Oxidation by Fe

The rate law of the oxidation by Fe(III) is dependent on the ratio of the concentrations of the reactants. When peroxide is in excess and when the acidity is sufficient to suppress hydrolysis of Fe(IlI) the rate expression... 

The oxidation by Fe(III) chloride is kinetically complex . Only in the presence of excess acid and Fe(II) was a rate law discernible, viz. 

These are oxidised by both Fe(III) and Cu(II) octanoates (denoted Oct) in nonpolar solvents at moderate temperatures . 80-90 % yields of the corresponding disulphide are obtained with Fe(III) and this oxidant was selected for kinetic study, the pattern of products with Cu(II) being more complex. The radical nature of the reaction was confirmed by trapping of the thiyi radicals with added olefins. Simple second-order kinetics were observed, for example, with l-dodecane thiol oxidation by Fe(Oct)3 in xylene at 55 °C (fcj = 0.24 l.mole . sec ). Reaction proceeds much more rapidly in more polar solvents such as dimethylformamide. The course of the oxidation is almost certainly... 

The two terms correspond to oxidation by Fe " and Fe(OH) which exist in a hydrolytic equilibrium. A radical intermediate CH3COC(OH)CH3, which suffers rapid oxidation by a second ferric ion, is proposed. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Schematic representation of the various reaction modes for the dissolution of Fe(III)(hydr)oxides a) by protons b) by bidentate complex formers that form surface chelates. The resulting solute Fe(III) complexes may subsequently become reduced, e.g., by HS c) by reductants (ligands with oxygen donor atoms) such as ascorbate that can form surface complexes and transfer electrons inner-spheri-cally d) catalytic dissolution of Fe(III)(hydr)oxides by Fe(II) in the presence of a complex former e) light-induced dissolution of Fe(III)(hydr)oxides in the presence of an electron donor such as oxalate. In all of the above examples, surface coordination controls the dissolution process. (Adapted from Sulzberger et al., 1989, and from Hering and Stumm, 1990.)...

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

To confirm that the effects of NO on O2 consumption are due to inhibition of hpid peroxidation, we also examined the effect of NO on TBARS, a product of lipid peroxidation. Cells were oxidized by Fe ", and 0.9 (jM N0 was added 1 min later. Cells were collected after 5 min for assay. Figure 5 shows that Fe " increased lipid peroxidation, and NO inhibited it by 63% (after subtracting basal levels). It is noteworthy that the percentage inhibition of O2 uptake by NO as measured by the change in O2 concentration under similar conditions was similar (78%) lending verification to these complementary methods. These results confirm the relationship of O2 consumption and lipid peroxidation in these experiments... 

There are various possible explanations for the decrease in oxidation state of the SOM carbon in most of the soils. One is that the SOM comprises different pools of organic matter with carbon in different oxidation states, and Z decreases as a result of preferential oxidation of more-oxidized pools of SOM, leaving a greater proportion of the more-reduced forms in the residue. The more oxidized SOM would include compounds in the original SOM and also compounds generated in the course of reduction, for example as a result of chemical oxidation by Fe(III) and other metal oxides. Alternatively, part of the organic matter could... 

The tertiary and primaiy hydroxyalkyl radicals are produced in the ratio 7.2 1, respectively. The former is rapidly oxidized by Fe (supporting the Fenton process) while a big proportion of the latter is accumulated in solution and terminates the redox chain by dimerization, according to the reactions (15) and (16), respectively ... 

In rare cases, the one-electron oxidized products are also readily oxidized, and the three-electron oxidized product is observed. A case in point is the oxidation of the 4-chlorobenzyl radicals by Fe(CN)63 to the corresponding benzaldehyde. The 4-chlorobenzylalcohol is not the intermediate that is further oxidized by Fe(CN)63, and thus the mechanism of the formation of 4-chlorobenzaldehyde is rather complex (Merga et al. 1996). Since Fe(CN)63 is commonly used as a simple and effective oxidant also in DNA free-radical chemistry, such potential complexities have to be kept in mind. 

