Reductive Dissolution Reactions

Although crude cyanoamidine (10) can be used for many reactions, reduction to the 2-formyl-A-norsteroid (11) is most satisfactory when purified material is employed. The crude cyanoamidine is stirred for about 15 min with boiling toluene (120 ml/g of steroid) to effect dissolution, the hot solution is filtered quickly through fluted paper, and the filtrate is cooled and diluted with an equal volume of petroleum ether. The mixture is cooled for 0.5 hr in ice, affording from 25 g of crude material about 18 g of colorless 2a-(A-pyrrolidinylcyanoiminomethyl)-A-nor-5a-androstan-17 -ol (10) mp 252-255° (anal, sample mp 262-263°, from benzene-hexane 250 m ... 

Several pathways may contribute to the overall dissolution reaction. Over the course of the dissolution, the relative importance of the contributing pathway may change, possibly due to accumulation of some reactant (Sulzberger et al., 1989 Siffert and Sulzberger, 1991). Many general reviews on reductive and oxidative... 

Since k2/k 2 corresponds to the equilibrium constant of the redox reaction (redox potential), Eq. (9.12) suggests that the dissolution reaction may depend both on the tendency to bind the reductant to the Fe(III)(hydr)oxide surface and (even if the electron transfer is not overall rate determining), on the redox equilibrium (see Fig. 9.4b). 

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

Electron transfer reactions of metal ion complexes in homogeneous solution are understood in considerable detail, in part because spectroscopic methods and other techniques can be used to monitor reactant, intermediate, and product concentrations. Unfavorable characteristics of oxide/water interfaces often restrict or complicate the application of these techniques as a result, fewer direct measurements have been made at oxide/water interfaces. Available evidence indicates that metal ion complexes and metal oxide surface sites share many chemical characteristics, but differ in several important respects. These similarities and differences are used in the following discussions to construct a molecular description of reductive dissolution reactions. 

Organic ligands without redox reactivity that coordinate metal oxide surface sites have been found to enhance rates of both reductive and non-reductive dissolution reactions ( 7). ... 

The high solubility and lability of reduced metal centers should make metal ion release fast relative to preceding steps in many reductive dissolution reactions. 

Effect of Oxide Mineralogy on Reductive Dissolution. Oxide/hydrox-ide surface structures and the coordinative environment of metal centers may change substantially throughout the course of a reductive dissolution reaction. Nonstoichiometric and mixed oxidation state surfaces produced during surface redox reactions may exhibit dissolution behavior that is quite different from that observed with more uniform oxide and hydroxide minerals. 

Fig. 9-15. Oxidative and reductive dissolution reactions of semiconductor electrodes of ionic compounds (a) cation dissolution coupled with anodic hole oxidation of surface anions, (b) anion dissolution coupled with cathodic electron reduction of surface cations.

In order that the oxidative and the reductive dissolution reactions may proceed, the affinity for the reaction is required to be positive the reaction affinity is represented by the difference between the Fermi level, ef(sc), of the electrode and the equivalent Fermi level Er(dK) of the ion transfer reaction (Refer to Sec. 4.4.). 

Fig. 9-18. Band edge levels and equivalent Fermi levels of oxidative and reductive dissolution reactions of compound semiconductors in aqueous sohitions at pH 7 en ) - F(a

Figure 9-18 illustrates the band edge levels of compound semiconductor electrodes in aqueous solutions and the equivalent Fermi levels of the following oxidative and reductive dissolution reactions ... 

Effects of Flooding and Redox Conditions on OClAIC. Reductive dissolution reactions of the sort indicated in Figures 2.6 and 2.7 will affect the amount of a solute in diffusible forms in the soil and the distribution of the diffusible forms between the soil solid and solution. These processes are discussed in detail in Chapter 3. 1 here exemplify their effects by reference to a study of phosphate diffusion in a soil under different water regimes. 

These equations can be solved simultaneously to obtain the new composition of the soil solution. Assume X ei and constant in spite of reductive dissolution reactions. 

