Synthesis, electrochemical

Electrochemical polymerization is less frequently employed for bulk P(ANi) production and more frequently for production of thin films for spectroelectro-chemical or similar characterization. In a typical procedure, ANi monomer is dissolved in an aqueous sulfuric acid solution (white ANi-sulfate first precipitated is shaken in solution until it redissolves). A potential of less than +0.9 V (vs. Pt quasireference) is then applied to a suitable electrode (e.g. Pt) on which the P(ANi) film is desired. Polymerization is rapid, and film thickness is controlled coulometrically. The polymer film is washed before further characterization. The cyclic voltammo-grams of such P(ANi) films display the dual-redox-peaks characteristic of poly-(aromatic amines) P(ANi) has also been electropolymerized in acetonitrile medium, but the overall properties of the polymer so obtained are considerably inferior to that obtained in aqueous medium. 

An interesting electropolymerization of P(ANi), in aqueous (0.2 M) selenic acid rather than sulfuric acid medium, was reported by Tang et al. [582]. The rate of polymerization of the aniline was however poor as compared to that in sulfuric acid, and was complicated by the deposition of metallic Se. 

Electrochemical mechanistic studies of melanin are an outcome of melanin research in 1980. Various authors have employed these methods using various catecholamines and related compounds as substrates (52,106, 224, 285, 286, 287). These studies have not only confirmed the validity of Raper-Mason s scheme of melanogenesis (see page 158 Fig. 5) but also provided information regarding the mechanism of the chemical steps that occur in the early stages of the melanization process, the identification of each electron-transfer process, and the determination of the rate constants of non-oxidative reactions. 

At an even higher pH ( 7.68), the absence of a cathodic peak permits an estimate of the half line of the corresponding quinone as being on the order of tens of milliseconds (2S7) simultaneous darkening around the anode is considered evidence for melanin formation by electrooxidation. The overall reaction sequence in the electrochemical process is, however, very slow involving only a few monolayers. 

A theoretical design of an enzymatic chemical mechanism (ECC), both on kinetic evidence and considering its pH dependence, has been suggested for electrochemical oxidation of chatecholamines (32, 224, 286). Thus, a-methyldopa (la) (Fig. 2) first undergoes a two-electron oxidation to a-methyldopa-quinone (2a 3a), which then cyclizes to 

No direct electron transfer between melanin particles suspended in aqueous buffers and electrodes has been observed. This permitted studies of charge-transfer processes between the chlorpromazine cation radical and catecholamines spectroelectrochemically in order to determine the biological function of chlorpromazine, 164). 

Electrochemical synthesis is becoming popular because of the method s safety and the mild conditions. The main drawback is the attrition of electrodes. In addition, die results depend on the nature of the electrodes.  

The germane-silane copolymers, 15, are not block copolymers. They were prepared with germanium mole fractions of 0.16 (M = 17,000) and 0.45 (M = 20,600) and exhibited values at 330 nm and 335 nm, respectively. An almost linear correlation exists between and the germanimn mole fraction, when compared to polygermane (X ax = 355 nm) 26,27.57 polysilane (X ax = 325 nm).  

The synthesis of three-dimensional copolymer networks containing both Ge and Si, has also been accomplished from the reaction of trichlorophenylgermane and trichlorocyclohexylsilane using the electrochemical method. For comparison, the same reaction was carried out employing the Wurtz approach. The electrochemical method produced a narrower bimodal distribution.  

Polygermanes are generally colorless solids that are soluble in a wide range of solvents, including THF, toluene, hexane, and chloroform. They are moderately resistant toward oxidation and hydrolysis with thermal stabilities 100°C. TGA analysis shows that weight loss begins above 200°C. For reasons imspecified, a weight retention of 50% to 500 °C was obtained.  

