Ferrous ions, reactions

B- and Q- bands, 36 212-213 d-d states, 36 212-213 high-spin heme, 36 213-214 low-spin heme, 36 212-213 Ferrous ions reaction with... 

Additionally, reaction (6) can also take place in this system if H2O2 is in excess. Moreover, hydroxyl radicals can also be trapped by excess of ferrous ions (reaction (7)). Thus, despite the advantages such as commercial availability of the oxidant, no mass transfer problems, and formation of hydroxyl radicals from H2O2, this process presents several serious drawbacks. One of the most important one is that H2O2 has to be continuously added in controlled amoimts as a source of hydroxyl radicals (Domenech et al., 2004). 

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 ... 

This choice of reactions is based upon the fact that at high R2 the predominant reaction of the hydroxyl radical will be with peroxide. Hence, as was the case in the ferrous ion reaction, chain termination by (3) is favored over (1). Using approximations which are justified by the numerical values of k2/kx and fc4//c3, the above scheme gives ... 

The dependence on peroxide concentration indicated by (g) was confirmed. It is suggested that Andersen s data of Eq. (d) have been obtained at values of R2 which are not low enough to eliminate reaction (3) entirely, and hence something intermediate between first and three halves power in peroxide concentrations might be expected. However, both these sets of data show a first order dependence on ferric ion concentration and inverse first power in ([H+] + Kh) which are at variance with Eq. (g). This discrepancy has not yet been resolved, and Barb et al. suggest that other entities might be present at these high ferric ion concentrations which play no part in the ferrous ion reaction or the ferric ion reaction at lower ferric ion concentrations. 

Cupric ions which for some time have been known to accelerate the ferric ion catalyzed decomposition of peroxide (Bohnson and Robertson, 91) have also been shown to affect the ferrous ion reaction (83). There is now good kinetic evidence that this arises from the cupric ion reacting more rapidly than the ferric ion with the 02 ... 

Figure 4 is the Pourbaix Diagram for iron and some of its ionic species and compounds in contact with water at 25 C. The equilibrium potential of the iron-ferrous ion reaction falls outside the region of stability of water (dashed lines). Therefore, any attempt to measure the equilibrium potential will fail since the solvent will undergo electrochemical reduc-... 

The once popular Emmerie-Engel analytical method involves the reduction of ferric to ferrous ions, reaction of the latter with 2,2 -dipyridyl and colorimetric measurement. Eor a long time this method remained the backbone of the majority of chemical assays for vitamin E. 

Figure 5.2 Current versus overpotential polarization plot of the ferric/ferrous ion reaction on palladium showing both the anodic and cathodic branches of the resultant current behavior.

Make acid yields coumaUc acid when treated with fuming sulfuric acid (19). Similar treatment of malic acid in the presence of phenol and substituted phenols is a facile method of synthesi2ing coumarins that are substituted in the aromatic nucleus (20,21) (see Coumarin). Similar reactions take place with thiophenol and substituted thiophenols, yielding, among other compounds, a red dye (22) (see Dyes and dye intermediates). Oxidation of an aqueous solution of malic acid with hydrogen peroxide (qv) cataly2ed by ferrous ions yields oxalacetic acid (23). If this oxidation is performed in the presence of chromium, ferric, or titanium ions, or mixtures of these, the product is tartaric acid (24). Chlorals react with malic acid in the presence of sulfuric acid or other acidic catalysts to produce 4-ketodioxolones (25,26). 

The most common water-soluble initiators are ammonium persulfate, potassium persulfate, and hydrogen peroxide. These can be made to decompose by high temperature or through redox reactions. The latter method offers versatility in choosing the temperature of polymerization with —50 to 70°C possible. A typical redox system combines a persulfate with ferrous ion ... 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

The Penniman-Zoph process involves the preparation of seeds or nucleating particles by the alkaU precipitation of ferrous sulfate. The reaction is carried out at alow temperature using an excess of ferrous ions. The hydroxide is then oxidized to the seeds of hydrated ferric oxide ... 

Acid. The reaction requires only enough acid to generate the ferrous ion which is needed to participate in the first step. Alternatively, a ferrous salt can be added directiy. Generally 0.05 to 0.2 equivalents of either hydrochloric or sulfuric acid is used, but both acids have their drawbacks. Hydrochloric acid can cause the formation of chlorinated amines and sulfuric acid can cause the rearrangement of intermediate aryUiydroxylamines to form hydroxyaryl amines. Occasionally an organic carboxyUc acid such as acetic or formic acid is used when there is a danger of hydrolysis products being formed. 

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. 

The ease with which the ferrous ion can be oxidized to a ferric ion in the electrowinning cell furthers this reaction. Attack on the copper is most apparent at the solution line, where it results in corrosion of the loops supporting the cathodes, leading to dropped cathodes. 