Situation (a) is rather unambiguously recognizable and believed to be a very common one. It includes the initial step in redox reactions between neutral or ionic organic molecules or radicals and the typical one-electron redox reagents, e.g., oxidation by Fe(III), Co(III), Mn(III), Tl(III), Ag(II), Ce(IV), Pb(IV),... 

If it is assumed that this reflects a mechanism in which NH2OH is oxidized, then the rate constant for electron transfer is 7.3 x 103 M 1 sec1. An upper limit of 1 x 1010 M-1 sec-1 for the reverse reaction establishes E° < 1.26 V for the NH2OH+/NH2OH couple. This result should be accepted with some caution because unpublished experiments by the present author indicate that the reaction is catalyzed by adventitious copper (291), as was the case in the oxidation by Fe(CN)63 (58). 

That various forms of Sb(IV) must be considered can be inferred from the observation that Ce(IV) oxidizes Sb(III) directly and in Cl -catalyzed paths (219), that the oxidation by Fe(CN)63 is second order in [Sb(III)] (176), and that reduction of SbCl6 by Fe2+ and Fe(CN)64-occurs with Fe(III) inhibition (25). The thermochemistry of these free radicals is unknown. 

An electron can be removed from an anion, and the process is known as the oxidation process. For example, the phenoxide ion is oxidized by Fe " " to give the phenoxy radical and the Fe " " is co-reduced to Fe +. This is known as the single-electron-transfer (SET) oxidation (Scheme 2.31). 

Connelly et al., (81) prepared a family of complexes of general formula [Cl3M NC)Mn CO)(dppm)2] (M = Mn, Co, Ni), the structures of which contain both tetrahedrally, M(II), and octahedrally Mn(II), coordinated metal ions (Fig. 12). The analogous compound with M = Fe (82) could not be obtained because of a redox process that resulted in an electron transfer from Fe(II) to Mn(II) and formation of [Cl3Fe (NC)Mn (CO)(dppm)2] with a diamagnetic low-spin Mn(I) center. This complex was oxidized by [Fe (Cp)(CpCOMe)](BF4) to yield the salt [Cl3Fe NC)Mn CO)(dppm)2](BF4), the cation of which contains two paramagnetic metal centers, namely, Fe(III) and Mn(II). 

Fe poses less of a problem although it in turn became less available with onset of oxidation by Fe Oj precipitation (yet, there is replenishment of Fe(ll) by both volcano eruptions (olivine) and aquatic photoreduction), with c - and x parameters of Cu(II) and Fe(III) being rather similar to each other. 

Hydrogen sulfide can be oxidized in less than an hour in seawater. This removal can be through oxidation by oxygen or iodate. There is a possibility of oxidation, by hydrogen peroxide, but it is probably a minor pathway (Radfordknoery and Cutter, 1994). Photo-oxidation is also possible (Pos et ai, 1998), along with oxidation by Fe(III) oxide particles. This latter process is dependent on the way in which the particle forms and on pH with a maximum near 6.5. The Fe(III) oxide route gives mostly elemental sulfur as a product, which may have implications for pyrite formation (Yao and Millero, 1996). 

A series of batch and mixed flow reactor experiments was performed at pH <3 to determine the effect of SO , Cl , ionic strength, and dissolved O2 on the rate of pyrite oxidation by Fe ". Of these, only dissolved O2 had any appreciable affect on the rate of pyrite oxidation in the presence of Fe. Williamson and Rimstidt (1994) combined their experimental results with kinetic data reported from the literature to formulate rate laws that are applicable over a range panning six order of magnitude in Fe " " and Fe " " concentrations, and for a pH range of 0.5-3.0, when fixed concentrations of dissolved O2 are present ... 

Galena may also be oxidized by Fe(III) under acidic conditions (Rimstidt et al., 1994) ... 

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