The earliest electrochemical synthesis is the so-called Kolbe reaction involving the oxidation of carboxylic acids in forming decarboxylated coupling products (alkanes). At present, the electrochemical synthesis has become an independent discipline. A large number of organic reactions (synthesis) have been achieved by this technique. The essential requirement for conducting an electrochemical reaction is the conductivity of the reaction medium. The most commonly 

The electrochemical synthesis are of two types anodic oxidative processes and cathodic reductive processes. During anodic oxidative processes, the organic compounds are oxidised. The nature of the product of anodic oxidation depends on the solvent used, pH of the medium and oxidation potential. 

In cathodic reductive processes, the cathode of electrolysis provide an electron source for the reduction of organic compounds. Generally the rate of reduction increases with the acidity of the medium. Electroreduction of unsaturated compounds in water or aqueous-organic mixtures give reduced products — this process is equivalent to catalytic hydrogenation. 

An electrochemical process uses a anode made of metal that resists oxidation, such as lead, nickel or most frequently platinum. The anode is usually in the shape of a cylinder made of wire guage. The usual electrolytes are dilute sulphuric acid or sodium methoxide prepared in situ from methanol and sodium. The direct current voltage is 3-100 V, the current density is 10-20 A/dm and the temperature of the medium is 20-80 C. 

Electrochemical reactions are practically as diverse as non-electrochemical reactions. Thus, the combination of electrochemical reactions with catalysts (electrochemical catalytic process), enzymatic chemistry (electroenzymatic reactions) are quite common. The readers may refer to the following references  

Processes of electrochemical reduction, oxidation, and synthesis of metalloorganic compounds have been investigated intensively. Many metalloorganic complexes and catalysts may be obtained via electrochemical methods. Electrolytic redox processes may be accompanied by reductive elimination or homolytic cleavage of the M — C bond and the formation of various products of radical reactions  

The cation undergoes homolytic decomposition to afford typical products of solvent-cage radical reactions.  

PROPERTIES OF COMPLEXES CONTAINING METAL-CARBON a BONDS 

A summary of materials which have been electrochemically synthesised is given in Table 1, and in subsequent sections specific materials are discussed separately in relation to the published literature, but it should be noted that this separation is 

Monomer (concn.) Electrode/solventa Electrolyte (concn.) 

Polyisothianapthene (PITN) isothianapthene acn, N2atm (0.23 M) isothianapthene acn (anhyd.) Bu4NPF6, LiBr, Bu4NBr, Ph4AsCl (0.3 M) Ph4PCl 

Mixed solvent systems are shown as e.g. acn-aq (0.01 M) where the number in parentheses indicates the concentration of the lesser constituent ITO-Indium/tin oxide-coated glass, Ar- Solutions purged with argon, Ar atm - Experiment performed under an argon atmosphere, N2 atm - Experiment performed under a nitrogen atmosphere b All potentials are measured vs. SCE unless otherwise stated  

Where only a current density was given in the original Ref. this is quoted in place of the polymerisation potential. c Counter ions are as incorporated during the electrochemical polymerisation process or by subsequent electrochemical doping unless suffixed with (chem.) which indicates the use of chemical doping. d Conductivities f-film, p-pressed pellet. 

Although it is thennodynamically favorable for the linkages to be formed at the 2 positions of the monomer umts as shown above, there is concern that the electrochemical polymerization results in a significant niunber (perhaps 30%) of linkages that are not of this 23 type. 23 Couplings do not extend conjugation, in fact. 

New polymers synthesized by this procedure continue to be reported as new oxidizable conjugated units become available. For example, the fused monomers shown in figure 8 have a higher degree of conjugation and are easier to oxidize than their smaller counterparts. 

The preparation of polypyrrole, polythiophene, polyaniline, and related conducting polymers demonstrates principles of electrochemical synthesis that are more widely applicable, and it is instructive to examine these in detail. 

An electrochemical cell is part of the complete electrical circuit of the system and thus electrons must travel through it just like any other electrical component. It has a resistance, and under altering or variable current and voltage conditions it will have a capacitance. To transverse the cell, electrons must leave the cathode leaving a reduced (electron-rich species) in the cell, while simultaneously electrons must pass into the anode from a species in the cell that becomes oxidized (electron-poor). To complete the circuit ions must cross the cell (cations towards the cathode competing with anions towards the anode). 

