Reduction with metals mechanism

Evans, D.A. Nelson, S.G. Gagne, M.R. Mud, A.R. J. Am. Chem. Soc., 1993,115,9800. It has been that shown in some cases reduction with metal alkoxides, including aluminum isopropoxide, involves free-radical intermediates (SET mechanism) Screttas, C.G. Cazianis, C.T. Tetrahedron, 1978, 34, 933 Nasipuri, D. Gupta, M.D. Baneijee, S. Tetrahedron Lett., 1984, 25, 5551 Ashby, E.C. Argyropoulos, J.N. Tetrahedron Lett.,... 

In terms of mechanism and stereochemical consequences, reductions by dissolving metals in liquid NH3 are very similar to reductions by the same metals in alcoholic media. However, reductions carried out in liquid ammonia do not suffer from the same inherent problems as those by metals in alcohols. There is no evidence for equilibration of the product alcohols, and ketones which undergo epimerization prior to reduction with metals in alcohols are reduced cleanly by metals in NH3. For example, menthone (4) on reduction with Li-NHs-ethanol gives a mixture of alcohols (6) and (7), with no trace of alcohols (8) and (9).22... 

The mechanism and stereochemistry of the reduction with metal hydride has been dealt with in many articles. It may be stated in general that the structure of the oxirane and the nature of the reducing agent exert great effects on the rate and pathway of the reaction. 

Not all free-radical mechanisms are chain reactions, and those that are not do not require initiators. Reductions with metals such as Li, Na, or Sml2 (often in liquid NH3) and light-promoted rearrangements of carbonyl compounds proceed by nonchain free-radical mechanisms. Compounds containing weak cr bonds (typically either heteroatom-heteroatom bonds or very strained bonds) can undergo intramolecular rearrangements by nonchain free-radical mechanisms upon heating. 

It is noteworthy that in some cases reduction with metal alkoxides, including aluminium isopropoxide, involves free-radical intermediates (single electron transfer (SET) mechanism). ... 

It is considered in adsorption-autocatalytic theory that the interaction happens at the interface of gas-solid. In some cases, it is foimd that of the auto-catalytic phenomenon happens at the early stages of reduction. The value of this theory indicates the necessity of direct contact of the reductant with metal oxides. It is possible to evaluate the mechanism and kinetics of reduction by use of law of physical chemistry, physics and surface chemistry. The great effect of product (H2O) on the rate of reaction confirms the important role of adsorption. Because H2O is a very active adsorbent, it can occupy the most active areas of oxide, and thus greatly reduce the rate and degree of reduction. 

The degradation which occurs on reduction with the alkali metals involve a mechanism that is considered to be firmly established and in the isoxazole series appears to proceed according to the usual scheme,... 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

The scheme of the interaction mechanism (Equation 88) testifies to an electro-affinity of MeFe" ions. In addition, MeFe" ions have a lower negative charge, smaller size and higher mobility compared to MeF6X(n+1> ions. The above arguments lead to the assumption that the reduction to metal form of niobium or tantalum from melts, both by electrolysis [368] and by alkali metals, most probably occurs due to interaction with MeF6 ions. The kinetics of the reduction processes are defined by flowing equilibriums between hexa-and heptacoordinated complexes. 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

This mechanism is based on the known importance of hydroxides in other deposition reactions, such as the anomalous codeposition of ferrous metal alloys [38-39], Salvago and Cavallotti claim an analogy with the mechanism of Ni2 + reduction from colloids in support of their proposed mechanism. There is no direct evidence for the hydrolyzed species, however. Furthermore, the mechanism does not explain two experimentally observed facts Ni deposition will proceed if the Ni2 + and the reducing agent are in separate compartments of a cell [36, 37] and P is not deposited in the absence of Ni2 +. The chemical mechanism does not take adequate account of the role of the surface state in catalysis of the reaction. It has no doubt been the extreme oversimplification, by some, of the electrochemical mechanism that has led other investigators to reject it. 

On the other hand, if 02 existed in the reaction system, the reaction mechanism would be affected by the reactions with 02 the reaction mechanism is dependent on the types of dissolved gases in the sample solution. The details for the effects of various parameters on the reduction of metal ions and formation of metal nanoparticles are described in the following sections. 

The organometallic complexes with d-metals are considered as promising electrocatalysts for oxygen electroreduction in air-metal electrochemical cells. Obviously, the first idea was to employ the catalytic mechanism of the oxygen reduction with porphyrin-like metal complexes [1] found in living beings (Figure 1). 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

The diversity of the substrates, catalysts, and reducing methods made it difficult to organize the material of this chapter. Thus, we have chosen an arrangement related to that used by Kaesz and Saillant [3] in their review on transition-metal hydrides - that is, we have classified the subject according to the applied reducing agents. Additional sections were devoted to the newer biomimetic and electrochemical reductions. Special attention was paid mainly to those methods which are of preparative value. Stoichiometric hydrogenations and model reactions will be discussed only in connection with the mechanisms. 

TPR of supported bimetallic catalysts often reveals whether the two metals are in contact or not. The TPR pattern of the 1 1 FeRh/SiOi catalyst in Fig. 2.4 shows that the bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the less noble iron. This forms evidence that rhodium and iron are well mixed in the fresh catalyst. The reduction mechanism is as follows. As soon as rhodium becomes metallic it causes hydrogen to dissociate atomic hydrogen migrates to iron oxide in contact with metallic rhodium and reduces the oxide instantaneously. 

Nitric acid synthesis, platinum-group metal catalysts in, 19 621 Nitric acid wet spinning process, 11 189 Nitric oxide (NO), 13 791-792. See also Nitrogen oxides (NOJ affinity for ruthenium, 19 638—639 air pollutant, 1 789, 796 cardioprotection role, 5 188 catalyst poison, 5 257t chemistry of, 13 443—444 control of, 26 691—692 effect on ozone depletion, 17 785 mechanism of action in muscle cells, 5 109, 112-113 oxidation of, 17 181 in photochemical smog, 1 789, 790 reduction with catalytic aerogels, l 763t, 764... 

---