Photoelectrochemistry of bromonitrobenzenes: Mechanism and photoelectrochemically-induced halex reactions

The theory for homogeneous ECE, ECCE and ECECE mechanisms at macroscopically large channel electrodes is derived. The photoelectrochemical reductions of para-bromonitrobenzene and 2,4-dibromonitrobenzene in acetonitrile solutions and at macroscopic platinum channel electrodes are studied using irradiation at 330 and 470 nm, corresponding to absorption bands in the corresponding radical anions. The former compound is shown to follow a homogeneous ECCE pathway in acetonitrile solutions containing tetrabutylammonium-based supporting electrolytes; by changing the supporting electrolyte to a salt of the tetramethylammonium cation, the mechanism is changed qualitatively and follows an ECEE pathway. The photoelectrochemical reduction of 2,4-dibromonitrobenzene in acetonitrile solution containing supporting electrolytes derived from the tetrabutylammonium cation is shown to follow an overall ECECE mechanism, with both chemical steps being chemically-reversible, and with the loss of the ortho-bromo in the dark. In the presence of chloride supporting electrolytes, it is shown that light-induced rupture of a C-Br bond occurs reversibly with the competing formation of a C-Cl bond. Unoptimised bulk photoelectrosynthesis indicates that some halogen exchange occurs, demonstrating the viability of a novel approach to halex reactions.