Now if there were two highly reactive species (for the oxidation and reduction process), it would require only a low voltage to drive the two electrode reactions. In fact a battery is a device that contains species that are sufficiently reactive so that instead of needing any externally supplied voltage to move electrons, the electrons are driven externally under the energy of reaction. Alternatively, two unreactive species (or even just one, if sufficiently unreactive) will require a higher voltage for electrolysis. 

The upshot is that the measured voltage necessary to drive a cell with the minimum two electrodes is a complex mixture of potentials at both electrodes together with various voltage losses in the system, and we do not know, if the required voltage should suddenly be seen to rise after a period of electrolysis, whether this is due to some time-dependent effect at the anode, the cathode or elsewhere in the system. 

this is not usually a problem for producing polypyrrole and related polymers. since a typical film on the electrode weighs only some tens of milligrams, and an electrolyte containing, for example, 0.01 M pyrrole can support its formation without appreciable depletion of the monomer. Constant-current electrolysis is therefore often used for these polymers although it should be noted that the exact properties of the film can vary with preparation conditions, and with this methodology the exact electrode potenhal is not known. 

Electrochemistry is widely used in industry, for example in effluent treatment, corrosion prevention and electroplating as well as in electrochemical synthesis. Electrochemical synthesis is a well-established technology for major processes such as aluminium and chlorine production there is, however, increased interest in the use of electrochemistry for clean synthesis of fine chemicals. The possible green benefits of using electrochemical synthesis include  

The electroreductive cyclization of chiral aromatic a-iminoesters 175, prepared from ( )-a-amino acids such as (6)-valine, (A)-leucine, and ( -phenylalanine, in the presence of chlorotrimethylsilane and triethylamine afforded mixed ketals of ar-2,4-disubstituted azetidin-3-ones 176 stereospecifically ( 99% de and 85-99% ee) (Equation 46) 2003JA11591 . The best result was obtained using tetrabutylammonium chlorate as a supporting electrolyte and a platinum cathode. 

By convention, the potentials of all half-reactions, E°, are found tabulated for the reduction process under standard conditions of temperature (298.15 K), pressure (1 atm), and solute concentrations (1 molar). For nonstandard conditions, the reduction potentials, and hence the cell voltage, will differ. The concentration dependence on the cell voltage is given by the Nemst equation  

In an electrolytic cell, electrical energy is used to force a current through the cell to produce a chemical change for which the cell potential is negative  

TABLE 4.2 Some Standard Electrode (Reduction) Potentials in Aqueous Solution at 25°C 

Many plating baths contain cyanide, phosphate, or carbonate anions to facilitate the half-reactions and to increase conductivity. In making predictions about overall cell reactions, all possible half-reactions should be considered, including oxidation or reduction of the anions present in solution or even of the solvent species itself. In general, the most easily oxidized species (the one with the most negative reduction potential) will be oxidized, and the most easily reduced species (the one with the most positive reduction potential) will be reduced. Of the possible half-reactions, the ones with the most positive (least negative) potential will usually take place. Hence, we can see from the reduction potentials listed 

Cathodic reduction is also used to purify some metals, such as copper. Slabs of impure copper serve as the anode, while a pure copper sheet serves as the cathode in an undivided electrolytic cell. The electrolytic bath is copper(II) sulfate. During electrolysis, Cu2+ ions leave the anode and plate on the cathode. Impurity metals more reactive than copper are oxidized and stay in solution. Less reactive metals collect at the bottom of the cell. After about a month, the enlarged copper cathodes are removed (Ebbing and Gammon, 2005). Metals can also be oxidized electrolytically at the anode (anodized). It is even possible to further oxidize some metals in a low oxidation state to a higher oxidation state. 

Phosgene has been produced at a gas diffusion anode when a sodium chloride solution was electrolysed whilst supplying carbon monoxide to the anode [2065]. 

