POLAROGRAPHIC AND ELECTROCHEMICAL STUDIES OF SOME AROMATIC AND HETEROCYCLIC NITRO-COMPOUNDS .9. SUBSTITUENT EFFECTS ON PROTONATION OF THE RADICAL ARNO2H. AND ITS REACTIONS WITH HYDROXYLAMINO AND NITROSO-COMPOUNDS IN BUFFERED MIXTURES OF WATER AND DMF

Whereas in aqueous solutions the polarographic reduction of most nitro benzenes occurs in a single four-electron step, in the presence of 20% or 40% dimethylformamide (DMF) separation of two waves can be observe d. The decrease of the first reduction step from a four-electron to a one-electron process with increasing pH is attributed to a decrease in the rate of protonation of the radical ArNO2H.. The decrease from a f our-electron to a one-electron wave has the shape of a dissociation cu rve with an inflexion point at pH = pK'. The dependence of the values of pK' on the electron-withdrawing substituents indicates the predomin ant role of adsorption on the rate of protonation of the radical ArNO2 H.. The effects of a change in concentration of the co-solvent (DMF) o n the values of pK' are attributed to changes in the solvation energie s of ArNO2H2+ and ArNO2H.. The ratio of currents of the reduction of t he nitro group in the forward sweep in cyclic voltammetry to those of the oxidation of the resulting arylhydroxylamine on the reverse sweep and to the peak current of the reduction of the nitro group on the sec ond sweep indicate deactivation of the arylhydroxylamine at the electr ode surface. The effects of electron-withdrawing substituents on such current ratios indicate the role of a surface reaction of arylhydroxyl amine with an intermediate of the reduction of the nitro group, probab ly ArNO2H..