Anodic oxidation of organic materials

The existence of materials now included among the conducting polymers has long been known. The first electrochemical syntheses and their characterization as insoluble systems took place well over a century ago. In 1862 Letheby reported the anodic oxidation of aniline in a solution of diluted sulphuric acid, and that the blue-black, shiny powder deposited on a platinum electrode was insoluble in HjO, alcohol, and other organic solvents. Further experiments, including analytical studies, led Goppelsroeder to postulate in 1876 that oligomers were formed by the oxidation of aniline. 

The rest of the chapter has been devoted to special topics and in materials science there are many possibilities. Those selected include the mechanism of the flotation of minerals in which the addition of a certain organic to the solution causes a specific mineral to become hydrophobic so that it is exposed to air bubbles, the bubbles stick to it and buoy the mineral up to the surface, leaving unwanted minerals on the bottom of the tank. It turns out that the mechanism of this phenomenon involves a mixed-potential concept in which the anodic oxidation of the organic collector, often a xanthate, allows it to form a hydrophobic film upon a semiconducting sulfide or oxide, but only if there is a partner reaction of oxygen reduction. This continues until there is almost full coverage with the dixanthate, and the surface is thereby made water-repelling. 

A variety of organic transformations in aqueous media using BDD anodes have been studied. The pronounced stability of the BDD material in the presence of water makes it obvious that is should be used in oxidation processes. However, the yields are usually low and therefore less attractive for synthetic purposes. The BASF company investigated the anodic oxidation of butyn-l,4-diol 32. The anodic treatment in an electrolyte of dilute sulfuric acid gave small amounts of the monoacid 33 and the acetylene dicarboxylic acid 34. The moderate product efficiency might be attributed to electrochemical incineration processes (Scheme 15). 

Comninellis et al. (Comninellis 1994 Comninellis and De Battisti 1996 Simond et al. 1997 Foti et al. 1999) found that the nature of the electrode material strongly influences both the selectivity and the efficiency of the process and, in particular, several anodes favored the partial and selective oxidation of pollutants (i.e., conversion), while others favored complete combustion to C02. In order to interpret these observations, they proposed a comprehensive model for the oxidation of organics at metal oxide electrodes with simultaneous oxygen evolution. 

As mentioned above, the nature of the electrode material influences the selectivity and the efficiency of an electrochemical process for the oxidation of organic compounds and for this reason, in literature, many anodic materials have been tested to find the optimum one. According to the model proposed by Comninellis (1994), anode materials are divided for simplicity into two classes as follows ... 

This paper has presented and briefly discussed the performance of different electrode materials for the electrochemical oxidation of organic pollutants for wastewater treatment. Literature results have demonstrated that anodes with low oxygen evolution overpotential, such as graphite, Ir02, Ru02, or Pt only permit the primary oxidation of organics (i.e., conversion), but not the complete mineralization, due to the accumulation of oxidation intermediates, mainly aliphatic acids, which are quite stable against further attack at these electrodes. 

The anodic oxidation of furans is one of the most extensively studied reactions because electrooxidation of furans in methanol yields 2,5-dimethoxy-2,S-dihydrofi] s (21 equation 41), which are useful starting materials in organic synthesis.44... 

As described in the previous section, the anodic oxidation of aliphatic amines is utilized only rarely in organic synthesis due to the instability of the generated intermediates, whereas amides tuid carbamates of aliphatic amines yield relatively stable intermediates which are sufficiently promising as starting materials in organic synthesis (equations 47 and... 

Electroluminescence (EL) is the phenomenon by which electrical energy is converted into luminous energy by the recombination of electrons and holes in the emissive material [8], The basic structure of an OLED consists of a thin film of organic material sandwiched between two electrodes, an anode of high-work-function material such as indium tin oxide (ITO) on a glass substrate, and a cathode of a low-work-function metal such as calcium (Ca), magnesium (Mg), or aluminum (Al) or an alloy such as Mg Ag. 

The anodic oxidation of volatile organic components (VOCs) using metal-oxide electrodes has attracted much attention and the group of Comninellis [170-174] has found that often the complete electrochemical oxidation of some organics in aqueous media occurs, without any loss in electrode activity, only at high potentials with concomitant evolution of O2. Furthermore, it has been found that the nature of electrode material strongly influences both the selectivity and the efficiency of the process. 

Photocatalytic oxidation of organic pollutants on TiO2-based materials has been extensively investigated. The catalyst is used in the form of a suspension of fine particles or thin film on robust substrates. Figure 12.2 shows the variation of open-circuit potential with time for TiO2 film electrodes, prepared by anodization of Ti... 

Polymetallorotaxanes 7.24 (M = Zn" or Cu ) have been prepared by electropolymerization, which involved anodic oxidation of the pre-assembled metallorotaxane precursors (Scheme 7.2) [48]. Importantly, studies of these materials have allowed an evaluation of the individual contributions of the organic backbone and the metal-centered redox process to the overall conductivity measured on interdigitated microelectrodes. The Zn and Cu polymers behave quite differently. The Zn polymer behaves in a similar fashion to the metal-free material 7.25, whereas the matching of the polymer and Cu-centered redox potentials in 7.24 (M=Cu ) leads to enhancement of the communication between these two units resistance drops by a factor of 10 for the Cu polymer 7.24 relative to metal-free 7.25. In a further development in this general area, two-step electropolymerizations have been used to generate three-stranded conducting ladder polymetallorotaxanes 49]. 

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