Direct anodic oxidation

The complete oxidation of large organic molecules requires a large number of electrons, i.e. coulombs mol thus in these cases energy consumption can be relatively high as will the cost of the cell hardware. Therefore the main application areas for electrochemical oxidation are 

Comninellis [63] has studied the direct oxidation of approximately two dozen organic aromatic compounds. Using platinum anodes, it was found that where one of the substituents is electron donating (e.g. —NH2), only 

Platinum anodes have a limited operational range of oxidation potentials and thus attention has focused on Sn02-coated titanium materials. The tin oxide material, when doped with Sb (approximately 5%) to impart the appropriate electrical conductivity, has oxygen overpotentials some 600 mV greater than those of platinum. Tin oxide gives higher oxidation efficiencies to those of platinum, lead dioxide, ruthenium and iridium oxide (DSA) electrodes and is reported to be stable to corrosion during anodic oxidation. 

4 kg COD h m at a current density of 300 A m . It is claimed that the process competes with wet oxidation and combustion processes at relatively high COD concentrations and with adsorption at the lower values of COD. 

Anodic oxidation has been researched as a pollution control device for several applications  

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The simplest applications involve direct anodic oxidation in aqueous media. 

The electrosynthesis of 4-methoxybenzaldehyde (anisaldehyde) from 4-methoxy-toluene by means of direct anodic oxidation is performed on an industrial scale [69]. Via an intermediate methyl ether derivative, the corresponding diacetal is obtained, which can be hydrolyzed to the target product. The different types of products - ether, diacetal, aldehyde - correspond to three distinct single oxidation steps. 

The direct anodic oxidation of methanol became much more attractive after it was shown that platinum-ruthenium alloys are catalytically much more active in this reaction than pure platinum (pure ruthenium is totally inactive in this reaction). 

An alternative to the direct anodic oxidation of organic contaminants are the methods of indirect oxidation with the aid of oxidizers formed electrochemically in situ. These oxidizers (or mediators) can be obtained in both anodic and cathodic processes. Anodic agents are the salts of hypochloric acid (hypochlorites), the permanganates, the persulfates, and even ozone. 

Germain and Commeyras have found that perfluoro sulfonic esters and fluorosulfates are formed in high yields by direct anodic oxidation of R,I in perfluoroalkane sulfonic acids and fluorosulfuric acid (Eqs. 12 and 13) [36]. 

In the addition to nonactivated alkenes, where the direct anodic oxidation is less, satisfactorily good yields can be achieved when Mn(OAc)2 is used as mediator (Table 8, entries 6, 7). Sorbic acid precursors have been obtained in larger scale and high current efficiency by a Mn(III)-mediated oxidation of acetic acid/acetic anhydride in the presence of butadiene [112]. 

Although cyclizations from the direct anodic oxidation of acyclic 1,3-dicarbonyl compounds have not been reported, the analogous mediated reactions have been studied [24]. Snider and McCarthy compared oxidative cyclization reactions using a stoichiometric amount of Mn(OAc)3 with oxidations using a catalytic amount of Mn(OAc)3 that was recycled at an anode surface (Scheme 11). In the best case, the anodic oxidation procedure led to a 59% yield of the desired bridged bicyclic product with the use of only 0.2 equivalents (10% of the theoretical amount needed) of Mn(OAc)3- Evidence that the reaction was initiated by the presence of the mediator was obtained by examining the electrolysis reaction without the added Mn(OAc)3. In this case, none of the cyclized product was obtained. For comparison, the oxidation using... 

Specifically, the mediators (Med ) used were the radical cations of tris-(4-bromophenyl)amine and 2,3-dihydro-2,2-dimethylphenothiazine-6(17/)-one [59], The results of the oxidative cyclizations under the homogeneous oxidation conditions are parallel to those obtained by direct anodic oxidation. 

These processes can occur by a direct electron transfer reaction to (reduction) or from (oxidation) the present organic pollutant, or by a chemical reaction of the pollutant with previously electrogenerated species. The mechanism is generally viewed as a direct anodic oxidation of organic pollutant involving its reduction by direct electron transfer from organic molecule to the electrode to form a radical cation that readily deprotonates, equation (37) ... 

Benzylideneamino)-phenols (49) can be oxidatively cyclized to form 2-phenyloxa-zols (50) (Eq. (13)) by direct anodic oxidation 2, by Pb(OAc)4AgjO " and nickel peroxide The oxidation of 49 proceeded disappointingly in t-butanoT. water at the nickel hydroxide electrode. 50 was isolated only in traces, benzaldehyde was the major product, which indicated that 49 hydrolysed under the reaction conditions. The hydrolysis could effectively be suppressed by electrolysis in an emulsion of water and cyclohexane, where the portion of water was kept low. The temperature was around 70 °C to secure a fast oxidation. With these reaction conditions good yields of 50 were obtained (Table 16). 

Contrary to the noncatalytic Kolbe synthesis or other anodically initiated radical reactions at Pt anodes, the anodic functionalization and anodic coupling of vinyl compounds by direct anodic oxidation of olefins in alcohols as... 

