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A Useful Mnemonic Rule

Before pericyclic reactions can be put to use in a predictable and controlled manner, a broad mechanistic understanding of the factors that influence these concerted transformations must be formulated. The simplest, albeit least rigorous, method for predicting the configurational path favored by a proposed pericyclic reaction is based upon a transition state electron count. In most of the earlier examples, pericyclic reactions were described by a cycle of curved arrows, each representing a pair of bonding electrons. The total number of electrons undergoing reorganization is always even, and is either a 4n+2 or 4n number (where n is an integer). Once this electron count is made, the following table may be used for predictions.

Thermal
Reactions

Transition State Class

Configurational Preference

4n + 2 (aromatic)

Suprafacial or Disrotatory

4n (antiaromatic)

Antarafacial or Conrotatory

Photochemical
Reactions

Transition State Class

Configurational Preference

4n + 2 (aromatic)

Antarafacial or Conrotatory

4n (antiaromatic)

Suprafacial or Disrotatory

Although this modest mnemonic does not make explicit use of molecular orbitals, more rigorous methods that are founded on the characteristics of such orbitals have provided important insight into these reactions. Since pericyclic reactions proceed by a cyclic reorganization of bonding electron pairs, it is necessary to evaluate changes in the associated molecular orbitals that take place in going from reactants to products. The following section describes approaches of this kind.

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