HETEROGENEOUS AND HOMOGENEOUS ELECTRON TRANSFERS TO AROMATIC HALIDES - ELECTROCHEMICAL REDOX CATALYSIS STUDY IN THE HALOBENZENE AND HALOPYRIDINE SERIES

Halobenzenes and halopyridincs undergo a two-electron irreversible reductive cleavage. It involves fast decomposition of the assumed anion radical intermediate, ARX-.(k1st order>>104 s-1), and transient formation of a neutral radical, Ar. easier to reduce than the starting molecule. With the exception of Phi, the electrochemical reaction is under the kinetic control of the initial electron transfer step. Standard application of the electrochemical techniques provides the transfer coefficients and the forward electron transfer rate constants but no information about the standard potentials and the standard rate constants. The latter quantities can be determined through the kinetic analysis of homogeneous redox catalysis of the electrochemical reduction. The following points are discussed: activation vs. diffusion control of the homogeneous electron transfer: existence of the anion radical as an actual reaction intermediate vs. simultaneous electron transfer and breaking of the carbon-halogen bond; correlation between homogeneous and heterogeneous electron transfers: determination of the standard potentials and the standard rate constants; relationships between structure and electron transfer thermodynamics and kinetics. The remarkable slowness of the electron transfer as compared to aromatic and heteroaromatic compounds of similar molecular size implies, besides solvent reorganization, a significant internal reorganization due to stretching of the carbon-halogen bond when passing from the starting molecule to the anion radical. Copyright © 1979, American Chemical Society. All rights reserved.