Electron-transfer reactions chlorine reduction

Reductive dechlorination or reductive hydrogenolysis is a common transformation of 1- and 2-carbon chlorinated aliphatics under methanogenic conditions [373,374]. 1,1,1-Trichloroethane (l,l,l-TCA),for example,is converted to 1,1-dichloroethane (1,1-DCA) [375], and Perchloroethylene (PCE) is successively converted to TCE, cDCE, VC, and ethane [274]. Each reductive dechlorination is a two-electron transfer reaction. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

This process involves a series of reactions, including dissolution, hydrogen reactions and chlorine withdrawal [20], The second type of reactions include reduction of protons at the catalyst by electron transfer yielding hydrogen radicals that are consumed by reaction or give elemental hydrogen otherwise. 

This approach has been taken for the reaction of chlorinated ethenes with Zn° [125,165] and Fe° [88,166], resulting in separate rate constants for all the reactions shown in Fig. 3. Care must be taken in using these parameters in predictive modeling, however, as it is not yet known how sensitive the relative values of these rate constants are to pH, thickness and composition of the oxide film, etc. The same caution applies where the approach represented by Eq. (25) is used to describe parallel mechanisms of transformation. For example, it has recently been reported that several experimental factors influence the relative contributions of dissociative electron transfer, hydrogen atom transfer, and reductive elimination to the dechlorination of carbon tetrachloride and TCE by Fe° [177],... 

The redox chlorins at the core of the various reaction centers form an obvious chain with separations of less than 6 A that ensure rates of 10 ps or less. The effect of this chain is to apparently make the photoinduced oxidant and reductant capable of residing briefly on any of the chlorins, subject principally to the free energy of that state and the energetic penalty of any uphill reverse electron transfer. When the free energy... 

Each of the photosystems ejects an electron from the excited chlorin complex to a quinone within a nanosecond, followed by electron transfer along chains leading out of the charge separation center within 100 ns. The high potential reaction of Tyr and Mn in PSII is quite rapid, beginning in the simulations on the same time scale as the quinone reduction reaction. However, it has been suggested that tyrosine oxidation may not be rate limited by tunneling, but by H+ transfer (Diner et al., 2001). 

At first sight, it may appear that a discussion of nucleophile-electrophile interactions does not fit into a chapter on redox processes, but the transfer of electron pairs from a nucleophile to an electrophile, such as in hydrolysis or in chlorination of a phenol, may be looked at as a reaction between an oxidant and reductant, although we are aware that mechanistically, in many real redox processes involving organic substances, the electron transfers often occur in one-electron steps. 

A great advantage of electrochemical reactions compared with chemical conversions is the effective contribution to pollution control. The direct electron transfer from the electrode to the substrate avoids the problem of separation and waste treatment of the frequently toxic end products of the chemical oxidants or reductants. Furthermore, by electrodialysis, organic acids or bases can be regenerated from their salts without the use of sulfuric acid or sodium hydroxide, for example, which lead to the coproduction of sodium salts or sulfates as waste [79]. At the same time, inorganic acids and bases, necessary for chemical production, are provided by this process. An application of electrodialysis has been demonstrated in the preparation of methoxyacetic acid by oxidation of methoxyethanol at the nickel hydroxide electrode [80]. Finally, unwanted side products can be converted into the wanted product, which increases the economy of the process and reduces the problem of waste separation and treatment. This is accomplished in the manufacture of chloroacetic acid by chlorination of acetic acid. There the side product dichloroacetic acid, formed by overchlorination, is cathodically converted to chloroacetic acid [81]. 

Reactions of Metals with Oxygen and Chlorine. With this problem, we tried to get information about how students describe their concepts of metal-oxygen reactions as oxygen transfer or as electron transfer, and how the chosen definition is applied to the metal-chlorine reactions. The other point of interest is to determine if the chosen definition is discussed by oxidation or reduction of substances or of particles. 

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