Oxidation/reduction formation

These often heterogenous reactions take place between different types of phases solid-liquid-gas. There is a whole hst of possible reactions dissociatiort, volatilization, reduction, notably by carbon, oxidation-reduction, formation of new compounds. 

Chemical Properties. Lignin is subject to oxidation, reduction, discoloration, hydrolysis, and other chemical and enzymatic reactions. Many ate briefly described elsewhere (51). Key to these reactions is the ability of the phenolic hydroxyl groups of lignin to participate in the formation of reactive intermediates, eg, phenoxy radical (4), quinonemethide (5), and phenoxy anion (6) ... 

Identification, isolation, and removal of (polyhydroxy)benzenes from the environment have received increased attention throughout the 1980s and 1990s. The biochemical activity of the benzenepolyols is at least in part based on thek oxidation—reduction potential. Many biochemical studies of these compounds have been made, eg, of enzymic glycoside formation, enzymic hydroxylation and oxidation, biological interactions with biochemically important compounds such as the catecholamines, and humic acid formation. The range of biochemical function of these compounds and thek derivatives is not yet fully understood. 

The mechanism for the formation of siUca is complex because oxidation, reduction, and hydrolysis pathways are all possible. 

The most suitable method of fast and simple control of the presence of dangerous substances is analytical detection by means of simplified methods - the so-called express-tests which allow quickly and reliably revealing and estimating the content of chemical substances in various objects. Express-tests are based on sensitive reactions which fix analytical effect visually or by means of portable instalments. Among types of indicator reactions were studied reactions of complex formation, oxidation-reduction, diazotization, azocoupling and oxidative condensation of organic substances, which are accompanied with the formation of colored products or with their discoloration. 

These reactions differ from those of sulfur tetrafluoride with carbonyl compounds in that a formal oxidation-reduction of the sulfur atoms m the thiocarbonyl compound and sulfur tetrafluoride molecule occurs, resulting in the formation of free sulfur and the complete utilization of the fluorine atoms in sulfur tetrafluoride. 

Lipoic acid exists as a mixture of two structures a closed-ring disulfide form and an open-chain reduced form (Figure 18.33). Oxidation-reduction cycles interconvert these two species. As is the case for biotin, lipoic acid does not often occur free in nature, but rather is covalently attached in amide linkage with lysine residues on enzymes. The enzyme that catalyzes the formation of the lipoamide nk.2Lg c requires ATP and produces lipoamide-enzyme conjugates, AMP, and pyrophosphate as products of the reaction. 

These results may easily be rationalized by assuming that the formation of hydroxy sulphoxides 91, 92 and 93 from hydroperoxysulphides 89 and 90 is an intramolecular oxidation-reduction reaction proceeding through a five-membered transition state 94. However, an alternative intermolecular mechanism in which the approach of the oxidant is directed by the hydroperoxy or the hydroxy function in the reductant cannot be excluded. 

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

Oxidation-reduction reactions which lead to formation of S02 in the exit gas... 

Formate is one of the most representative hydrogen sources for the biocatalytic reduction because CO2 formed by the oxidation of formate is released easily from the reaction system [4]. For example, for the reduction of aromatic ketones by the... 

A discussion of ligand exchange reactions of organometallic compounds associated with oxidation-reduction processes leading to free-radical formation will be found in Volume 14 (Free-radical polymerization). 

Besides induced oxidation-reduction reactions we often speak of induced dissolution, induced precipitation, as well as of induced complex formation there is even a reference to an induced reaction caused by neutralization. It is only necessary to examine briefly the latter cases. 

There are also examples of induced complex formation, an essential step of which is always an oxidation-reduction reaction. Rich and Taube found that the rate of exchange between PtCl and Cl was considerably increased by addition of cerium(rV). In the presence of this oxidizing agent a labile complex of Pt(III) is formed, the chloride of which is easily exchangeable. Exchange of platinum between PtCl and PtClg is similarly rapid via the intermediate labile PtCIs complex formed by cerium(IV). 

Summarizing, it can be said that induced reactions are connected mostly with oxidation-reduction processes and that this is true for the induced complex formation, too. On the other hand, induced precipitation has nothing to do with genuine induced reactions therefore it is advisable not to use this collective name in order to keep the concept of chemical induction clear. 

For the majority of redox enzymes, nicotinamide adenine dinucleotide [NAD(H)j and its respective phosphate [NADP(H)] are required. These cofactors are prohibitively expensive if used in stoichiometric amounts. Since it is only the oxidation state of the cofactor that changes during the reaction, it may be regenerated in situ by using a second redox reaction to allow it to re-enter the reaction cycle. Usually in the heterotrophic organism-catalyzed reduction, formate, glucose, and simple alcohols such as ethanol and 2-propanol are used to transform the... 

Reductive dechlorination in combination with the elimination of chloride has been demonstrated in a strain of Clostridium rectum (Ohisa et al. 1982) y-hexachlorocyclohexene formed 1,2,4-trichlorobenzene and y-l,3,4,5,6-pentachlorocyclohexene formed 1,4-dichlorobenzene (Figure 7.69). It was suggested that this reductive dechlorination is coupled to the synthesis of ATP, and this possibility has been clearly demonstrated during the dehalogenation of 3-chlorobenzoate coupled to the oxidation of formate in Desulfomonile tiedjei (Mohn and Tiedje 1991). Combined reduction and elimination has also been demonstrated in methanogenic cultures that transform 1,2-dibromoethane to ethene and 1,2-dibromoethene to ethyne (Belay and Daniels 1987). 

Evaluating the results a clear kinetic picture of the catalysts has been obtained. In the steady state the active sites in Fe- and Cu-ZSM-5 are nearly fully oxidized, while for Co only -50% of the sites are oxidized. The former catalysts oporate in an oxidation reduction cycle, Fe /Fe and CuVCu. Coi in zeolites is hardly oxidized or reduced, but ESR studies on diluted solid solutions of Co in MgO indicate that Co -0 formation is possible, rapidly followed by a migration of the deposited oxygen to lattice oxygen and reduction back to Co [36]. For Fe-ZSM-5 such a migration has been observed, so a similar model can be proposed for the zeolitic systems. Furthermore, it is obvious that application of these catalysts strongly depends on the composition of the gas that has to be treated. 

Using electrons for the electrolytic reduction of metal salts, Reetz and coworkers have introduced a further variation to the tetraalkylammoniumhalide-stabilization mode [192-198]. The overall electrochemical process can be divided into the following steps (i) oxidative dissolution of the sacrificial Metbuik anode, (ii) migration of Met ions to the cathode, (iii) reductive formation of... 

Birss VI, Chang M, Segal J. 1993. Platinum oxide film formation reduction— An in-situ mass measurement study. J Electroanal Chem 355 181-191. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

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