Voltammetry under high mass transport conditions. The application of the high speed channel electrode to the reduction of pentafluoronitrobenzene

The electroreduction of pentafluoronitrobenzene in dimethylformamide solution resulting in the formation of the dimer, octafluoro-4,4'-dinitro-biphenyl, is studied. The dimeric species is reduced further at the potentials applied to the corresponding di-anion. Microband electrodes are used in conjunction with a high speed channel flow cell to explore the mechanism of this process and to quantify the associated very fast homogeneous kinetics. In particular, the formation of the dimer is seen to proceed via the rate determining loss of fluoride anion from the initially formed radical anion of pentafluoronitrobenzene with a first order rate constant of 4.2 x 10(5) s(-1). The reduction of the dimer to its di-anion is seen to occur via a DISP2 mechanism in which the rate determining step is the disproportionation of the mono-anions of octafluoro,4,4'-dinitro-biphenyl with a rate constant of 2 x 10(8) M(-1) s(-1). The merits of high speed channel electrodes for the measurement of fast homogeneous kinetics are examined and compared with those of microdisc voltammetry in stationary solution.