Conjugated polyene, electrocyclic molecular orbitals

Conjugated polyene, electrocyclic reactions of, 1181-1186 molecular orbitals of, 1179-1180 Conjugated tricne, electrocyclic reactions of, 1182 Conjugation, 482... 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

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