The Substitution. Electrodes and Electrons as Catalysts

FIGURE 2.39. Example of the cyclic voltammetric observation of an SrnI reaction upon addition of the nucleophile. Upper scheme ECE-DISP reduction of the substrate in the absence of nucleophile. Lower scheme SrnI mechanism. 

COUPLING OF ELECTRODE ELECTRON TRANSFERS WITH CHEMICAL REACTIONS 

As discussed in Section 2.5.1, aryl radicals are easily reduced at the potential where they are generated. This reduction that can take place at the electrode surface (ECE) or in the solution (DISP) opposes the substitution process. This three-cornered competition between substitution (SUBST) electron + proton transfer (ECE or DISP) depends on two competition parameters that are closely similar to the HAT-ECE-DISP parameters described in the preceding section  

The preceding analysis of the effect of side reactions, or termination steps, applies in poor H-atom donating solvent such as liquid ammonia. This is the favorite solvent for 5rn1 reactions, not only because the first 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

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