Oxidation of halide ions

Other inorganic reactions shown to be photo-induced at colloidal semiconducting metal oxide surfaces include the synthesis of ammonia from water and nitrogen (19) and the oxidation of halide ions 1 ,... 

At a platinum electrode, highly purified FLINAK has a voltammetric window extending from about +1.5 to -2.0 V vs. the nickel reference electrode [7]. The positive limits of the alkali halide melts discussed herein arise from the oxidation of halide ions, whereas the negative limits are due to reduction of the alkali metal ions. Because chloride ion is substantially easier to oxidize than fluoride ion, the potential window of the LiCl-KCl melt is approximately 1.5 V smaller than that for FLINAK. 

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

With estimated ip(X ), the percentage of oxidation of halide ions (X ) can be determined as a ratio of iq to the total disk photocurrent measured ... 

The importance of the first factor, the concentration, is clear because the anodic photocurrent due to oxidation of halide ions should be proportional to the product of the concentration of positive holes at the electrode surface and that of halide ions in solution. When ip becomes large, the supply of halide ions to the electrode surface by diffusion becomes unable to follow ip, resulting in a decrease of (X ), which depends on the concentration of X". 

Another model for giving an explanation of the pH dependence of the reactivity of halide ions may be that surface cations serve as effective sites for adsorption of reaction intermediates which are produced in the course of the anodic oxidation of halide ions. Usually, the anodic oxidation of halide ions is believed to... 

Photogenerated holes can also be simultaneously utilized to oxidize substrates at the semiconductor surface [43,100-102]. The holes trapped at the Ti02 surface can survive for a duration of microseconds to milliseconds and can react with OH- to generate OH radicals in aqueous medium. These OH radicals can further induce secondary oxidation. Oxidation of halide ions (X-) has been investigated in various colloidal semiconductor suspensions by laser flash photolysis [103,104]. 

Haloperoxidases catalyze the hydroperoxide-dependent oxidation of Cl-, Br- and/or I to electrophilic halogenating species that halogenate organic substrates (equation 1). As predicted by the relative ease of oxidation of halide ions (I- > Br- > Cl" >> F-), chloroper-oxidases oxidize Cl-, Br- and I-, bromoperoxidases oxidize Br- and I-, iodoperoxidase oxidize only I- while no peroxidase can oxidize F-14. 

As with Ti02, CdS can be used to photocatalyse reactions other than water cleavage. Oxidation of halide ions " proceeds smoothly at chalcogenide electrodes and n-type CdS can be used to photo-oxidize NO in the presence of iron(n) complexes. Similar studies have described the photoassisted reduction of CO2 to CO and the photo-oxidation of formic acid, formaldehyde, and methanol. ... 

The oxidation of halide ions to molecular halogen is relatively easy consequently, the mechanism of anodic halogenation, brought about by oxidation of organic substrates in the presence of halides is not always clear [249-254]. In many cases it may involve halogenation by anodically generated halogen. In other cases, where the substrate is easily oxidized, the halide has the role of a nucleophile and attacks a radical cation. 

Hydrogen peroxide is a substrate for myeloperoxidase, a multisubunit heme protein of M.W. 150,C, present in primary neutrophilic granules. The active prosthetic groups are two hemes covalently attached to the apoen-zyme. This enzyme catalyzes many kinds of oxidation reactions, but oxidation of halide ions to hypohalite ions appears to be the most important. Hypochlorite ion is the principal compound formed, although Br , I, and SCN (a pseudohalide) can also serve as substrates. The reaction catalyzed is... 

The principles of redox catalysis applied successfully here to photodecomposition of water, can also be profitably employed to carry out efficiently other multi-electron reactions. Of special interest in the area of solar energy conversion are processes such as reduction of C02, N2 and NAD+ and oxidation of halide ions (Cl-, Br- or I-). With the latter, there exists the interesting possibility of complete decomposition of hydrogen halides with the reverse reaction used in a fuel cell to generate power. 

Enzymatic halogenation catalyzed by haloperoxidases and perhydrolases involves the oxidation of halide ions to a halonium ion species which leads to the formation of hypohalous acids (Fig. 16.9-1). The products obtained by enzymatic halogenation with these enzymes are the same as the products obtained by chemical electrophilic halogenation with hypohalous acids. The differences in the para ortho ratios in the halogenation of some aromatic compounds could be due to a mixture of halogenation at or near the active site and in solution. 

A. Kirschning, Md. A. Hashem, H. Monenschein, L. Rose, and K.-U. Schoning, Preparation of novel haloazide equivalents by iodine(III) promoted oxidation of halide ions, /. Org. Chem., 64 (1999) 6522-6526. 

Biosynthetic halogenation can occur through multiple pathways, but many halogenase enzymes use electrophilic halogenating species that are produced by oxidation of halide ions. 

The activity of haloperoxidase and halogenase in the oxidation of halide ion reveals that Fe(lll)-hydroperoxo complexes oxidize halide ions via oxygen atom transfer while the corresponding Fe(lV)-oxo complex reacts via electron transfer. ... 

The elemental halogens are usually obtained by the oxidation of halide ions. Chlorine, bromine and iodine are made industrially by the oxidation of the aqueous halides in brines or seawater. With chlorine, the oxidation is electrolytic with bromine and iodine, chlorine is the oxidizing agent. Industrial fluorine is made by the electrolytic oxidation of a conducting, liquid blend of KF and HF. 

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