Electrochemical processes, indirect

HCIO4, one of the strongest of the mineral acids. The perchlorates are more stable than the other chlorine oxyanions, ie, chlorates, CIO chlorites, CIO or hypochlorites, OCf (3) (see Chlorine oxygen acids and salts). Essentially, all of the commercial perchlorate compounds are prepared either direcdy or indirectly by electrochemical oxidation of chlorine compounds (4—8) (see Alkali and chlorine products Electrochemical processing). 

Redox reactions can proceed by direct transfer of electrons between chemical species. Examples include the rusting of iron and the metabolic breakdown of carbohydrates. Redox processes also can take place by indirect electron transfer from one chemical species to another via an electrical circuit. When a chemical reaction is coupled with electron flow through a circuit, the process is electrochemical. Flashlight batteries and aluminum smelters involve electrochemical processes. 

Flash rusting exhibited in neutral to alkaline water borne formulations appears to occur through a localised corrosion process probably Involving grit "activity" present from blasting, either directly or indirectly, in an electrochemical process. At such pH the rapid oxidation of ferrous to ferric ion produces... 

Indirect electrochemical processes are hybrids in a certain sense they combine an electrochemical and therefore heterogeneous electron transfer reaction with a homogeneous redox process. The redox reagent undergoes a homogeneous reaction with the substrate and is subsequently regenerated in its active form at the electrode (see Fig. 1). 

Fig. 2. Flow diagram of an indirect electrochemical process with external regeneration of the redox catalyst (ex-cell process)...

In the following, the potentiality and advantages of indirect electrochemical processes are compared with available alternatives 3.13.25,26) Naturally, exact estimations are only possible using concrete examples ... 

Anodic addition to an electron-rich heteroaromatic compound is used to transform furan to 2,5-dimethoxy-2,5-dihydrofuran, a valuable synthetic intermediate. Again, an indirect electrochemical process occurs. The bromide ion as redox catalyst is electrochemically oxidized to give bromine, which then acts as chemical oxidant for furan [7] ... 

In the case of A-acylated amino acid derivatives, the oxidation has to take place via an indirect electrochemical process using chloride ions as redox catalysts. In methanol as solvent, methylhypochlorite is formed as oxidant, which yields, via -chlorination, HC1 elimination, and methanol addition, the a-methoxylated amino acids as amidoalkylation reagents [27] ... 

In an indirect electrochemical process, vitamin B,2 has been used as catalyst for the cathodic alkylation of a,p-unsaturated carbonyl compounds by alkyl iodides as exemplified in the formation of (+)- X0-brevicomine [30] ... 

Figure 22.1 Schematic representation of an indirect electrochemical process (given for an oxidation Med0Jt, Medred oxidized and reduced forms of the redox catalyst = mediator Sox, Sred oxidized and reduced forms of the substrate).

Wellmann J, Steckhan E. Indirect electrochemical processes 2. Electro-catalytic direct oxidation of aromatic compounds by hydrogen peroxide. Chem Ber 1977 110 3561-3571. 

Similarly, carbanions from 1,3-dicarbonyl compounds can act as nucleophiles [153]. In a somewhat related reaction, enamines of cyclohexanone or cycloheptanone are oxidized in an iodide-mediated indirect electrochemical process leading to a Favorskii-type rearrangement. Thus, bicyclo[(n — 1).1.0] systems containing a nucleophile like a cyano or a methoxy function are formed [Eq. (27)] [154]. 

As pointed out before, to couple the redox enzyme to an electrode, a redox catalyst usually has to be applied that shuttles the electrons between the electrode and the protein (Fig. 3) (indirect electrochemical processes [19]). In certain cases, the transfer of the redox equivalents between the mediator and the cofactor also has to be catalyzed by an enzyme (see later discussions). 

The indirect electrochemical process for the regeneration of NAD" from NADH is very fast, which is indicated by a strong catalytic current in the presence of glycerol dehydrogenase, the oxidized cofactor, and 7e.so-l,2-cyclohexanediol as substrate [109]. 

According to the reaction scheme given in Fig. 26, the anaerobic oxidation of l-glycerol 3-phosphate was performed under the conditions of an indirect electrochemical process using a water-soluble ferrocene derivative as mediator in the presence of o-fruc-tose-1,6-diphosphate aldolase from rabbit muscle for the in situ generation of the carbohydrate product. With a mediator concentration fo 1.5 mM using about 80 U of immobilized enzyme, after 25 h a 75% turnover of the substrate L-glycerol phosphate... 

Pyruvate oxidase (Pyox) is a FAD- and thiamine diphosphate (ThDP)-dependent enzyme that catalyzes the reaction of pyruvate to give acetyl phosphate or vice versa (see Fig. 15). If used in the oxidative way, it can be activated and reactivated under nonaerobic conditions using ferrocene mediators. Kinetic parameters of the indirect electrochemical process using the enzyme incorporated into a biomimetic supported bilayer at a gold electrode have been reported [142]. Similarly, FAD-dependent amino oxidases may also be applied. 

Within a potential range where radicals (like CF ) are not readily reduced or/and using indirect formation in solution by means of electron carrier, a chemistry—rather similar to that already developed by the Kolbe method by oxidation of CF3COO at a platninum anode—can be developed. Thus, Medebielle [193] showed that radical additional onto electron-rich olefins was possible and applied this for convenient synthesis of fluorinated heterocyclic compounds. Let us stress that the electrochemical process is in this case stoichiometric in electrons. 

The indirect, ex-cell chromic acid oxidation is the oldest industrial organic electrochemical process. 

H Marzouk. EDF, France. The Modified Grignard Reactor for Electrosynthesis Scale-Up of the Indirect Oxidation of Galacturonic Acid Electrochemical Processing, The Versatile Solution, Conference, ICI and EA-Technology. Barcelona, Spain, April 14-18, 1997. 

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