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

Abstract

A simple kinetic method based on competition kinetics is presented for the measurement of cleavage rate constants of radical anions over the range of 10⁷ − 5 × 10⁹ 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 detection techniques and is an extension of the redox catalysis approach described previously in the literature. The applicability of the method is illustrated through a systematic study of the cleavage reactions for a number of short-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 corresponding heterogeneous reduction at an electrode surface proceeds by a concerted electron transfer-bond cleavage process. The nature of the cleavage reactions is discussed in terms of the activation–driving force plot of the cleavage rate constants versus the relevant Gibbs energies. While the exergonic cleavage reactions follow a simple decay mechanism taking place at the halogen site, the endergonic processes are best described as intra-molecular electron transfers from the substituent to the carbon–halogen bond. Nevertheless, 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 σ* radical anion is formed as an intermediate prior to the formation of the dissociated products, the aryl radical and the halide. The above conclusions were supported by semi-empirical PM3 calculations of structures and charge distributions in the radical anions.