Forbidden mechanism

A reaction pathway disallowed by orbital-symmetry selection rules under specified conditions. It may become possible under different conditions or through a pathway that avoids the symmetry restriction.

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Woodward–Hoffmann rules: Selection rules that predict whether concerted pericyclic reactions are symmetry-allowed. They determine whether a proposed pericyclic pathway is forbidden by orbital symmetry.
Symmetry-allowed reaction: A reaction pathway that satisfies the applicable orbital-symmetry selection rules. It contrasts with a forbidden pathway that cannot proceed concertedly under the same conditions.
Electrocyclic reaction: A pericyclic reaction that forms or breaks a sigma bond at the ends of a conjugated pi system. Its allowed conrotatory or disrotatory motion depends on orbital symmetry and conditions.
Pericyclic reaction: A concerted reaction whose bonding changes occur through a cyclic arrangement of interacting orbitals. Orbital-symmetry rules chiefly address this family of reactions.
Molecular orbital: A wavefunction describing an electron in a molecule, often spread across several atoms. Orbital phases and symmetries are the basis for classifying reaction pathways.
Thermal reaction: A chemical reaction driven by heat rather than by absorbed light. Thermal and photochemical conditions can give opposite symmetry predictions for a pericyclic pathway.
Cycloaddition: A reaction joining unsaturated molecules to form a ring through new sigma bonds. Orbital symmetry predicts which cycloaddition modes are allowed thermally or photochemically.
Hückel's rule: A rule predicting aromaticity in planar, cyclic, fully conjugated systems with particular pi-electron counts. It reflects orbital symmetry patterns also used to reason about reaction pathways.
Orbital symmetry: The behavior of molecular orbitals under symmetry operations, including how their phases transform. A pathway is forbidden when its orbital symmetry cannot be conserved along the reaction coordinate.
Stepwise reaction: A reaction proceeding through two or more elementary steps and intermediate species. A stepwise route can bypass a forbidden concerted pathway.