Ferric ion catalysis

An interesting observation which must be accounted for in any theory of the ferric ion catalysis was made by Bohnson and Robertson, who found that the catalytic decomposition by ferric salts is considerably enhanced if cupric salts are present (51,63,64). The resulting rate is much higher than that expected from the sum of the individual ferric and cupric ion rates. For a constant ferric ion concentration the enhanced rate at first increases with added cupric ion but ultimately reaches a limit beyond which further cupric ion has little effect. Bohnson and Robertson assumed that in this promoted catalysis the rate was first order in peroxide concentration as had been found for the ferric ion alone, but there is as yet no experimental evidence for this. Analysis of their numerical data (43,62) shows that when the maximum promotion by cupric ion has been reached the rate of peroxide decomposition is proportional to [Fe+++]H/[H+]. 

The ferric ion catalysis has been considered by Haber and Weiss (4) in terms of the reactions of radicals along the lines adopted to explain the ferrous ion reaction. They proposed the following mechanism ... 

Barb et al. have also considered the ferric ion catalysis kinetics at low values of R2 where, as stated above, deviations from, the von Bertalan equation (c) occur. They conclude that with decreasing R2 reactions (3) and (1) will become of comparable importance as chain-terminating reactions, since peroxide will no longer be of such a concentration as to eliminate the ferrous reaction in the competition for the hydroxyl radical. At... 

The accelerating effect of cupric ions on the ferric ion catalysis which was observed by Bohnson and Robertson is considered by Barb el al. to be due to reaction (5 ), as was the analogous effect of cupric salts on the ferrous ion catalysis. For conditions in which Scheme A applies reaction (4 ) is in effect catalyzed by (5 ). At high cupric ion concentrations (5 ) will eliminate (3), since effectively all the radical HO2 will react in (5 ). In these conditions the enhancement reaches a limit as was observed by Bohnson and Robertson. However (1) now becomes the operative chain-terminating step, and hence the kinetics of Scheme B should apply (Eq. g). Unfortunately no data on the peroxide dependence is available, but analysis of the data (43,66) shows the rate to be proportional to [Fe+++]w as required by (g). There is the same discrepancy in hydrogen ion dependence as was found in the simple ferric reaction. 

In view of the long period for which the ferric ion catalysis has been under investigation it is surprising that it was only recently that deviations from the kinetic equation of von Bertalan have been examined in any detail. It would appear that the system is even yet not completely understood, and clearly more data at low peroxide and high ferric ion concentrations are required as well as a more extensive investigation of the copper enhanced reaction. One other factor which would seem to merit attention is the part played, if any, by the complex FeH02++, which has been shown by Evans, George, and Uri (2) to be formed from Fe+++ and H02. Barb et al. have pointed out that the accepted initiation step (i) could very well be replaced by... 

The catalytic activity of these systems was interpreted by Haber and Weiss (4) in terms of their reaction mechanism for the ferric ion catalysis. They consider that the reaction of ferrous ion with the base to form the complex ion lowers the stationary concentration of ferrous ion and hence leads to longer reaction chains by decreasing the rate of the termination reaction... 

Kiss and Lederer (93) have made the only significant quantitative study of the decomposition of hydrogen peroxide by cupric ions alone and this is not very extensive. The reaction is much slower than the ferric ion catalysis at the same concentrations and acidities. For three... 

Such oxidation reactions may be responsible in part for the enhancement of the chromate catalysis which is produced by Mn++, Co++, Cu++, Ce+++, Ni++ (120,121). Alternatively this promotion may arise from the reaction of these ions with perchromate compounds, and it is possible that chain reactions may occur similar to those in the ferric ion catalysis with the perchromate replacing the peroxide. Uri has suggested such a scheme for promotion in the molybdate and tungstate catalyses (see Sec. IX.3). However the data are too fragmentary for any definite conclusions to be drawn. 

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

A small solvent isotope effect was found by Bell and Onwood kuiolkoiO = 1.08) in contradiction to that of only 0.38 reported by Taylor and Halpern . Over one-third of the oxygen present in the carbonate originated from the oxidant when 0-labelled permanganate was used . The reaction is subject to pronounced catalysis by ferric ions . 

Indeed, when present in concentrations sufficient to overwhelm normal antioxidant defences, ROS may be the principal mediators of lung injury (Said and Foda, 1989). These species, arising from the sequential one-electron reductions of oxygen, include the superoxide anion radical, hydrogen peroxide, hypochlorous ions and the hydroxyl radical. The latter species is thought to be formed either from superoxide in the ptesence of iron ions (Haber-Weiss reaction Junod, 1986) or from hydrogen peroxide, also catalysed by ferric ions (Fenton catalysis Kennedy et al., 1989). 

Catalysis of these reactions has been reported . At pH 3.5, cupric ion is said to catalyse the process (1) and ferric ion, the process (2). The latter appears to be third order in chlorite. Nickel and cobalt salts had less selective action. The first-order reactions reported by Ishi involve rate coefficients comparable to those above and thus the orders may be wrong. The maximum yield of CIO2 near pH 2, which was found by Buser and Hanisch , has been confirmed . A low order (1.69) in chlorate was found with acetate buffers and may be explained by the above mechanism. Further studies in this pH region are required, but it is likely that the process initiated by (11) is predominant, and produces CIO2 more rapidly than either (3) or (10) . 

