Electrosynthesis indirect

During an indirect electrode reaction, a redox couple is used as a catalyst or dectron carrier for the oxidation or reduction of another species in the system. In other words, the electrode is used to continuously reconvert the redox reagent 

One example of an indirect electrolysis-the methoxylation of the substituted furan-has already been described. The anode reaction was the oxidation of bromide ion to bromine which subsequently reacted with the furan. Here, two further examples will be presented. The first is the conversion of anthracene to anthraquinone via eicctrogenerated chromic add. A concentrated solution of chromiurnfm) in aqueous sulphuric acid is partly oxidized to chromic acid at a lead dioxide anode  

The reaction is part of a method for the manufacture of ascorbic acid (vitamin C). 

Br7Br2, Cr/ClO, Ni(OH)2/NiOOH For reductions Sn VSn Cr 7Cr Ti VTi Zn VZn, Na VNaHg It has been proposed that indirect electrosynthesis may be carried out  

In-cell, i.e. the reaction between the organic substrate and the redox reagent in its active oxidation state occurs within the cell. 

Ex-cell, i.e. the chemical reaction is carried out in a reactor separate to the cell. This approach has the advantages that the conditions for the electrode reaction and the chemical step may be optimized separately and also that the electrolyte may be conditioned/purified between the reactor and the cell. On the other hand, more unit processes are necessary and economic driving forces will encourage the process designer to combine the cell and reactor as closely as possible. 

It should be noted also that the redox couple may be solution-soluble or may be immobilized within an electrode coating and that, in the former case, the chemical step may be carried out (1) with the organic substrate dissolved within the electrolyte (2) at the interface between the electrolyte and the substrate present as a separate phase or dissolved in a solvent immiscible with the electrolyte (3) within a solvent immiscible with the electrolyte, the redox reagent in its active oxidation state being extracted into the other solvent (maybe by phase-transfer catalysis) and (4) at the interface between the electrolyte and the organic substrate present as a solid. 

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Indirect electrosynthesis of reactive formyl transition metal compounds involves an initial step of reduction of metal carbonyls to radicals followed by transfer of a hydrogen atom from trialkyltin hydrides190. Electroreduction of metal carbonyls yields products of dimerization and loss of CO from the radical anion. Electroreduction in the presence of R3SnH yields the formylmetalcarbonyls ... 

Pletcher has reviewed papers on indirect electrosynthesis when phase transfer catalysis is used to allow the chemical step to occur homogeneously in an organic solvent immiscible with the aqueous electrolyte [10,127],... 

Selected examples of indirect electrosynthesis which have found technical or pilot plant scale applications are discussed in the following ... 

Intermediate enols can also be oxidized by indirect electrosynthesis (vide supra) halide anions are used as oxidation mediators. A multistep reaction converts ketones into a-hydroxylated acetals when oxidized electrochemically in the presence of iodine as the redox catalyst (Scheme 48) [196],... 

In direct electrosynthesis, the desired product is formed by means of an electrochemical reaction. In indirect electrosynthesis, a redox couple is used as an electron carrier for the oxidation or reduction of another species in the system. In other words, the electrode is used to reconvert the redox reagent continuously to an oxidation state where it is able to react with another compound in a desirable, generally homogeneous redox reaction. Most suitable redox couples are inorganic whereas the desired product is typically organic. The final redox step may be carried out inside or outside the electrochemical cell. 

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