Disrotatorisk

Disrotatorisk is a term used in organic chemistry to describe a stereochemical pathway in certain electrocyclic reactions in which the terminal p orbitals rotate in opposite directions as a π-system closes to form a ring or opens to a chain. In a disrotatory process, substituents attached to the rotating termini end up on opposite faces of the forming ring, reflecting the opposite sense of rotation of the two ends. The term is the Swedish equivalent of the English “disrotatory.”

The mode of rotation in disrotatoriske processes is explained by orbital symmetry considerations, as summarized in the Woodward–Hoffmann rules. The preferred pathway depends on the number of π electrons in the system and whether the reaction occurs thermally or photochemically. For thermal electrocyclic reactions, systems with 4n π electrons tend to proceed disrotatorily, while those with 4n+2 π electrons tend to proceed conrotatorily; under photochemical conditions, the roles are reversed. These rules help predict the stereochemical outcome of ring closures and openings in conjugated hydrocarbon systems.

An illustrative example is the thermal electrocyclization of 1,3-butadiene (a 4π system) to cyclobutene, which proceeds via a disrotatory pathway. The concept also applies to the reverse process, ring opening, and to other π-conjugated systems undergoing electrocyclic transformations.

Disrotatorisk is distinguished from conrotatorisk (conrotatory), where the terminal p orbitals rotate in the same direction. Together, these concepts describe how stereochemistry is controlled in pericyclic reactions and assist in planning synthesis routes that yield specific ring geometries.