The reaction is approximately first-order with respect to each reactant (the second-order rate coefficient increases with increase of substrate concentration), and catalysis by hydroxide ions is observed. Henderson and Winkler studied the ferrous ion-catalysed oxidation of thioglycolicacidto dithioglycolic acid. The rate is sensitive to traces of metal ions, and reproducible results could not be obtained in the absence of the catalyst. The oxidation is first-order with respect to both peroxodisulphate and ferrous ions, and zero-order with respect to the substrate. The second-order rate coefficient is approximately equal to that determined in the absence of the substrate, so Henderson and Winkler suggested that the ratedetermining step is the oxidation of ferrous to ferric ions, as in reaction (96), and that this is followed by reaction (97) and then rapid oxidation of thioglycolic acid by ferric ions. 

As another example of novel catalysis employing montmorillonite, the clay was found to show excellent catalytic activity for the addition reaction of trimethylsilyl ketene acetal to a, -acetylenic esters (ynoates), which contrasted strikingly with the reactions induced by a homogeneous acid catalyst, trimethylsilyl triflate (TMSOTf), as well as the addition reactions of lithium enolates with ynoates [Eq. (17)] (89). Table XXIII summarizes the results of the reactions of the silicon and lithium enolates of methyl propionate (21) with ynoates (22a-c). Except for the reaction of 22c, ferric ion-exchanged montmorillonite (Fe-Mont), which is more acidic than Al-Mont, catalyzed exclusive 1,2-additions of trimethylsilyl ketene acetal to 22a and 22b to give 23 in... 

Polarographic catalytic currents" are well known, especially in inorganic electrochemistry, with classic examples, such as the reoxidation of ferrous ions—fonned cath-odically from ferric ions—by hydrogen peroxide or hydroxylamine. However, it appears obvious that the term catalysis is used too often quite ambiguously. Therefore it use in organic electrochemistry needs to be clarified a distinction should be made between the two main kinds of catalysis. This differentiation has been emphasized by Andrieux and Saveant [1,2]. 

A question occurs as to why the bacterial enzyme has such a complicated structure, because hydroxylamine is oxidized to nitrite by the catalysis of ferric ion under aerobic conditions. In the nonenzymatic reaction, molecular oxygen is incorporated into nitrite formed by the oxidation of hydroxylamine, while the oxygen atom of water is incorporated into nitrite formed by the enzymatic oxidation of hydroxylamine (see below) (Yamanaka and Sakano, 1980 Andersson and Hooper, 1983). The mechanism in the bacterial oxidation of hydroxylamine will have been devised to reserve efficiently the energy of the reaction for the biosynthesis of adenosine triphosphate (ATP). 

The above compensating reactions are attractive because of the success of similar schemes in the halide catalysis, but proof in this case is more difficult. Thus it was possible to show in the halide systems that halogen and halide are present simultaneously. Evidence for the presence of ferrous ion in the ferric catalysis would support a similar interpretation. Manchot and Lehmann (44) claimed to have proved that ferrous ion is formed from ferric ion in the presence of peroxide since the addition of <, < -dipyridyl to the mixture resulted in the slow formation of the red ferrous tris-dipyridyl ion Fe(Dipy)3++. However, later work (65,66), which will be discussed when these systems are considered in more detail (IV,6), indicates that the ferrous complex ion may be formed by reduction not of the ferric ion, but of a ferric dipyridyl complex. Similar conclusions on the presence of ferrous ion were drawn by Simon and Haufe (67) from the observation that on addition of ferri-cyanide to the system Prussian blue is formed. This again is ambiguous, since peroxide is known to reduce ferricyanide to ferrocyanide and the latter with ferric ion will of course give Prussian blue (53). 

Barb, Baxendale, George, and Hargrave (43) have applied the results of their analysis of the ferrous reaction to the ferric catalysis. They conclude that for high ratios of peroxide to ferric ion concentrations (Ri) the significant reactions are ... 

The experiments hitherto described dealt with catalytically active electrons and positive holes released by light. They allow only indirect conclusions regarding thermal catalysis. It is felt that direct observations are necessary in the present stage more than ever. Some work along these lines has been mentioned in the Introduction. Other observations on semiconductors of the ferrite type (d) have shown that the carbon monoxide oxidation, a donor reaction, is catalyzed best by inverse spinels, in which ferric ions, situated in octahedral positions, chemisorb carbon monoxide. Zinc ferrite, in which all the occupied octahedral positions carry ferric ions, showed a... 

Purple acid phosphatases contain a dinuclear Fe " M + centre in their active site (M = Fe or Zn +). To resolve the specific role of the ferric ion in catalysis, a series of metal-substituted forms of bovine spleen purple acid phosphatase (BSPAP) of general formula M Zn°-BSPAP has been prepared, in which the trivalent metal ion was systematically varied (M = Al, Fe, Ga and In). The activity of the AlZn-BSPAP form was only slightly lower kcat 2000 s- ) than that of the previously reported GaZn and FeZn forms (fecat 3000 s- ), The InZn form was inactive. This... 

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