reactionselectrocyclic

Reactionselectrocyclic refers to electrocyclic reactions, a class of pericyclic reactions in which a conjugated polyene undergoes ring closure or ring opening with the conversion of a pi bond into a sigma bond, or vice versa. The process is concerted, proceeding through a single transition state without discrete intermediates. These reactions are governed by orbital symmetry, described by Woodward–Hoffmann rules, which predict the stereochemical outcome based on electron count and reaction conditions.

Key features include the distinction between thermal and photochemical conditions. Under thermal conditions, 4n pi electrons drive ring closure or opening in a conrotatory mode, while 4n+2 pi electrons favor a disrotatory mode. Light (photochemical activation) inverts these patterns: 4n pi electrons favor disrotatory closure for photochemistry, and 4n+2 pi electrons favor conrotatory closure. The terms conrotatory and disrotatory refer to the directions in which the terminal p orbitals rotate during bond formation or cleavage. Substituents and stereochemical requirements of the starting polyene can influence rate and outcome but do not alter the fundamental orbital-symmetry rules.

Common examples include the thermal ring opening of cyclobutene to butadiene (4 pi electrons, conrotatory) and the thermal ring closure of hexatriene to cyclohexadiene (6 pi electrons, disrotatory). Electrocyclic reactions are utilized in synthesis to construct or fragment cycloalkenes and in photoresponsive materials, where controlled ring formation or cleavage can alter physical properties.