Organic reaction mechanisms electron transfer reactions

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

Here, the relative stability of the anion radical confers to the cleavage process a special character. Thus, at a mercury cathode and in organic solvents in the presence of tetraalkylammonium salts, the mechanism is expected16 to be an ECE one in protic media or in the presence of an efficient proton donor, but of EEC type in aprotic solvents. In such a case, simple electron-transfer reactions 9 and 10 have to be associated chemical reactions and other electron transfers (at the level of the first step). Those reactions are shown below in detail ... 

Energetic electron transfer reactions between electrochemically generated, shortlived, radical cations and anions of polyaromatic hydrocarbons are often accompanied by the emission of light, due to the formation of excited species. Such ECL reactions are carried out in organic solvents such as dimethylformamide or acetonitrile, with typically a tetrabutylammonium salt as a supporting electrolyte. The general mechanism proposed for these reactions is as follows. 

These processes can occur by a direct electron transfer reaction to (reduction) or from (oxidation) the present organic pollutant, or by a chemical reaction of the pollutant with previously electrogenerated species. The mechanism is generally viewed as a direct anodic oxidation of organic pollutant involving its reduction by direct electron transfer from organic molecule to the electrode to form a radical cation that readily deprotonates, equation (37) ... 

H. Taube, Electron Transfer Reactions of Complex Ions in Solution, Academic Press, New York 1970. >C.K. Ingold, Structure and Mechanism in Organic Chemistry, 2. Aufl. S. 406-417, Cornell University... 

It may seem unnecessary to stress that it is important to know the product or the product composition of any electron transfer reaction before it is meaningful to discuss the kinetics and mechanism (see Chapter 2). Nevertheless, this simple rule is often violated. One reason probably is that it is not always a trivial task to isolate and identify the products from an electrochemical reaction using the usual arsenal of methods available to the organic chemist. A major problem is that often it is not an easy task to separate the product from the supporting electrolyte. In such cases, direct analysis of the product mixture without the need of a work-up, e.g. by LC-UV/vis-MS, is desirable. Analysis by this method of the product mixture resulting from the oxidation of 1,2,5-trimethylpyrrole is shown in Fig. 6.27. It is... 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

Argriello, J.E. and Penenory, A.B. (2003) Fluorescent quenching of 2-naphthoxide anion by aliphatic and aromatic halides, mechanism and consequences of electron transfer reactions. Journal of Organic Chemistry, 68, 2362-2368. 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

The mechanism of electron transfer reactions in metal complexes has been elucidated by -> Taube who received the Nobel Prize in Chemistry for these studies in 1983 [xiv]. Charge transfer reactions play an important role in living organisms [xv-xvii]. For instance, the initial chemical step in -> photosynthesis, as carried out by the purple bacterium R. sphaeroides, is the transfer of electrons from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 1.7 mm away. This electron transfer occurs very rapidly (2.8 ps) and with essentially 100% efficiency. Redox systems such as ubiquinone/dihydroubiquinone, - cytochrome (Fe3+/Fe2+), ferredoxin (Fe3+/Fe2+), - nicotine-adenine-dinucleotide (NAD+/NADH2) etc. have been widely studied also by electrochemical techniques, and their redox potentials have been determined [xviii-xix]. 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

Previously the electron transfer reactions attracted more attention of researchers [1011,1012], Electrochemical data mainly in common with ESR spectroscopy data are the important source of the information about the reaction mechanism and also about structure, reactivity, properties of intermediate free radicals of different classes of organic, organometallic, and inorganic reactions. Elucidation of the mechanism and problems of reactivity in the chemistry of one-electron transfer can be of main significance in such fields as synthesis and catalysis, radical chemistry, photochemical synthesis, biochemistry of in vivo organism. 

Effective molarities of intramolecular reactions, 17, 183 Electrical conduction in organic solids, 16, 159 Electrochemical methods, study of reactive intermediates by, 19, 131 Electrochemistry, organic, structure and mechanism in, 12, 1 Electrode processes, physical parameters for the control of, 10, 155 Electron spin resonance, identification of organic free radicals by, 1, 284 Electron spin resonance studies of short-lived organic radicals, 5, 23 Electron-transfer reaction, free radical chain processes in aliphatic systems involving an, 23, 271... 

The empirical models are of two kinds. The course of organic reaction mechanisms is mapped out by curved arrows that represent the transfer of electron pairs. Electrochemical processes, on the other hand are always analyzed in terms of single electron transfers. There is a non-trivial difference involving electron spin, between the two models. An electron pair has no spin and behaves like a boson, for instance in the theory of superconductivity. An electron is a fermion. The theoretical mobilities of bosons and fermions are fundamentally different and so is their distribution in quantized potential fields. 

Volume 2 is dedicated to a detailed description of the most important classes of electron transfer reactions involving organic molecules (Part 2.1) and organometallic and inorganic compounds (Part 2.2). In several cases the reactions described are important not only from the viewpoint of fundamental research on reaction mechanisms, but also for their catalytic and synthetic applications. The emerging fields of electron transfer reactions of fullerenes, electron-reservoir complexes, and biomi-metic electron transfer chemistry of porphyrins are discussed in depth. 

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