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

摘要

A simple kinetic method based on competition kinetics is presented for the measurement of cleavage rate constantsnof radical anions over the range of 107 u0001 5 × 109 su00011 in aprotic solvents. The standard potential for the formation ofnthe radical anions may be extracted from the kinetic analysis as well. The method employs electrochemical steadystatenor optical detection techniques and is an extension of the redox catalysis approach described previously in thenliterature. The applicability of the method is illustrated through a systematic study of the cleavage reactions for annumber of short-lived haloaromatic radical anions. Interestingly, the radical anion of iodobenzene is found to benan intermediate in the homogeneous reduction of iodobenzene, even though recent investigations have shown thatnthe corresponding heterogeneous reduction at an electrode surface proceeds by a concerted electron transfer-bondncleavage process. The nature of the cleavage reactions is discussed in terms of the activation–driving force plot ofnthe cleavage rate constants versus the relevant Gibbs energies. While the exergonic cleavage reactions follow a simplendecay mechanism taking place at the halogen site, the endergonic processes are best described as intra-molecularnelectron transfers from the substituent to the carbon–halogen bond. Nevertheless, the overall intrinsic barrier isnfound to be relatively small (27–39 kJ molu00011) and it is suggested that the endergonic reactions may proceed byna stepwise mechanism, in which a σ* radical anion is formed as an intermediate prior to the formation of thendissociated products, the aryl radical and the halide. The above conclusions were supported by semi-empiricalnPM3 calculations of structures and charge distributions in the radical anions.