The Relation between Energy Levels of Substituent Groups and Electron Migration Effects in Some Monosubstituted Benzenes

Abstract

Determinations of the highest occupied (HB, HS ) and the lowest vacant levels (VB, VS ) were made with benzene and some substituent groups using experimental knowledge of their ionization potentials and their near ultraviolet absorption spectra. On the basis of the relative height of these levels, monosubstituted benzenes can be classified into following three groups: (1) | VS –HB | < | VB –HS | for meta‐directing molecules: (2) | VS –HB | > | VB –HS | for ortho‐, para‐directing molecules: (3) | VS –HB | ≈ | VB –HS | for styrene. By the aid of a simple quantum‐mechanical treatment, it becomes sure that in the case of (1) the interaction between levels VS and HB is predominant and the migrating electron flows out of benzene into the substituent: on the other hand in the case of (2) the interaction between levels VB and HS is the stronger and the migrating electron moves in the reverse direction: and finally, in the case of (3), both interactions referred to in the foregoing should be taken into account with almost equal importance. Furthermore, it was shown that the difference in the ionization potential between the meta‐ and the ortho‐, para‐directing monosubstituted benzenes can be interpreted by taking into consideration the interaction of HB with HS or VS , and that the absorption bands in the wavelength region 2300∼2600A, observed with meta‐directing molecules, may be regarded as due to the transition between two levels which occur by the interaction between levels VS and HB .