OVER the past decade or two the MO theory has added considerably to our understanding of relative chemical reactivities of conjugated systems such as polycyclic hydrocarbons and heterocycles. It has thrown light on some previously puzzling properties of heterocycles t-4 and has stimulated investigations of reaction mechanisms. 5 Overall the 7r-electron concept is now firmly embedded in organic chemical thought. When so much has been achieved in a field it is salutary to review the basic assumptions periodically, perhaps to seek a clue to profitable lines of future development. In the present article I propose to consider some of the fundamental propositions on which the MO theory (or theories) of electrophilic aromatic substitution is based.
Various theoretical quantities---n--electron densities, 6 polarizabilities, 6 localization energies, 7 superdelocalizabilities, 8 Z-numbers, 9 etc.--have been used as criteria of reactivity in electrophilic aromatic substitution. They all represent attempts to estimate the variable part of the activation energy for the reaction of a variety of conjugated substrates with one particular electrophil. Their use carries with it the implicit assumptions (i) that activation entropies are constant for a given electrophil and reaction conditions, or at most that they vary smoothly with activation energies, (ii) that the solvent, though manifestly essential for achieving electrophilic aromatic substitution, similarly has a constant effect, or its effect varies smoothly with activation energy.
The present status of these theoretical quantities as criteria of chemical reactivity will be considered in this article from a critical viewpoint. Attention will be paid to factors which have not been adequately incorporated in present theories rather than to