Electrochemical synthesis Electrolysis

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

This direct electrochemical synthesis has proved efficient in the preparation of several other complexes, among these the tin derivatives of 3-hydroxy-2-phenylflavone (2) and 2-ethoxyphenol (3), respectively. The use of sacrificial electrodes proved very efficient the produced complex precipitates during electrolysis and is easy to isolate135. 

Kalhammer FR (1991) Energy, electrochemistry and electrochemical synthesis Exploratory themes at the EPRI research 5th International Forum on Electrolysis, Ford Lauderdale, FL,... 

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

Recently, Ce43 75+0(0Pr )i3( Pr0H) was obtained by irradiation of Ce(OPr )4 PrOH solutions in (-CH2OMe)2 and PrOH by the visible light [1803]. Partial reduction ofalkoxides occurs in the course of electrolysis of metal alkoxides (see above). Oxoalkoxoniobates NbO(OR)3 (R=Me, Et, Bu), the products of oxidation of the Nb(OR)4, were isolated in the course of electrolysis performed with Nb anode and Pt cathode [911]. Although the reduction products are highly unstable (they cannot be used for synthesis of oxides) their formation in electrochemical synthesis should be taken into consideration because their subsequent oxidation by 02 (or even its traces) leads to ox-oalkoxides. 

Direct electrochemical synthesis is carried out according to the next procedure. Sheets of copper, nickel, or zinc are used as sacrificial anodes, and platinum is used as the cathode. Methanol is used as a solvent and LiC104 as a supporting electrolyte. The ligand (0.5 g) is dissolved in methanol (30 mL) by heating and then the obtained solution is cooled to room temperature. The electrolysis is carried out for 1 hr (current 20 mA applied voltage 20-30 V). The formed solid is filtered, washed with hot methanol (3x5 mL), and dried in air. 

Th anode Br2, CH3CN ThBr4(CH3CN)4 Direct electrochemical synthesis using a sacrificial Th anode. Only 0.60 g of metal is dissolved during 6 hr of electrolysis (8 V, 50 mA) 370 ... 

One of the most important, yet latent, applications of controlled-potential electrolysis is electrochemical synthesis. Although electrolysis has been used for more than a century to synthesize various metals from their salts, application to other types of chemical synthesis has been extremely limited. Before the advent of controlled-potential methods, the selectivity possible by classical electrolysis precluded fine control of the products. The only control was provided by appropriate selection of electrode material, solution acidity, and supporting electrolyte. By these means the effective electrode potential could be limited to minimize the electrolysis of the supporting electrolyte or the solvent. Today potentiostats and related controlled-potential-electrolysis instrumentation are commercially available that provide effective control of the potential of the working electrode to 1 mV, and a driving force of up to 100 V for currents of up to several amperes. Through such instrumentation electrochemical syn-... 

The electrochemical synthesis of organometallics (Lehmkuhl, 1973, 1973a) by conducting the electrolysis at a consumable electrode, e.g. as in the Nalco process (39), is again not readily accommodated in a single formal scheme. 

In addition to yields, current density and anode life are also important in evaluating an electrochemical synthesis. Although the current density should drop as water (a strong electrolyte in HF) is consumed, it does not always do so. Instead, for the first 15-30 minutes of electrolysis it increases in both continuous and interrupted electrolysis. This may be caused by a breakdown in a resistive anode coating. Once a maximum current is reached, the current density remains constant however, it drops as the last few tenths percent of water are consumed. Also, high water levels (>3%) cause low current densities. The current density maximum was at 0.5-1.0 mole % water. 

Since its accidental discovery by electrolysis of sulfuric acid in 1840 [59], electrochemical synthesis of ozone has evolved fairly slowly, primarily because of low current efficiencies observed at practical operating temperatures. 

A simple electrochemical synthesis of P-lactams has been developed by constant current electrolysis of suitable VOC-supporting electrolyte solutions and subsequent addition of haloamides. The utilization of bases (NaH, EtjN, Cs COj etc.). 

The rare 1,2,3-dithiazine ring system was prepared by the relatively unusual method of electrochemical synthesis <89BSF433,90BSF427). The electrolysis is performed using a carbon-sulfur cathode, which acts as a rich source of highly nucleophilic sulfur radical anions. These attack the substrate a,)S-unsaturated nitriles (166) or (168) to give radical intermediates which lead to the 1,2,3-dithiazine products by attack on the solvent (167) or by dimerization (169) (Scheme 17). 

The electrochemical synthesis of chlorine takes place through the electrolysis of aqueous sodium chloride. The electrode reactions are ... 

In 1862, Letheby [30] discussed the production of a blue substance by the electrolysis of sulfate of aniline, nowadays considered as the first reported electrochemical synthesis of a conjugated polymer, in his case poly(aniline). Since then a great number of different monomers were synthesized that form conducting polymers. The synthesis method of choice for preparing these polymers involves the anodic oxidation of a suitable monomer M into a cation radical... 

Polypyrrole and many of its derivatives can be synthesized via simple chemical or electrochemical methods [120]. Photochemically initiated and enzyme-catalyzed polymerization routes have also been described but less developed. Different synthesis routes produce polypyrrole with different forms chemical oxidations generally produce powders, while electrochemical synthesis leads to films deposited on the working electrode and enzymatic polymerization gives aqueous dispersions [Liu. Y. C, 2002, Tadros. T. H, 2005 and Wallace. G. G, 2003]. As mentioned above the electrochemical polymerization method is utilized extensively for production of electro active/conductive films. The film properties can be easily controlled by simply varying the electrolysis conditions such as electrode potential, current density, solvent, and electrolyte. It also enables control of thickness of the polymers. Electrochemical synthesis of polymers is a complex process and various factors such as the nature and concentration of monomer/electrolyte, cell conditions, the solvent, electrode, applied potential and temperature, pH affects the yield and the quality of the film... 

Interestingly, Osa et al. reported an electrochemical synthesis of enantioenriched binaphthyl type dimers (up to 98% ee) via constant potential electrolysis of 2-naphthol, 2-methoxylnaphthalene, and 9-phenanthrol at 0.6 V vs. Ag/AgCl on a TEMPO-modified graphite felt electrode in the presence of a stoichiometric amount of (-)-sparteine (TEMPO = 2,2,6,6-tetramethylpiperidin-l-yloxyl). ... 

Electrochemical synthesis and subsequent isolation of gram or greater amounts of a pure sample of product B, via the electrode process A B + e , requires exhaustive or bulk electrolysis of electroactive material A at a large-size working electrode, although, of course, exhaustive electrolysis of A frequently is achieved on the microscale (mg to pg) level in mechanistic studies at smaller-sized electrodes. Thus, bulk electrolysis can be a useful large-scale synthetic tool and, indeed, is widely used for commercial production of metals such as Cu, Zn, and Al. Bulk electrolysis experiments, when the data are analyzed in a coulometric form, also enable the n value in a voltammetric electrode process to be determined, provided no additional reactions occur on the longer time scale (typically minutes to tens of minutes) associated with such experiments. 

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