Halide ions, electrochemical oxidation

Such solutions are necessarily contaminated with halide ions and with the products of any subsequent decomposition of the hypohalite anions themselves. Alternative routes are the electrochemical oxidation of halides in cold dilute solutions or the chemical oxidation of bromides and iodides ... 

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]. 

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). 

The electrochemical oxidation of 2,5-dimethylthiophene in various electrolytes has been investigated (71JOC3673). In non-halide electrolytes such as ammonium nitrate or sodium acetate, the primary anodic process is the oxidation of the thiophene to the cation-radical (159). Loss of a proton, followed by another oxidation and reaction with solvent methanol, leads to the product (160) (Scheme 31). When the electrolyte is methanolic NaCN, however, nuclear cyanation is observed in addition to side-chain methoxylation. Attack by cyanide ion on the cation-radical (159) can take place at either the 2- or the 3-position, leading to the products (161)-(163) (Scheme 32). 

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 limited reversibility of some electrode reactions might require consideration of consumable (cheap) ionic liquids in the anode compartment for technical applications and commercial electroplating. For example, the electrochemical oxidation of oxalate delivers carbon dioxide, hydride could be oxidized to hydrogen, halides to the halogen or trihalide salt in the case of iodide ionic liquids and so on. Since ionic liquids can readily form biphasic systems an alternative may be to have the anodic reaction in an immiscible solvent. In that case a common ion would be needed that can be transferred from one phase to the other. 

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]. 

Instead of nucleophiles such as H2O, MeOH, RNH2 and halide ions, both the aryl group and olefinic double bond will react with an electrogenerated phenoxonium ion to give carbon-carbon coupled products. In particular, electrooxidative coupling reactions of a,ft)-diarylalkanes leading to cyclic diaryl ethers have been known to take place in a radical or cationic manner depending on the oxidation potential, the nature and location of substituents, the solvent systems and other factors, as cited in many books Electrochemical carbon-carbon bond formations will be described here. 

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. 

A quantitative separation of the halide ions F , Cr, Br , and 1 has been obtained by means of a column of Sephadex G-15 gel/ Thin-layer voltammetric data for Pt electrodes indicate that these ions form chemisorbed layers which withstand rinsing with typical aqueous electrolytes/ The chemisorbed species are much less reactive towards electrochemical oxidation than the aqueous ions. 

As this oxidation is energetically, relatively easy the overall electrochemical process (which includes reduction of metal and oxidation of phenol) requires comparatively little energy. On the other hand, if the only oxidisable species in solution (other than the metal) is difficult to oxidise (for example a halide ion) then the overall electrochemical process requires more energy. The message is that the whole content of the efflu-... 

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