Ionic iron reactions

Further reduction of H2O2 yields the hydroxyl radical, OH, indeed a powerful but indiscriminant and very unstable oxidant. Superoxide anion may itself serve as the reductant of H2O2 to form OH in the presence of ionic iron in a reaction called the Haber-Weiss reaction... 

Singlet molecular oxygen, Oj, is also thought to arise, perhaps in reaction 4 or 5 (in place of dioxygen). The addition of 02 completes the cast of characters comprising molecules which may mediate the effects of O , since whenever O is formed, H2O2, OH and O2 all may exist in aqueous solutions in the presence of micromolar concentrations of ionic iron which contaminate many buffers. Armed with the capacity to form species with such a variety of affinities for electrons, the PMN is endowed with the ability to initiate a formidable array of chemical reactions, not merely between the various species themselves but also with its own constitutents and those of ingested microbes. 

Hemoglobin and myoglobin in their ferric forms show rudimentary peroxidatic and catalatic activity, but ferrous peroxidase does not combine reversibly with molecular oxygen. Ionic iron also gives the hydrogen peroxide reactions but not the combination with oxygen. 

Many experimental studies now support a free radical mechanism for these reactions of ionic iron, and it is the main theme of the report to examine the data on the hemoprotein reactions to see whether similar reaction mechanisms are first of all possible and if so whether there is evidence that the reactions do proceed in this way. 

Certain other aspects of these mechanisms will be discussed later in comparison and contrast with the reactions of ionic iron. 

In the absence of direct proof of a valency change or free radical intermediates participating, it is valuable to survey the data on the reactions of ionic iron in similar systems and then examine the possibility of similar mechanisms occurring in hemoprotein reactions. 

There are some further aspects of the ionic iron-hydrogen peroxide system which have a possible bearing on hemoprotein reactions. Apart from the reactions of the HO radical listed above (1 and 2) and reactions with oxidizable or polymerizable substrates, there is some experimental evidence that two additional types of reaction may occur. The first... 

It is possible to extend the free radical mechanism so that the derived rate equation has the required first order dependence by assuming that there is an auxiliary electron accepting group in hemoglobin and myoglobin that can act as cupric ions do in the ionic iron-hydrogen peroxide system catalyzing the reaction between Fe3+ and 02. ... 

In comparing the reactions of hemoglobin, myoglobin, peroxidase, and catalase with molecular oxygen or hydrogen peroxide in relation to the similar reactions of ionic iron the following conclusions can be drawn ... 

It is the reaction of the ferric forms of these hemoproteins with hydrogen peroxide that provides the sharpest contrast to ionic iron. Complexes are formed in both systems. Although ferric ion only gives an ion pair complex Fe02H2+, whereas peroxidase can give three com-... 

A brown precipitate is formed when the two drops of sodium hydroxide or ammonia solution are added to the iron chloride solution. An ionic precipitation reaction has occurred forming insoluble iron(lll) hydroxide which does not react with excess sodium hydroxide or ammonia solution. Ammonia reacts with water to produce ammonium ions and hydroxide ions which react with the ions. The brown precipitate indicates the presence of iron(... 

Kutal et reviewed the chemistry of several iron (II) metallocenes that are effective photoinitiators for ionic polymerization reactions. Photoexcitation of ferrocene and 1,1-dibenzoyl -ferrocenes in solutions of ethyl-a-cyanoacrylate produces anionic species that initiate the polymerization of electrophilic monomers. Irradiation of CsHs-Fe (t] - arene) in epoxide containing media generates several cationic species capable of initiating ringopening polymerizations. It was concluded that iron(II) metallocenes exhibit a diversity of photoinitiation mechanisms. 

Hydrogenolysis can be decreased, if necessary, by the addition of any of a variety of salts that increase the ionic strength of the medium U62). Iron salts have been used specifically for this purpose 102J73). Yields may depend markedly on the conditions of the reaction 92). 

The Friedel-Crafts acylation reaction has also been performed in iron(III) chloride ionic liquids, by Seddon and co-workers [96]. An example is the acetylation of benzene (Scheme 5.1-66). Ionic liquids of the type [EMIM]Cl/FeCl3 (0.50 < X(FeCl3) < 0.62) are good acylation catalysts, with the added benefit that the ketone product of the reaction can be separated from the ionic liquid by solvent extraction, provided that X(FeCl3) is in the range 0.51-0.55. 

The ability of iron(III) chloride genuinely to catalyze Friedel-Crafts acylation reactions has also been recognized by Holderich and co-workers [97]. By immobilizing the ionic liquid [BMIM]Cl/FeCl3 on a solid support, Holderich was able to acetylate mesitylene, anisole, and m-xylene with acetyl chloride in excellent yield. The performance of the iron-based ionic liquid was then compared with that of the corresponding chlorostannate(II) and chloroaluminate(III) ionic liquids. The results are given in Scheme 5.1-67 and Table 5.1-5. As can be seen, the iron catalyst gave superior results to the aluminium- or tin-based catalysts. The reactions were also carried out in the gas phase at between 200 and 300 °C. The acetylation reac-... 

The cyclodimerization of 1,3-butadiene was carried out in [BMIM][BF4] and [BMIM][PF(3] with an in situ iron catalyst system. The catalyst was prepared by reduction of [Fe2(NO)4Cl2] with metallic zinc in the ionic liquid. At 50 °C, the reaction proceeded in [BMIM][BF4] to give full conversion of 1,3-butadiene, and 4-vinyl-cyclohexene was formed with 100 % selectivity. The observed catalytic activity corresponded to a turnover frequency of at least 1440 h (Scheme 5.2-24). 

In quantitative analysis we are chiefly concerned with reactions which take place in solution, i.e. ionic reactions. We shall therefore limit our discussion of oxidation-reduction to such reactions. The oxidation of iron(II) chloride by chlorine in aqueous solution may be written ... 

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