Pericyclic reaction

A concerted reaction in which bonding changes occur through a cyclic arrangement of interacting orbitals, without discrete ionic or radical intermediates.

Woodward–Hoffmann rules: Rules that predict whether concerted pericyclic reactions are allowed by orbital symmetry. They connect orbital symmetry to the thermal and photochemical pathways available to pericyclic reactions.
Molecular orbital: A wavefunction describing the spatial distribution and energy of an electron in a molecule. Pericyclic mechanisms are described by the changing shapes and phases of molecular orbitals.
Diels–Alder reaction: A [4+2] cycloaddition between a conjugated diene and a dienophile that forms a six-membered ring. This widely used cycloaddition constructs six-membered rings and multiple stereocenters in one step.
Nucleophilic substitution: A reaction in which a nucleophile replaces a leaving group at an electrophilic center. Unlike a pericyclic pathway, substitution commonly involves a localized reaction center and may proceed stepwise.
Frontier molecular orbital theory: A model of chemical reactivity based on interactions between the highest occupied and lowest unoccupied molecular orbitals. It explains pericyclic selectivity through constructive and destructive overlap of frontier orbitals.
Conjugated system: A sequence of atoms with overlapping p orbitals that allows electrons to delocalize. Conjugated p orbitals provide the continuous orbital array used in many pericyclic reactions.
Cope rearrangement: A thermal rearrangement of a 1,5-diene that exchanges its central carbon–carbon sigma bond. It illustrates how a pericyclic rearrangement can reorganize a carbon skeleton without a catalyst.
Carbocation: A positively charged carbon species with an electron-deficient valence shell. A discrete carbocation intermediate marks an ionic pathway rather than a concerted pericyclic one.
Orbital symmetry: The pattern of signs and transformations of molecular orbitals under symmetry operations. Pericyclic reaction pathways depend on whether interacting orbitals can maintain symmetry-compatible overlap.
Suprafacial and antarafacial: Terms describing bonding changes occurring on the same face or opposite faces of a π system. These labels specify the orbital-face relationships that determine pericyclic stereochemistry.

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