Consider the thermal reaction 1,3,5-hexatriene → 1,3-cyclohexadiene.
(a) Use the symmetry of the polyene HOMO to predict whether the reaction path is conrotatory or disrotatory. (The HMOs need not be found explicitly; all that is needed are the signs of the AOs in the HOMO, and these can be found from the pattern of nodes; the number of vertical nodal planes is zero for the ground state and increases by 1 for each higher state.)
(b) Do the same as in (a) when the reaction occurs photochemically.
(c) State the general rules for the cyclization of the polyene (17.28) with nπ π electrons.