Halide ions, anodic oxidation

Halide-free anodic oxidation of alcohols mediated by an anionic WS-TEMPO (Figure 12.3) was also accomplished. It was suggested that the anionic WS-TEMPO might form an anionic oil-in-water emulsion including alcohol, and the attractive force between the sulfonate ion and the positive charge of anode would facilitate direct electron transfer from the N-oxyl moiety to the anode. 

Bombi and co-workers.The standard potential of CUIC, which could not be determined experimentally due to the oxidation of NOg" by chlorine, was estimated. On the other hand, Delimarskii and Shilina - claimed that the halide ions were oxidized to halogens by UNO3 in the ternary eutectic. They studied the voltammetric behavior of the dissolved halogen in the melt. Irreversible cathodic waves corresponding to X2 + 2e 2X were observed, but no anodic waves. The diffusion coefficients of CI2, Br2, and I2 were reported the order observed was Dci, >... 

It must be noted that impurities in the ionic liquids can have a profound impact on the potential limits and the corresponding electrochemical window. During the synthesis of many of the non-haloaluminate ionic liquids, residual halide and water may remain in the final product [13]. Halide ions (Cl , Br , I ) are more easily oxidized than the fluorine-containing anions used in most non-haloaluminate ionic liquids. Consequently, the observed anodic potential limit can be appreciably reduced if significant concentrations of halide ions are present. Contamination of an ionic liquid with significant amounts of water can affect both the anodic and the cathodic potential limits, as water can be both reduced and oxidized in the potential limits of many ionic liquids. Recent work by Schroder et al. demonstrated considerable reduction in both the anodic and cathodic limits of several ionic liquids upon the addition of 3 % water (by weight) [14]. For example, the electrochemical window of dry [BMIM][BF4] was found to be 4.10 V, while that for the ionic liquid with 3 % water by weight was reduced to 1.95 V. In addition to its electrochemistry, water can react with the ionic liquid components (especially anions) to produce products... 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

In the course of biogenesis-type syntheses of lupin alkaloids, reduction of protected macrocyclic acyloin 50 (Scheme 15) has been found to be a second route to bicyclic aminoalcohol 14a (69JA7372). Another 11-membered ring compound, namely caprinolactam (51), on anodic oxidation in the presence of halide ions produces 6/7 bicyclic lactam 52 together with two isomeric 5/8 bicyclic lactams (87CJC2770). 

Electrochemical oxidation of aldoximes using halide ions as mediators afforded the corresponding nitrile oxides in the anode compartment, which were simultaneously reduced to nitriles by cathodic reduction (equation 15). Sodium chloride affords the best result among the supporting electrolytes (Cl > Br > 1 > C104 > TsO ). Accordingly, the electrochemical reaction of oximes carried out in the presence of dipolephiles yielded isooxazolines (equation 16). 

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 ... 

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... 

The anodic oxidation of iodide, bromide and chloride ions at illuminated ZnO electrodes, which occurs in competition with the anodic decomposition of the electrode itself, was studied as functions of halide ion concentration, illumination intensity and solution pH in order to investigate factors which affect the degree of competition. The reactivity of halide ions, obtained under fixed conditions, was in the order of I >Br >Cl, reflecting the importance of the redox potential in determining the reactivity. 

Anodic oxidation of TTF in the presence of halide ions leads to the formation of mixed-valence salts, TTFXo.y, and the electrochemical behavior of these salts was studied by means of carbon paste electrodes [179]. 

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. 

So far the self-organized anodic Ti02 has been synthesized by halide ions containing solution, either aqueous or non-aqueous. The overall reactions for anodic oxidation of titanium can be represented as... 

Anion effects on the electrooxidation of Ru(OOOl) were explored by introducing Cr and Br into the 0.1 M HCIO4 solution. A sharp rise of anodic current occurs near 0.2 V, which is at a more negative potential than the onset of surface oxidation in sulfuric acid. Hence, even though halide ions are strongly adsorbed, they do not better protect the Ru from surface oxidation than do bisulfate ions. It is likely that a different redox process occurs with halide ions because they form compounds with Ru in several different oxidation states. 

Shono and coworkers have examined the electrochemical oxidation of sulfonamides [25], presumably with the intent of generating a-alkoxy sulfonamides. However, anodic oxidation of short chain acyclic sulfonamides, like 12, in the presence of halide ion surprisingly afforded the a-sulfonamido acetals 13 (Scheme 6) [25a]. It is believed that oxidation of 12 occurs to initially produce a-methoxy sulfonamide 14. Under the reaction conditions, however, 14 eliminates methanol to produce N-sulfonyl aldimine 15, which can tautomerize to ene sulfonamide 16. Reaction of 16 with a positive halogen species, generated elec-trochemically, probably leads to 17, which can rearrange via an intermediate aziridine to the observed acetal product 13. 

There is no evidence that compounds containing Al1 exist at ordinary temperatures. Anodic oxidation of aluminum at high current densities evidently produces lower-valent aluminum ions, either Al1 or Al11, or both, but they are ephemeral. There is no doubt that gaseous Al1 halide molecules exist at high temperatures, and their spectroscopic properties are well known. In the chloride system the equilibrium... 

Electrolytic procedures employing gallium metal in the form of a sacrificial anode can also be conveniently used to synthesize salts of the gallium(Ill) complex halide ions, [GaX4]. A straightforward procedure described here involves electrolysis of gallium in aqueous acid, HX, followed by heating to oxidize any Ga(II) species to Ga(III) ... 

The most common species determined by cathodic stripping voltammetry are anions such as halides or sulphide, at a mercury electrode. This involves formation of a film of mercury(I) salts on the electrode in the deposition step. The anodic oxidation process involved in the deposition step is in fact the oxidation of mercury metal to mercury(I) ions. These immediately precipitate insoluble mercury(I) salts with the halide ion etc, on to the surface of the electrode. The anodic deposition potential required depends on the anion concerned. The subsequent cathodic stripping peak for the mercury(I) salt of each anion has at its own individual potential. 

Among the fluorides, EtjN-SHF is the most effective in promoting this anodic oxidation reaction and halide ions other than fluoride ion are not effective at all. It is notable that Bu4NF-3H20 is also effective and does not cause formation of a sulfoxide through water capture of the initially formed, sulfur-centered cation radical. 

Anodic oxidation of enantholactam in the presence of halide ions gave the bicyclic compound <87CJC2770>. Ozonolysis on a-substituted and A-substituted enantholactams has been reported... 

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