Theory of Substitution in Conjugated Molecules

A theory concerning the mechanism of substitution in conjugated molecules is stated. The energy of the whole system is divided into σ- and π-parts. When certain relations hold between some integrals of σ-electrons, the σ-part of the energy of the system has a maximum in the process of the reaction, by which the transition state is defined. In the transition state a hyperconjugation may take place between the initial π-electron system and the pseudo-π-orbital in the vicinity of the reaction center. By means of the perturbation method, the π-part of the energy of the system involving the pseudo-π-orbital is obtained. The perturbation energy is always negative and by the magnitude of its absolute value, the reactivity of conjugated molecules can be discussed in a definite type of reaction with a definite reagent. From an approximate expression of this perturbation energy, the frontier electron theory is derived, which has previously been proposed by the present authors, whose coincidence with chemical experi...