The direct anodic oxidation of 1,4-butynediol at Pb02 anodes in sulfuric acid yields the desired acetylenic dicarboxylic acid [11] ... 

The mechanism given in equation 47 has been proposed for this reaction. The initially formed cation radical reacts with molecular oxygen to generate an intermediate, which may couple with a neutral cyclic silane to form species A. The intermediate A decomposes to the final product B and its cation radical B+", which could also be generated by direct anodic oxidation of the siloxane B. A further oxygen insertion step could take place via intermediate C+. ... 

Malkowsky IM, Rommel CE, Wedeking K, Frohlich R, Bergander K, Nieger M, Quaiser C, Griesbach U, Putter H, Waldvogel SR (2006) Facile and highly diastereoselective formation of a novel pentacyclic scaffold by direct anodic oxidation of 2,4-dimethylphenol. Eur J Org Chem 241-245... 

Direct anodic oxidation of alkanes may be performed if they have ionization potentials lower than about 10 eV. Such oxidations can be classifted into two types of reactions, cleavage of C—bonds (equation 4) and cleavage of C—C bonds (equation S). 

The direct anodic oxidation of aliphatic saturated alcohols to the corresponding carbonyl compounds is not always effective, because the high oxidation potentials of these alcohols make difficult Ae direct removal of an electron from the lone pair electrons on the oxygen atom. 

Whenever the system being titrated forms a reversible redox couple with its reaction product, the second electrode used in the generation reaction must be shielded from the bulk of the sample solution. For example, in the titration of iron(II) with anodically generated cerium(TV), the cathode is placed in a separate compartment to prevent the reduction of iron(III). In this example, iron(II) undergoes direct anodic oxidation during the bulk of the titration until the bulk concentration of iron(II) is so low that its rate of mass transfer can no longer sustain the applied current. At this point the intermediate oxidation of cerium(III) permits 100% current efficiency to be maintained to the end point. 

Direct anodic oxidation, whereby organics are oxidized at the electrode surface. 

In this section, recent advances in the field of polymer electrolyte direct methanol fuel cells, i.e., PEFCs based on direct anodic oxidation of methanol are discussed. A schematic of such a ceU is shown in Fig. 48, together with the processes that take place in the cell. The DMFC has many facets, electrocatalysis materials and components which deserve a detailed treatment. The discussion here will be confined, however, to the very significant performance enhancement demostrated recently with polymer electrolyte DMFCs, and, as a result, to possible consideration of DMFCs as a nearer term technology. 

The prototype cyclodimerization of 1,3-cyclohexadiene catalyzed by tris(4-bromo-phenyl)aminium ion (Ar3N ) proceeds smoothly in CH2CI2 at room temperature or below to a 70% yield of the Diels-Alder dimer [Eq. (51)] [116]. The direct anodic oxidation of 1,3-cyclohexadiene results in formation of the Diels-Alder product only in low yield, with the major product being a polymer linked through the 1,4-positions [119]. 

In Scheme 8, five possible electrochemical pathways for the formation of V-acyli-minium ions are represented. Pathway a (see Sec. VIII.A) describes the direct anodic oxidation of amides and carbamates to the intermediated V-acylium ions via removal of one electron from the nitrogen lone pair followed by deprotonation in a-position of the nitrogen atom and further one-electron oxidation. In pathway b (see Sec. VIII.C), a decarboxylative methoxylation of an V-acylated amino acid (Hofer-Moset reaction) leads to the same intermediate. The radical that is formed after anodic decarboxylation is immediately further oxidized to the cation due to the electron donation of the nitrogen. Pathway c (see Sec. VIII.B) describes the anodic oxidation of an V-acylated amino... 

If the direct anodic oxidation of tertiary formamides, which do not possess an a-hydrogen, is performed, the formyl function is transformed into a carboxymethyl group [233]. 

In the case of basic amino acids like lysine or ornithine, the regioselectivity of the direct and indirect electrochemical oxidation can be demonstrated. Direct anodic oxidation of A/, A -dimethoxycarbonylated L-lysine and L-ornithine in methanol regioselec-tively resulted in the methoxylation at the xt>-amino group, while indirect... 

Direct anodic oxidation of aldehydes to acids has long been known, but has been largely neglected in recent years due to its limited preparative value [156,157]. Likewise, the anodic oxidation of ketones leads mainly to C-C bond cleavage reactions or to Ritter-type products when conducted in acetonitrile, though the reaction in the latter case is of some mechanistic interest [158-160]. 

Direct anodic oxidation of PhSeSePh at a Pt anode in MeCN-Bu4NBF4 containing alkenes results in 2-acetamido selenides in good yields [61]. 

S-tert-buty thioates can be deprotected by indirect electrochemical oxidation using bromide/bromine as mediator the reaction probably goes through a bromosulfonium intermediate. By direct anodic oxidation, 4-methoxyphenylthiomethyl esters are readily converted to carboxylic acids. The initially formed radical cation cleaves on reaction with water to the acylated hemiacetal and further to the acid [144,145]. 

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