Application of a new kinetic method in the investigation of cleavage reactions of haloaromatic radical anions

A simple kinetic method based on competition kinetics is presented for the measurement of cleavage rate constants of radical anions over the range of 10(7) - 5 x 10(9) s(-1) in aprotic solvents. The standard potential for the formation of the radical anions may be extracted from the kinetic analysis as well. The method employs electrochemical steady-state or optical detect ion techniques and is an extension of the redox catalysis approach describe d previously in the literature. The applicability of the method is illustra ted through a systematic study of the cleavage reactions for a number of sh ort-lived haloaromatic radical anions. Interestingly, the radical anion of iodobenzene is found to be an intermediate in the homogeneous reduction of iodobenzene, even though recent investigations have shown that the correspo nding heterogeneous reduction at an electrode surface proceeds by a concert ed electron transfer-bond cleavage process: The nature of the cleavage reac tions is discussed in terms of the activation-driving force plot of the cle avage rate constants versus the relevant Gibbs energies. While the exergoni c cleavage reactions follow a simple decay mechanism taking place at the ha logen site; the endergonic processes are best described as intra-molecular electron transfers from the substituent to the carbon-halogen bond. Neverth eless, the overall intrinsic barrier is found to be relatively small (27-39 kJ mol(-1)) and it is suggested that the endergonic reactions may proceed by a stepwise mechanism, in which a sigma* radical anion is formed as an in termediate prior to the formation of the dissociated products, the aryl rad ical and the halide. The above conclusions were supported by semi-empirical PM3 calculations of structures and charge distributions in the radical ani ons.