An electrocyclic reaction is a reversible reaction that involves ring closure of a conjugated
polyene to a cycloalkene, or ring opening of a cycloalkene to a conjugated polyene. For
example, ring closure of 1,3,5-hexatriene forms 1,3-cyclohexadiene, a product with one more σ
bond and one fewer π bond than the reactant. Ring opening of cyclobutene forms 1,3-butadiene,
a product with one fewer σ bond and one more π bond than the reactant.