If the ring opening of 3,4-dimethylcyclobutene were carried out under photochemical conditions the resulting electrocyclization would be occur via a disrotatory mode instead of a conrotatory mode as can be seen by the correlation diagram for the allowed excited state ring opening reaction . Only a disrotatory mode , in which symmetry about a reflection plane is maintained throughout the reaction , would result in maximum orbital overlap in the transition state .
The correlation diagram for the allowed excited state ring opening reaction shows why : Only a disrotatory mode , in which symmetry about a reflection plane is maintained throughout the reaction , would result in maximum orbital overlap in the transition state .