Orbital-symmetry rules associated with Robert Burns Woodward and Roald Hoffmann for concerted reactions. Ground-state and photochemically excited occupations can have different allowed stereochemical modes; an allowed mode does not determine every kinetic detail or guarantee a large reaction rate.
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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.