Electron Transfer and Coupling Reactions

The stable triphenylcyclopropenium cation (81) undergoes an electron-transfer reaction when photolyzed in acidic medium (van Tamelen et (d., 1968,1971). Irradiation of 81 for 4 hours in 10% aqueous sulfuric acid resulted in a 49% yield of hexaphenylbenzene (82). The reaction is presumed to proceed by initial charge transfer to produce the cyclopropenyl radical 83, which then couples to give 84. This compound in 

The di-n-propyl cyclopropenyl cation failed to photolyze either in aqueous acid or organic solvents, with or without sensitizers. A possible explanation in the discrepancy between the triphenyl system and this one lies in the calculated energy differences between the cations and their corresponding radicals. In the triphenyl system this energy difference is 0-5 3 or 16 kcal/mol, while in the di-n-propyl case it is I OO or 32 kcal/ mol, based on calculated delocalization energies for the two species. 

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Chen XX, Discher BM, Pilloud DL, Gibney BR, Moser CC, and Dutton PL. De novo design of a cytochrome b maquette for electron transfer and coupled reactions on electrodes. J. Phys. Chem. B 2002 106 617-624. 

The chronocoulometry and chronoamperometry methods are most useful for the study of adsorption phenomena associated with electroactive species. Although less popular than cyclic voltammetry for the study of chemical reactions that are coupled with electrode reactions, these chrono- methods have merit for some situations. In all cases each step (diffusion, electron transfer, and chemical reactions) must be considered. For the simplification of the data analysis, conditions are chosen such that the electron-transfer process is controlled by the diffusion of an electroactive species. However, to obtain the kinetic parameters of chemical reactions, a reasonable mechanism must be available (often ascertained from cyclic voltammetry). A series of recent monographs provides details of useful applications for these methods.13,37,57... 

The greatly reduced double-layer capacitance of microelectrodes, associated with their small area, results in electrochemical cells with small RC time constants. For example, for a microdisk the RC time constant is proportional to the radius of the electrode. The small RC constants allow high-speed voltammetric experiments to be performed at the microsecond timescale (scan rates higher than 106V/s) and hence to probe the kinetics of very fast electron transfer and coupling chemical reactions (114) or the dynamic of processes such as exocytosis (e.g., Fig. 4.25). Such high-speed experiments are discussed further in Section 2.1. 

Another important feature of mass transfer processes is related to the very physical nature of the phenomenon. As such it is easily quantifiable and predictable. Thus the rate of mass transfer to and from an electrode may be determined a priori for a given electrochemical system. As a result this rate may be used as natural built-in clock by which the rate of other electrochemical processes may be measured. Such a property was apparent in our earlier discussions related to electrode kinetics (electron transfer and coupled chemical reactions). Basically it proceeds from the same idea as that frequently used in organic chemistry for relative rate constant determinations, when opposing a chemical reaction of known (or taken as the reference in a series of experiments) rate constant against a chemical reaction whose rate constant (or relative rate constant) is to be determined. Many such examples exist in the organic literature, among which are the famous radical-clocks ... 

Reflections on the Electron-Transfer and Coupling Steps in Reactions ofRMgCl and R2Hg with Th +Cl04 ... 

The employment of suitable organic solvents, such as acetonitrile and acetic acid, with oxidation-resistant supporting electrolytes permits the anodic formation of reactive radical cations from many organic materials. Most aromatic compounds and olefins, as well as those alkanes which have particularly weak C—H bonds, are oxidised in acetonitrile containing fluoroborate or hexafluorophosphate electro-lytes. °" 2 Some aromatic radical cations can be further oxidised to dications within the available potential range. Radical cations in general either deprotonate or attack nucleophiles present in the medium reactions with pyridine, methanol, water, cyanide ion, acetate ion or acetonitrile itself produce addition or substitution products. The complete reactions involve a second electron transfer and coupled chemical... 

On the other hand, what are the problems which presently prevent the widespread commercial exploitation of organic electrosynthesis First we must recognize that organic electrosynthetic processes are chemically much more complex than any other processes considered in this book. Usually the overall electrode reaction is not simple electron transfer, but is a sequence of electron transfers and coupled chemical processes either on the electrode surface or in... 

Importantly, and unlike potentiometry, voltammetric methods are dynamic and give information on kinetics, that is, rates of electron transfer and coupled (EC) reactions the latter include those in which electron transfer drives a reaction such as ion/proton transfer, or is gated , that is, the case in which the electron-transfer event is controlled by a preceding chemical process. Redox reactions can be quantified in both the potential and time domains, and these may be separated and resolved for example, steady-state catalytic studies of adsorbed enzymes reveal how catalytic electron transport varies as a function of potential, which can be important if the rate is sensitive to the oxidation state of a particular site in the molecule [1]. 

On the other hand, what are the difficulties which prevent the universal exploitation of organic electrosynthesis Firstly, one must recognize that electrosynthetic processes are chemicatty much more complex than any other processes considered in this book. Already, it has been noted that the overall chemical change at the electrode results from a sequence of both electron transfers and chemical reactions. Indeed, it is ohtn convenient to think of electrode reactions occurring in two distinct steps (1) the electrode reaction converts the substrate into an intermediate (e.g. carbenium ion, radical, carbanion, ion radical) by electron transfer and (2) the intermediates convert to the final product. Controlling the electrode potential wifi influence only the nature of the intermediate produced and its rate of production. The electrode potential does not influence the coupled chemistry directly, particularly if it occurs as the intermediates diffuse away from the electrode. Rather, the reaction pathways followed by the intermediate are determined by the solution environment and it is often difficult to persuade reactive intermediates to follow a single pathway. 

Hirst J. Elucidating the mechanisms of coupled electron transfer and catalytic reactions by protein film voltammetry. Biochim Biophys Acta 2006 1757 225 239. 

A classic reaction involving electron transfer and decarboxylation of acyloxy radicals is the Kolbe electrolysis, in which an electron is abstracted from a carboxylate ion at the anode of an electrolysis system. This reaction gives products derived from coupling of the decarboxylated radicals. 

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