The Near Ultraviolet Absorption of Substituted Benzenes

Abstract

The intensification of ultraviolet absorption in substituted over unsubstituted benzene is here attributed to the destruction of the sixfold symmetry of the benzene by the partial migration of an electron to the ring or out of it. Toluene, aniline, phenol, and fluorobenzene are treated. The electric moment produced by a migration of an electron from the radical into the benzene ring is calculated by the method of antisymmetric molecular orbitals and the extent of migration by a minimum condition. The results are in agreement with empirical data. It is concluded that a radical will produce a large intensification of the absorption if it has a low ionization potential, a pair of nonbonding P electrons, and a not too large ring‐radical distance. On this basis the SH group is predicted to be very effective.