are reactions that involve cyclic transition states and occur via a concerted mechanism (simultaneous bond-breaking and bond-forming processes). Named after chemists Robert Burns Woodward and Donald Albert Hoffmann, these rules provide a framework to determine whether a pericyclic reaction is thermally allowed or forbidden based on molecular orbital (MO) theory.