OUTER-SPHERE DISSOCIATIVE ELECTRON-TRANSFER TO ORGANIC-MOLECULES - A SOURCE OF RADICALS OR CARBANIONS - DIRECT AND INDIRECT ELECTROCHEMISTRY OF PERFLUOROALKYL BROMIDES AND IODIDES

As an example of the general problem posed in the title, the reduction of CF3Br, CF3I, C6F13I, and C8F17I by outer-sphere heterogeneous (glassy-carbon electrodes) and homogeneous (aromatic anion radicals) reagents is investigated, in a protic solvents containing tetralkylammonium salts, by cyclic voltammetry and preparative-scale electrolysis. A RF ⋅ chemistry is thus triggered in all cases with the exception of CF3Br where a mixed RF ⋅/RF - chemistry is obtained by direct electrochemical reduction or by electron transfer from aromatic anion radicals having a close reduction potential. Quantitative analysis of the reduction kinetics of RFX (X = Br, I) and of RF ⋅ led to the following conclusions. RFX most likely undergoes a concerted electrontransfer-bond-breaking reduction involving a substantial overpotential (ca. 1.5 V for C-Br and 1.0 V for C-I), mainly governed by the C-X bond dissociation energy. RF ⋅ radicals are strongly stabilized by interaction with the solvent but are nevertheless easier to reduce than alkyl radicals on thermodynamical grounds, exhibiting however a high intrinsic barrier of the same order as for alkyl radicals. Unlike alkyl radicals, but similar to aryl radicals, RF ⋅ radicals are good H-atom scavengers and do not couple with aromatic anion radicals. © 1990, American Chemical Society. All rights reserved.