Electrochemical Reactions with Stepwise Electron Transfer

3144 Te29B nsl ig = 210 F s 9B485 Fig. 5.3 Polarization curve (cathode region) in the Tafel coordinates 

Step mechanism. In addition, we implemented significant simplifications, ignoring the structure of double layer at the interface. More sophisticated approaches to the subject can be found in specialized literature. 

3 Electrochemical Reactions with Stepwise Electron Transfer 

Simple calculations show that when three electrons are transferred during the slow electrochemical step and the transfer number is 0.5, a slope of the Tafel correlation between p and Ig i must equal 2.3RT/(anF) = 39 mV. However, completely different slopes can be measured experimentally. Data for the same electrochemical system can heavily depend on the electrolyte composition. This leads to a conclusion that electrooxidation or electroreduction of charged particles can proceed through a stepwise electron transfer mechanism. 

Consider with Maple an analysis of the following kinetic scheme that represents stepwise ionization of iron  

---

Reaction between RuClj-xHjO and Na2[H2edta] gives the yellow paramagnetic [Ru (Hedta)-(0H2)] 4H20 and [RuCl(H2edta)(0H2)] 3.5H20. In water, stepwise hydrolysis of the former occurs to give [Ru( ta)(OH2)] and [Ru(OH)(edta)] respectively. Electrochemical studies reveal that electron transfer is rapid and reversible for the [Ru(edta)(OH2)] couple and that [Ru" (edta)(OH2) is oxidized by perchlorate ion. Reaction of the latter with formate ion aflbrds the carbonyl complex [Ru(edta)(CO)] ... 

How can these photochemical and electrochemical data be reconciled With the benzylic molecules under discussion, electron transfer may involve the n or the cr orbital, giving rise to stepwise and concerted mechanisms, respectively. This is a typical case where the mechanism is a function of the driving force of the reaction, as evoked earlier. Since the photochemical reactions are strongly down-hill whereas the electrochemical reaction is slightly up-hill at low scan rate, the mechanism may change from stepwise in the first case to concerted in the second. However, regardless of the validity of this interpretation, it is important to address a more fundamental question, namely, whether it is true, from first principles, that a purely dissociative photoinduced electron transfer is necessarily endowed with a unity quantum yield and, more generally, to establish what are the expressions of the quantum yields for concerted and stepwise reactions. 

Upon reaction with a metal, the NMR spectrum of a neutral hydrocarbon first disappears, due to one-electron transfer from the metal. This step can be studied by EPR spectroscopy [la, 5j. The reduction of carbon-rich compounds usually occurs stepwise by one-electron transfers as depicted in Scheme 13.1 [lb]. The same set of events occurs in electrochemical processes and in alkali metal reductions. This reaction has become the main synthetic route to i-conjugated anions. 

---