Reductive dissolution reaction scheme

Reductive Dissolution of Oxides by Organic Reductants 164 Reaction Scheme and Mechanism 164 Specific Studies 166... 

The following reaction sequence in reductive dissolution is plausible and is exemplified (Suter et al., 1991) here for the reaction of Fe(III)(hydr)oxide with ascorbate. It follows the general scheme given in Eqs. (9.4a) - (9.4c). 

By following the reaction scheme proposed by dos Santos Afonso and Stumm (22) for the reductive dissolution of hematite surface sites (Scheme 1), we were able to explain perfectly the observed pH pattern of the oxidation rate of H2S. The rate is proportional to the concentration of inner-sphere surface complexes of HS" formed with either the neutral (>FeOH) or the protonated (>FeOH2+) ferric oxide surface sites. 

Ferritin induced nanoparticle synthesis was adapted from a number of different synthetic strategies reliant upon the physical nature of ferritin. For instance, ferritin can readily exist in two stable forms (native ferritin with an intact iron oxide core or apo-ferritin lacking a mineral core) owing to the enhanced structural integrity of the protein shell. As a result, two general reaction schemes were adopted. The first route utilized the iron oxide core of native or reconstituted ferritin as a precursor to different mineral phases and types of iron nanoparticles, while the second invokes mineralization within the empty cavity of apo-ferritin. In the latter approach, the native protein must be demetallated by reductive dissolution with thioglycolic acid to yield apo-ferritin. Ultimately, apo-ferritin provides a widely applicable means to the synthesis of various nanoparticle compositions under many conditions. 

An explanation for the transition of the dissolution reaction from valence 4 to valence 2 with increasing light intensity was given in a model proposed by Kooij and Vamnaekelbergh. They assume the presence of an intermediate Si(l), which is an electron-deficient surface silicon atom and has a catalytic effect on the hydrogen reduction reaction. The essential steps in the reaction scheme are described by the following equations ... 

Figure 6 gives the rate of the reductive dissolution of a-Fe203 (hematite) by H2S. The reaction mechanism (27) implies that, in line with the scheme given in equations 14a-l4c, surface complexes of =FeS and of =FeSH are formed and then undergo electron transfer. The dissolution rate, R (mol/m2 per hour), is given by... 

Figure 16 illustrates the reaction scheme that accounts for the reductive dissolution of Fe(III) (hydr)oxides by ascorbate. Figure 17 gives experimental results illustrating the zero-order dissolution rate with varying ascorbate... 

The two main cathodic reactions involved in dissolution (and corrosion) reactions are the reduction of to H2 and the reduction of dissolved O2 to 0H . The following reaction schemes are the ones which have been postulated. 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Both models apply the same chemical scheme of mercury transformations. It is assumed that mercury occurs in the atmosphere in two gaseous forms—gaseous elemental HgO, gaseous oxidized Hg(II) particulate oxidized Hgpart, and four aqueous forms—elemental dissolved HgO dis, mercury ion Hg2+, sulphite complex Hg(S03)2, and aggregate chloride complexes HgnClm. Physical and chemical transformations include dissolution of HgO in cloud droplets, gas-phase and aqueous-phase oxidation by ozone and chlorine, aqueous-phase formation of chloride complexes, reactions of Hg2+ reduction through the decomposition of sulphite complex, and adsorption by soot particles in droplet water. 

The electrochemical reduction of acid chlorides takes a very different course when carried out in an undivided cell equipped with nickel cathode and anode79. The product is a symmetrical ketone (57) 57 is formed by a complex sequence involving both electrodes (Scheme 12). This is really a chemical reaction induced by a highly reactive form of nickel produced by dissolution of the anode and plated onto the cathode. We have already encountered similar chemistry involving highly reactive zinc (Section V.A.l). 

A formal total synthesis of ( )-morphine has been achieved by adopting the above synthetic route (Scheme 18). The tetrahydropyridine 91, prepared from the reaction of A/ -methyl-4-piperidone with 2,3-dimethoxy-phenyllithium, followed by dehydration, was converted to the bicyclic en-amine 92 by treatment with the ylic dibromide. Kinetic protonation of 92 with perchloric acid gave the trans-fused immonium salt, which upon dissolution in methanol equilibrated to the thermodynamically prefered cis isomer 93. Treatment of 93 with diazomethane brought about the formation of the aziridinium salt 94, which was readily transformed into the a-amino aldehyde 95 by its oxidation with dimethyl sulfoxide. It is also worth noting that the Komblum oxidation of aziridinium salts leads to the construction of a-amino aldehydes efficiently. Lewis-acid-catalyzed cyclization of 95 afforded the morphinan carbinol 96 in 80% yield. Successive mesylation and reduction of the mesylate derived from 96 with LiBEtjH afforded morphinan (97) in excellent yield. In this instance, direct conversion of 93 to 97 by treatment with diazomethane gave approximately 1 % of the desired product. Lemieux-Johnson oxidation of 97 under acidic conditions furnished the ketone 98, which was previously transformed into ( )-morphine by Gates. In order to confirm the structure of 98, its conversion to the known... 

Electrochemical corrosion of metals follows the scheme indicated in Fig. 1.1 in two reactions, i.e. anodic dissolution of the metal and cathodic reduction... 

Copper turnings were used to effect "slow corrosion" i.e. controlled production of Cu(I) by dissolution. The actual catalyst is Cu(I) and the proposed mechanism has been given by Scheme 2 [58]. Complexation of Cud) by the amine is followed by reaction with dioxygen to yield an oxocopper(III) species, whose reduction is accompanied by H-abstraction from the a-carbon. This mechanism is supported by kinetic measurements and a primary deuterium isotope effect of = 3.6... 

Metals associated with various binding sites on sediments have been assessed using extraction procedures applied as single digests or as a set of sequential steps. Selective dissolution of trace metals from the particle surfaces is followed by determination using atomic absorption spectrometry (AAS), ICP-MS, or total reflection X-ray fluorescence. The use of sequential extraction schemes for the operational definition of metal species in sediments has proved contentious. They have been criticized on the basis that the reactions are not sufficiently phase selective and labile phases could be transformed during sample preparation, causing a marked reduction... 

Disulfides, such as the diallyl disulfide shown in Rgure 8.24, can be obtained by the reaction of sodium disulfide (NaiSi), itself prepared by the dissolution of an equivalent of sulfur in a concenctrated sodium sulfide (Na2S) solution, with the corresponding alkyl halide. Reduction of the disulfide (e.g., with zinc dust in acetic acid) results in cleavage of the sulfur-sulfur bond and thiol formation (Scheme 8.105). Alternatively, as might be expected, thiols are rather easily oxidized (e.g., by hypohalites [NaOCl]) and the products are disulfides (Equation 8.60). 

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