The enhanced reactivity of 3-aminophenylboronic acid in the presence of fluoride was reportedly due to interaction between fluoride and boron. The complexation of fluoride with the boronic acid moiety substantially reduced the oxidation potential required for polymerization, thereby avoiding deleterious side reactions that occur at more positive potentials. The binding of fluoride was confirmed by Fabre et al. [31] with NMR spectroscopy of monomer solution before and after addition of fluoride, since the chemical shift is sensitive to the hybridization of the boron. In monomer solution without fluoride, a NMR signal was observed at 27.7 ppm with reference to borontrifluoride etherate. However, after addition of fluoride, a new signal was observed at 3.4 ppm. The B chemical shift of a tetrahedral boronate formed in the presence 

BORONIC ACID SUBSTITUTED SELF-DOPED POLYANIUNE 

Shifts measured relative to reference borontrifluoride etherate. 

Similarly, self-doped PABA can be prepared using excess of saccharide and one equivalent of fluoride to monomer. Complexation between saccharides and aromatic boronic acids is highly pH dependent, presumably due to the tetrahedral intermediate involved in complexation [25]. Because the pKa of 3-aminophenylboronic acid is 8.75, complexation requires pH values above 8.6. This pH range is not compatible with the electrochemical synthesis of polyaniline, which is typically carried out near a pH value of 0. However, Smith et al. have shown that the addition of fluoride can stabilize the complexation of molecules containing vicinal diols with aromatic boronic acids [23]. Based on this work, it was postulated that the electrochemical polymerization of a saccharide complex with 3-aminophenylboronic acid in the presence of one molar equivalent of fluoride at pH values lower than 8 is possible if a self-doped polymer is produced in the process. 

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Yu Y efa/1997 Gold nanorods electrochemical synthesis and optical properties J. Phys. Chem. B 101 6661... 

Electrochemical Synthesis. Electrochemical methods have also been investigated for the synthesis of polysilanes, but these have so far yielded low molecular weight materials (113,114). 

Oxygen-Evolving Anode. Research efforts to iacorporate the coated metal anode for oxygen-evolving appHcations such as specialty electrochemical synthesis, electro winning, impressed current, electrodialysis, and metal recovery found only limited appHcations for many years. 

TYZOR TPT and the tetraethyl titanate, TYZOR ET [3087-36-3], have also been prepared by direct electrochemical synthesis. The reaction involves anode dissolution of titanium in the presence of the appropriate alcohol and a conductive admixture (3). 

Electrochemical synthesis of P3ATs was accompHshed in 1986 (81). In the same year, a technique was reported for the chemical synthesis of P3ATs as shown in equation 7. 

Concern for the conservation of energy and materials maintains high interest in catalytic and electrochemistry. Oxygen in the presence of metal catalysts is used in CUPROUS ION-CATALYZED OXIDATIVE CLEAVAGE OF AROMATIC o-DIAMINES BY OXYGEN (E,Z)-2,4-HEXADIENEDINITRILE and OXIDATION WITH BIS(SALI-CYLIDENE)ETHYLENEDIIMINOCOBALT(II) (SALCOMINE) 2,6-DI-important industrial method, is accomplished in a convenient lab-scale process in ALDEHYDES FROM OLEFINS CYCLOHEXANE-CARBOXALDEHYDE. An effective and useful electrochemical synthesis is illustrated in the procedure 3,3,6,6-TETRAMETHOXY-1,4-CYCLOHEX ADIENE. ... 

Mediated electrolyses make use of electron transfer mediators PjQ that shuttle electrons between electrodes and substrates S, avoiding adverse effects encountered with the direct heterogeneous reaction of substrates at electrode surfaces (Scheme 6). In recent years this mode of electrochemical synthesis has been widely studio and it is becoming increasingly well understood. A review is given in vol 1 of the present electrochemistry series... 

Partially successful attempts towards chiral electrochemical synthesis have involved chiral supporting electrolytes chiral solvents and chiral adsorbates, mostly alkaloids With the latter method enantiometric excess values >40% have... 

In the following, selected results will be presented on the conventional electrochemical synthesis of metal chalcogenide binary and ternary systems, conducted by employing variants of the methods outlined in the previous sections. A brief account of chemical bath deposition principles exemplified will be addressed at the end of this chapter, as being closely related to electrochemical deposition of thin films. 

