PHOTOELECTROCHEMICAL REDUCTION OF P-HALONITROBENZENES

A detailed and comparative mechanistic study of the photoelectrochemic al dehalogenation of the four p-halonitrobenzenes, X-C6H4-NO2 (X = F, Cl, Br, or I) in acetonitrile solution is reported. In the case of p-i odonitrobenzene, iodide loss is accomplished by electrochemical reduct ion alone. The formation of I- is shown to take place via an ECE proce ss with the ultimate generation of the radical anion of nitrobenzene. Dual photo- and electro-chemical activation of p-chloronitrobenzene an d p-bromonitrobenzene leads to halide loss through a photo-ECE mechani sm. This proceeds via absorption of light by the radical anions, [X-C6 H4-NO2].-, which is followed by fragmentation forming the .C6H4NO2 ary l radical. The latter is shown to react with the solvent system formin g nitrobenzene which is further reduced at the electrode with the gene ration of [C6H4-NO2].-. The aryl radical is demonstrated to be able to undergo (partial) recombination with added Br- or Cl-. The effectiven ess of different electronic transitions in the radical ions, [X-C6H4-N O2].- (X = Br, Cl) towards dehalogenation are compared; both radical a nions exhibit transitions centred near 330 and 470 nm in the near UV-V IS part of the spectrum. For the chloro-compound only the former band is effective in stimulating chloride release, whereas for the bromo-co mpound the excitation of either band causes bromide loss. The long wav elength band of the latter is quantified as being some 5.6 times more effective towards fragmentation on a per photon absorbed basis and thi s is rationalised using spin selection rules. No loss of fluoride from [F-C6N4-NO2].- was observed at any wavelength in the visible region o f the spectrum.