Dissolving metals mechanism

As shown in Fig. 24, the mechanism of the instability is elucidated as follows At the portion where dissolution is accidentally accelerated and is accompanied by an increase in the concentration of dissolved metal ions, pit formation proceeds. If the specific adsorption is strong, the electric potential at the OHP of the recessed part decreases. Because of the local equilibrium of reaction, the fluctuation of the electrochemical potential must be kept at zero. As a result, the concentration component of the fluctuation must increase to compensate for the decrease in the potential component. This means that local dissolution is promoted more at the recessed portion. Thus these processes form a kind of positive feedback cycle. After several cycles, pits develop on the surface macroscopically through initial fluctuations. 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

Whatever the best explanation may be, an indication that allylic alkali metal compounds or allylic carbanions do in fact form the less stable of the two possible acids on neutralization is found in the results of the reduction of aromatic compounds by dissolving metals.376The detection of a paramagnetic intermediate in a similar system and polaro-graphic evidence indicate a one electron transfer in the rate and potential determining step.878 376 The mechanism therefore involves ions (or organometallic intermediates) like the following ... 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

We therefore use a dissolving metal reduction in strong acid. This reaction, the Clemmensen reduction, may use the principle we have outlined here, but its mechanism is unknown in detail. R2C = 0 + Zn/Hg + cone. HC1— )RaCH2... 

The electron transfer to the acetylenic bond forms the frans-sodiovinyl radical 20 that, after protonation, produces tram radical 21. At low temperature (—33°C) in the presence of excess sodium, the conversion of the trans radical to sodiovinyl intermediate 22 is slightly more rapid than the conversion of the tram radical to the cis radical 23 (21 —> 22 > 22 —> 23). As a result, protonation yields predominantly the trans alkene. However, low sodium concentration and increased temperature lead to increasing proportion of the cis alkene. Although other dissolving-metal reductions are less thoroughly studied, a similar mechanism is believed to be operative.34 Another synthetically useful method for conversion of alkynes to trans alkenes in excellent yields is the reduction with CrS04 in aqueous dimethylforma-mide.198... 

After Abe s work the problem again lay dormant for a number of years until it was taken up by Wilmarth and his co-workers. Claeys, Baes, and Wilmarth (29) in 1948 reported that a liquid ammonia solution of potassium metal rapidly catalyzed o-p H2 conversion, a half-time in solution of 37 sec. being obtained at —53°. In order to establish that this result was due to dissolved metal and not to amide ion impurity, Claeys, Dayton, and Wilmarth (30) studied the o-p H2 conversion in the presence of potassium amide in liquid ammonia. Rates were obtained comparable with those occurring with the metal solution. The mechanism of the conversion was different for the two cases, however, since the amide solution also catalyzed exchange between gaseous deuterium and liquid ammonia, while the metal solution did not. It was assumed that the metal acted by a paramagnetic mechanism and the amide ion by a chemical mechanism. In the same note Claeys, Dayton, and Wilmarth (30) reported confirmation of Wirtz and Bonhoeffer s results on the aqueous alkali system and questioned the validity of Abe s objections. 

Research conducted at Washington State University, as well as in situ applications by commercial entities, has indicated that stabilization of hydrogen peroxide is necessary for effective subsurface injection [39]. Without stabilization, added peroxide decomposes rapidly through interaction with iron oxyhydroxides, manganese oxyhydroxides, dissolved metals, and enzymes (e.g., peroxidase and catalase). Some of these peroxide decay pathways involve nonhydroxyl radical-forming mechanisms, and therefore are especially detrimental to Fenton oxidation systems. 

If deactivation of black-oil conversion catalysts is a result of pore plugging, two mechanisms are plausible. Pore plugging by dissolved metals proceeds by successive decomposition of molecules of a metal organic complex until a plug develops by accretion. Pore plugging by particulates is accomplished by the trapping of a single particle or a small number of particles in a pore of a size comparable with the particle diameter. 

The mechanism has a good deal in common with a whole class of reductions, of which the Clemmensen is a member, known as dissolving metal reductions. We shall now look at these as our third (after metal hydrides and catalytic hydrogenation) important class of reducing agents. 

Generation of the radical cation of aromatic substrates in the presence of sodium boron hydride offers another path for reduction, alternative to that via the radical anion seen in Sect. 2.1.2, and, as one may expect in view of the different mechanism, with a different regiochemistry [174-175], Thus, e.g. irradiation of the xylenes in the presence of m-dicyanobenzene and NaBH4 yields the corresponding 1,4-dihydro derivatives rather than the 2,5-dihydro derivatives obtained with dissolved metals [175]. 

The desired product 6 was isolated in only 25% yield. Bearing in mind the mechanism of the dissolving metal ring opening reaction, suggest a structure for the major product (36%) isolated from the reaction mixture. 

Write a stepwise mechanism analogous to that of the dissolving metal reaction that reduces alkynes to trans alkenes for the following reaction ... 

The stereochemical course of these, and other similar reductions, led Barton to suggest that dissolving metal reductions of ketones and oximes to secondary alcohols and primary amines would lead to mixtures of products rich in the thermodynamically more stable product. However, in the early 1960s a number of reports appeared in which the reduction of ketones gave primarily the thermodynamically less stable epimeric alcohol. These observations have prompted a continuing series of investigations into the mechanism of these reductions. 

It was suggested in the 1950s that the reduction of aliphatic ketones by dissolving metals proceeded by two sequential one-electron additions to provide a dianion (equation 1). This mechanism was based on the observation that benzophenone affords a dianion on reaction with excess Na in liquid NH3, and it was inferred that aliphatic ketones would behave similarly. A number of workers presented mechanistic rationalizations for the stereochemical course of the dissolving metal reductions of cyclic aliphatic ketones based on this dianion concept. However, in a 1972 review, it was noted that the reduction potentials of alkali metals were not sufficient to effect the addition of two electrons to an aliphatic carbonyl group, and an alternative mechanism was suggested which with some modification is now generally accepted. ... 

A variety of other less common reaction conditions, which employ a variety of either dissolving metals or low-valence metal cations, have been used to effect the reduction of carbonyl groups to primary or secondary alcohols. Although the mechanisms of these reactions have not been explored in detail, they almost certainly proceed by mechanisms similar to those outlined above. 

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... 

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