Illustrated Glossary of Organic Chemistry

Woodward-Hoffmann rules: A series of rules which predict whether a pericyclic process occurs under thermal or photochemical conditions, as well as the stereochemical outcome of the reaction. The rules are based on the conservation of orbital symmetry.

The Woodward-Hoffmann rules predict that a [4+2] cycloaddition reaction (such as the Diels-Alder reaction of 1,3-butadiene with maleic anhydride) is thermally allowed but photochemically forbidden.

The Woodward-Hoffmann rules predict that ring closure of (E,E)-hexa-2,4-diene under thermal conditions produces trans-3,4-dimethylcyclobut-1-ene, whereas the same reaction under photochemical conditions gives cis-3,4-dimethylcyclobut-1-ene.