The “Forbidden” World of Chemistry

Hückel’s rule1, the Woodward-Hoffmann rules2, and, in a broader sense, Hückel Molecular Orbital (HMO) theory3 have had profound impact on chemistry in a way which is now well recognized and admired. Perhaps, the most important and time-lasting contribution of HMO theory has been the revelation of what in Valence Bond (VB) terms we call parity control of stereoselection4, or, in more familiar language, the revelation of the fact that a ground state molecule, a transition state, or, any molecular system, in general, can be thought of as the product of a “forbidden” or “allowed” union of two component fragments with the latter mode of union being energetically more favorable than the former one.