Iodine is present in the environment predominantly in the oxidation states —1 (1, iodide) and - -5 (lOs", iodate). Reduction of lOs" to 1 occurs at pe = 13.3 at pH 5 and pe° = 11.3 at pH 7. Hence 1 is expected to predominate in the soil solution except in oxic alkaline soils (Whitehead, 1984). However Yuita (1992) found predominantly IO3 in acid Japanese soils contaminated with iodine the concentrations in solution were some 20 times those of 1 and I2. On flooding the soils, the total concentration of 1 in solution increased 10- to 50-fold, predominantly as I. The concentrations of sorbed 1 were not measured, but both lOs and 1 are expected to be bound to organic matter and oxides and hence their concentrations in solution are expected to increase with reductive dissolution reactions. Further, for a given concentration in solution, 1 is more rapidly absorbed by plants than IO3 (Mackowiak and Grossl, 1999). Hence flooding is expected to increase accumulation in plants both through increased solubility and increased absorption. 

This assumption is based on three relevant indications. First, this wave results in a limiting-current. This means that steady-state transport phenomena control the rate of this reaction, which is not compatible with a possible oxidation of metallic copper to Cu(I) or Cu(II). If the latter were to be valid, a peak-shaped response should have been obtained because of the limited available amount of metallic copper (initially deposited by reduction of Cu(II) or Cu(I) in the reduction wave). In addition, the second voltammetric oxidation wave in the backward scan direction is actually compatible with such a dissolution reaction. 

Cathodic limits on mercury. In aqueous or other protic solvents the reduction of hydronium ion or solvent generally will limit the negative potential range. The nature of some electrode reactions at highly negative potentials on mercury has been examined.63 For example, K(OH2) and Na(OH2)4 ions are reduced reversibly in aqueous solutions, but the process is accompanied by a parallel irreversible reaction due to an amalgam dissolution reaction of the alkali metal with water that produces hydrogen. 

Zinc-Manganese Dioxide. In 1866 Leclanche invented a galvanic cell in which the reduction of Mn02 is the cathodic reaction in the cell s discharge. The corresponding anodic dissolution reaction is the oxidation of zinc. The Leclanche cell is a (so-called) dry cell, i.e., the ammonium electrolyte is immobilized in the form of a paste. There are three forms of the zinc-manganese dioxide batteries ... 

Fig. 1.1. A possible mechanism for (he reductive dissolution of hematite by oxalic acid in the presence of light (after Stumm et al.6). See Section 3.4 for additional discussion of reductive dissolution reactions.

Equilibria involving reductive dissolution reactions add to the complexity of mineral solubility phenomena in just the way that pE-pH diagrams are more complicated than ordinary predominance diagrams, like that in Fig. 3.7. The electron activity or pE value becomes one of the master variables whose influence on dissolution reactions must be evaluated in tandem with other intensive master variables, like pH or p(H4Si04). Moreover, the status of microbial catalysis under the suboxic conditions that facilitate changes in the oxidation states of transition metals has to be considered in formulating a thermodynamic description of reductive dissolution. This consideration is connected closely to the existence of labile organic matter and, in some cases, to the availability of photons.26... 

The kinetics of reductive dissolution reactions is made complicated (relative to the conceptual picture developed in Section 3.1) by electron transfer processes, similar to the way in which these processes bring complexity to... 

The dissolution reaction in Eq. 3.59b can be regarded as an example of a ligand-promoted process, in that adsorbed bicarbonate species are likely to play a role as intermediates in the kinetic analysis of the reaction.5 Ligand-promoted dissolution reactions are a principal basis for the reductive dissolution processes described in Section 3.4 (see Eq. 3.46). The sequence of steps is analogous to that in proton-promoted dissolution ... 

A. T. Stone and J. J. Morgan, Reductive dissolution of metal oxides, Chap. 9 in Aquatic Surface Chemistry, ed. by W. Stumm, Wiley, New York, 1987. This chapter provides a comprehensive discussion of rate laws for reductive dissolution reactions, especially mathematical models. 

Stumm, W., Aquatic Surface Chemistry, Wiley, New York, 1987. This edited volume offers advanced reviews of adsorption, surface oxidation-reduction, and mineral dissolution reactions with emphasis on metal oxyhydroxide adsorbents. 

There are many reactions in soil-water systems pertaining to nutrient availability, contaminant release, and nutrient or contaminant transformations. Two processes regulating these reactions are chemical equilibria (Chapter 2) and kinetics. The specific kinetic processes that environmental scientists are concerned with include mineral dissolution, exchange reactions, reductive or oxidative dissolution, reductive or oxidative precipitation, and enzymatic transformation. This chapter provides a quantitative description of reaction kinetics and outlines their importance in soil-water systems. 

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