The electrochemical behaviour of 1-F-2,4-dinitrobenzene, 1-Cl-2.4-dinitrobenzene and 1-Br-2,4-dinitrobenzene in DMF is described. The 1-F-2,4-dinitrobenzene radical anion dimerises before cleaving, whereas 1-Cl-2,4-dinitrobenzene and 1-Br-2,4-dinitrobenzene radical anions dimerise after cleavage. This change in mechanism allows the obtention of 2,2',4,4'-tetranitrobiphenyl in a selective way, and with good efficiency. The electrochemical oxidation of sigma -complexes is shown to be an alternative means of obtaining products that are difficult to obtain through traditional procedures.