Pericyclic reactions mirror plane

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

Since pericyclic reactions proceed with a cyclic reorganization of bonding electron pairs, it is necessary to evaluate changes in the associated MOs that take place in going from reactants to products. These orbitals may be classified by two independent symmetry operations a mirror plane (m) perpendicular to the functional plane and bisecting the molecule, and a twofold axis of rotation (C2). 

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