To construct a correlation diagram for the electrocyclic reaction, we need first to identify the MOs of the reactant and product that are actually involved in the reaction. For a cyclobutene, these are the s and s* of the breaking bond and the p and p* of the double bond. In the product, the orbitals are simply Y1 through Y4 of butadiene.

We proceed to make sketches of these orbitals:

Next, we check the symmetry of each with respect to the C₂ axis for conrotatory processes, and the sigma plane for disrotatory. The results are tabulated adjacent to the orbital pictures.

In the reactant cyclobutene, the s and p MOs are occupied. During a conrotatory process, the electrons in the C₂ symmetric s can be placed in the C₂ symmetric Y2; likewise, the p of the cyclobutene correlates with Y1 of butadiene, as shown by the arrows. Since all reactant electrons can be placed in bonding product orbitals of the same symmetry, this process is allowed.

If we were to attempt a disrotatory ring opening, no bonding MO of the product is available for the s-symmetric p electrons of the cyclobutene. We would be forced to place these electrons either in the antibonding Y3, or in a bonding orbital of the wrong symmetry, Y1. This mode of ring opening therefore is forbidden.

For the hexatriene - cyclohexadiene interconversion

The correlation of two s-symmetric and one s-antisymmetric orbitals for the disrotatory process follows from the same kind of analysis. Here the conrotatory opening is forbidden and the disrotatory is allowed.

Two useful insights are obtained using correlation diagrams:

• all electrons involved in the reaction are accounted for; and

• "forbidden-ness" can be seen not to be an absolute prohibition, but rather the requirement of a higher energy transition state.