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Electrocyclic reaction
The rules apply to the observed stereospecificity of electrocyclic ring-opening and ring-closing reactions at the termini of open chain conjugated polyenes either by application of heat (thermal reactions) or application of light (photochemical reactions). In the original publication in 1965, three rules are stated as:


 * In an open-chain system containing 4n-electrons, the orbital symmetry of the highest occupied ground-state orbital is such that a bonding interaction between the termini must involve overlap between orbital envelopes on opposite faces of the system and this can only be achieved in a conrotatory process. An example of such reaction type is the electrocyclic ring closure of 1,4-dimethylbuta-1,3-diene pictured at the left.




 * In open systems containing 4n + 2 electrons, terminal bonding interaction within ground-state molecules requires overlap of orbital envelopes on the same face of the system, attainable only by disrotatory displacements.


 * In a photochemical reaction an electron in the HOMO of the reactant is promoted to an excited state leading to a reversal of terminal symmetry relationships and reversal of stereospecificity.

Organic reactions that obey these rules are said to be symmetry allowed. Reactions that take the opposite course are symmetry forbidden and require a lot more energy to take place if they take place at all.

The rules predict the outcome of several ground-state reactions:
 * Cyclopropyl cation → allyl cation: disrotatory
 * Cyclopropyl radical → allyl radical: conrotatory
 * Cyclopropyl anion → allyl anion: conrotatory
 * Cyclopentenyl cation → pentadienyl cation: conrotatory