Reports on electrochemical synthesis of indium chalcogenides have been scarce. 

Loizos Z, SpyreUis N, Mamin G (1991) Electrochemical synthesis of semiconducting CdSe thin films. Thin Solid Films 204 139-149... 

Schneemeyer LF, Cohen U (1983) Electrochemical synthesis of photoactive M0S2. J Electrochem Soc 130 1536-1539... 

Herrero J, Ortega J (1987) Electrochemical synthesis of photoactive ln2Se3 thin films. Sol Energy Mater 16 477-485... 

As an illustrative example of this method for electrochemical synthesis of sulfide compounds consisfed in utilizing a sulfur-modified mefal surface as a template for fhe elecfrodeposition of mefal sulfide films, Tacconi and Rajeshwar described fhe firsf aflempl fo obfain all-elecfrodeposifed indium sulfide tiiin films, by a dual batii procedure [95] (cf conventional deposition of indium chalcogenides). The... 

Chemical and electrochemical techniques have been applied for the dimensionally controlled fabrication of a wide variety of materials, such as metals, semiconductors, and conductive polymers, within glass, oxide, and polymer matrices (e.g., [135-137]). Topologically complex structures like zeolites have been used also as 3D matrices [138, 139]. Quantum dots/wires of metals and semiconductors can be grown electrochemically in matrices bound on an electrode surface or being modified electrodes themselves. In these processes, the chemical stability of the template in the working environment, its electronic properties, the uniformity and minimal diameter of the pores, and the pore density are critical factors. Typical templates used in electrochemical synthesis are as follows ... 

Xi D, Zhang H, Furst S, Chen B, Pei Q (2008) Electrochemical synthesis and photovoltaic property of cadmium sulfide-polybithiophene interdigitated nanohybrid thin films. J Phys ChemC 112 19765-69... 

She G, Zhang X, Shi W, Cai Y, Wang N, Liu P, Chen D (2008) Template-free electrochemical synthesis of single-crystal CuTe nanoribbons. Cryst Growth Des 8 1789-1791... 

Khan, M., Oldham, G. and Tuck, D.G. (1981) The direct electrochemical synthesis of triphenylphosphine adducts of Group IB monohalides. Canadian Journal of Chemistry, 59, 2714-2718. 

Synthesis Basically, two methods are available, which both start (evidently) from suitable monomers (1) chemical synthesis, followed by doping, and (2) electrochemical synthesis directly in a doped state. 

Great promise exists in the use of graphitic carbons in the electrochemical synthesis of hydrogen peroxide [reaction (15.21)] and in the electrochemical reduction of carbon dioxide to various organic products. Considering the diversity in structures and surface forms of carbonaceous materials, it is difficult to formulate generalizations as to the influence of their chemical and electron structure on the kinetics and mechanism of electrochemical reactions occurring at carbon electrodes. 

Maybe the best well-estabhshed method for the preparation of Bfxs is the oxidative ring closure of o-nitroanihne derivatives. AlkaUne hypochlorite, NaOCl/KOH, has been the most commonly used reagent but electrochemical synthesis [17] and bis(acetate-0-)phenyUodine as oxidizer have also been employed [18]. The procedure is particularly useful where the other... 

Spreader-Bar Structures as Molecular Templates for Electrochemical Synthesis of Nanoparticles... 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

Hancock died in Budapest, Hungary, in September 1993 while on official travel. He had served as Division Director since 1990, and since 1987 had provided direction either as Acting Division Director or Deputy Division Director. He guided the development of joint programs with the Division of Chemical and Thermal Systems in Electrochemical Synthesis and in Environmentally Benign Synthesis and Processing. Hancock recognized very early the opportunities for U.S. scientists in Eastern Europe... 

Within the Division of Chemistry, several initiatives to improve intersectoral cooperation have already been established. New in 1992 were (1) a cooperative program with the Electric Power Research Institute on electrochemical synthesis joint review and joint funding and (2) a coopera-... 

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