Theory of Substitution in Conjugated Molecules

Abstract 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 experiences has already been ascertained. The mechanism stated above, therefore may supply us with a key to grasp a feature of the transition state and at the same time may present us with a physical meaning of the frontier electron theory.