G-20 Dicarboxylic Acids. These acids have been prepared from cyclohexanone via conversion to cyclohexanone peroxide foUowed by decomposition by ferrous ions in the presence of butadiene (84—87). Okamura Oil Mill (Japan) produces a series of commercial acids based on a modification of this reaction. For example, Okamura s modifications of the reaction results in the foUowing composition of the reaction product C-16 (Linear) 4—9%, C-16 (branched) 2—4%, C-20 (linear) 35—52%, and C-20 (branched) 30—40%. Unsaturated methyl esters are first formed that are hydrogenated and then hydrolyzed to obtain the mixed acids. Relatively pure fractions of C-16 and C-20, both linear and branched, are obtained after... 

The hydroxyl ions migrate inward, attracted by the positive charge that is produced by the ferrous ion generated near the corroding surface (Fig. 3.4). Other anions such as carbonate, chloride, and sulfate also concentrate beneath the shell. Carbonate may react with ferrous ions to form siderite (FeCOa) as in Reaction 3.4 (Fig. 3.7) ... 

Quite a number of mixed sulfur-nitrogen macrocycles have been prepared, but these have largely been by the methods outlined in Chaps. 4 and 5 for the respective heteroatoms. An alternative method, involves the formation of a Schiff base, followed by reduction to the fully saturated system, if desired. An interesting example of the Schiff base formation is found in the reaction formulated in (6.12). Dialdehyde 14 is added to ethylenediamine in a solution containing ferrous ions. Although fully characterized, the yield for the reaction is not recorded. To avoid confusion with the original literature, we note the claim that the dialdehyde [14] was readily prepared in good yield by reaction of the disodium salt of 3-thiapentane-l, 5-diol . The latter must be the dithiol rather than the diol. 

Emmons proposes as the chain starting reaction a direct attack of the ferrous ion on the oxazirane ring with the formation of an 0-radieal (24) Eq. (20)]. This radical (24) starts a reaction chain fEq. (21 ). By the attack of a further molecule of oxazirane, forma-... 

The most commonly employed reagent for the hydroxylation of aromatic compounds is that consisting of ferrous ion and hydrogen peroxide. The suggestion that hydroxyl radicals are intermediates in this reaction was first made by Haber and Weiss, who proposed the following radical-chain mechanism for the process ... 

Heterocyclic compounds have in most cases been hydroxylated by modified forms of Fenton s reagent. For instance, EDTA or pyrophosphate have been added to the system to complex the ferrous ions. It has been shown in the reactions of bcnzenoid compounds, however, that addition of complexing agents does not affect the distribution of isomers obtained by Fenton s reagent,and therefore the hydroxyl radical must still be the hydroxylating species. 

The yield of 3-hydroxyquinoline relative to the amount of quinoline consumed is low but is increased markedly by the presence of ascorbic acid. This was attributed to the regeneration of ferrous ions by reduction of the ferric ion formed in the first step of the reaction. 

Udenfriend et al. observed that aromatic compounds are hydroxyl-ated by a system consisting of ferrous ion, EDTA, ascorbic acid, and oxygend Aromatic and heteroaroinatic compounds are hydroxylated at the positions which are normally most reactive in electrophilic substitutions. For example, acetanilide gives rise exclusively to the o-and p-hydroxy isomers whereas quinoline gives the 3-hydroxy prod-uct. - The products of the reaction of this system w ith heterocyclic compounds are shown in Table XIII. 

This view is supported by the observation that small quantities of pyridines which have lost the 2-methyl group are present in the reaction mixture. It may also be noted that y-collidinc gave traces of a mixture of several 2,2 -bipyridines, which reacted with ferrous ions... 

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

A simple calculation based on the solubility product of ferrous hydroxide and assuming an interfacial pH of 9 (due to the alkalisation of the cathodic surface by reaction ) shows that, according to the Nernst equation, at -0-85 V (vs. CU/CUSO4) the ferrous ion concentration then present is sufficient to permit deposition hydroxide ion. It appears that the ferrous hydroxide formed may be protective and that the practical protection potential ( —0-85 V), as opposed to the theoretical protection potential (E, = -0-93 V), is governed by the thermodynamics of precipitation and not those of dissolution. 

In the laboratory you have observed the reaction of ferrous ion, Fe+i(aq), with permanganate ion, MnOiYaqJ, and also the reaction of oxalate ion, CiOi2(aq), with permanganate ion, MnO (aq). These studies show that the rate of a reaction depends upon the nature of the reacting substances. In Experiment 14, the reaction between IO and HSO3" shows that the rate of a reaction depends upon concentrations of reactants and on the temperature. Let us examine these factors one at a time. 

Both ferrous ion, Fe+2(aq) and oxalate ion, Cf)t2(aq), have the capability of decolorizing a solution containing permanganate ion at room temperature. Yet, there is a great contrast in the time required for the decoloration. The difference lies in specific characteristics of Ft+2(aq) and Cf 2(aq). On the other hand, Fe+2(aq) is also changed to Fe+ (aq) by reacting either with MnO fagJ or with ceric ion, Ce+i(aq). One of these reactions is simple and the other involves... 

The reaction between ferric ion, Fe+3, and cuprous ion, Cu+, to produce ferrous ion, Fe+2, and cupric ion, Cu+2, is plainly an oxidation-reduction reaction ... 

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