Electrochemical Routes

In contrast to the electrochemical route to other important conducting polymers such as polypyrrole and polyaniline, which are obtained anodically and oxidize at the electrode to conducting forms that allow current to be passed continuously, PPV is obtained cathodically and the films formed at solid electrodes are insulating and halt the electrochemical process. Thus, the reduction of a,a,a, a -tetrabromo-p-xylene in aprotic solvents at indium-tin oxide (ITO) or Pt cathodes gives thin PPV films suitable only for optical studies [60, 61]. On the other hand, if the reduction is carried out using a stirred mercury pool cathode [62], the process may be operated at a preparative scale, because any film formed at the electrode is continuously broken up. 

The main advantage of this route is that it offers the possibility of obtaining organic soluble and insoluble polymers with sub- 

The scope of this electrochemical method can be seen in Ik-ble III, which presents some selected examples of PPVs and analogous polymers prepared by this route. 

Other electrochemical methods for preparing PPV thin films on solid electrodes have been described, such as the cathodic reductions of p-jgdylene bis(triphenylphosphonium bromide) [68] and of Wessling s sulfonium precursors [22]. 

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The electrochemical route to duoroaromatics (90) based on controlled potential electrolysis in the absence of hydrogen duoride (platinum anode, +2.4 V acetonitrile solvent tetraalkylammonium duoride electrolyte) has not been commercialized. However, considerable industrial interest in the electrochemical approach stiU exists (91—93). 

Seb cic Acid. Sebacic acid [111-20-6] C QH gO, is an important intermediate in the manufacture of polyamide resins (see Polyamides). It has an estimated demand worldwide of approximately 20,000 t/yr. The alkaline hydrolysis of castor oil (qv), which historically has shown some wide fluctuations in price, is the conventional method of preparation. Because of these price fluctuations, there have been years of considerable interest in an electrochemical route to sebacic acid based on adipic acid [124-04-9] (qv) as the starting material. The electrochemical step involves the Kolbn-type or Brown-Walker reaction where anodic coupling of the monomethyl ester of adipic acid forms dimethyl sebacate [106-79-6]. The three steps in the reaction sequence from adipic acid to sebacic acid are as follows ... 

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

The synthesis of conducting polymers can be divided into two broad areas, these being electrochemical and chemical (i.e., non-electrochemical). Whilst the latter may be considered to be outside the scope of this review, it is worth noting that many materials which are now routinely synthesised electrochemically were originally produced via non-electrochemical routes, and that whilst some may be synthesised by a variety of methods many, most notably polyacetylene, are still only accessible via chemical synthesis. In view of this it is useful to have an appreciation of the synthesis of these materials via routes which do not involve electrochemistry. 

Aniline black was first prepared in the last century [164] when it was found that anodic oxidation of aniline at a Pt electrode in aqueous H2S04 gave a dark green powdery product. Aniline blacks have subsequently produced under a variety of conditions via both chemical and electrochemical routes with a low level of interest... 

Particularly desirable among film deposition processes are solution-based techniques, because of the relative simplicity and potential economy of these approaches. However, the covalent character of the metal chalcogenides, which provides the benefit of the desired electronic properties (e.g., high electrical mobility), represents an important barrier for solution processing. Several methods have been developed to overcome the solubility problem, including spray deposition, bath-based techniques, and electrochemical routes, each of which will be discussed in later chapters. In this chapter, a very simple dimensional reduction approach will be considered as a means of achieving a convenient solution-based route to film deposition. 

The second class One or more conventional routes compete with the electrochemical route. Typical advantages and disadvantages are decisive for the process. 

Adiponitrile is still the only large-tonnage organic product currently prodcued electrochemically. The world output of adiponitrile by electrolysis now exceeds 200000 t year 1. The electrochemical route for conversion of acrylonitrile to... 

L-Cysteine is a high value a-amino acid used world-wide in a scale of 1200-15001 year-1 as additive in foodstuffs, cosmetics or as intermediate or active agent (as antidote to several snake venoms) in the pharmaceutical industry. Chemical routes generally lack the efficiency of electrochemical techniques, or they produce mixtures of l- and d- forms rather than the L-isomer. The most common electrochemical route is the cathodic reduction of L-Cystine in acid (usually HC1) solution to produce the stable hydrochloride. In Table 10, the charateristic data for a laboratory bench, laboratory pilot and a product pilot reaction using a DEM filter press are compared [13]. A production scale study was carried out in a filterpress reactor divided by a cation exchange membrane with a total area of 10.5 m2. The typical product inventory was 450 kg/24-hour batch time. For more details see Ref. [13]. 

Scheme 29 An elegant electrochemical route to the vinblastine alkaloids.

The use of porous membranes as templates for electrode structures was pioneered by Martin and coworkers nearly 20 years ago, and this approach has since been extended to include numerous electrode compositions and geometries " and applications beyond energy storage, including sensing and separations. In this approach, chemical and electrochemical routes are used to fill in the cylindrical, uniform, unidirectional pores of a free-standing membrane with electrochemically active materials and... 

The electrochemical route also achieves intermolecular coupling of phenolic tet-rahydroquinolines. For the relatively unhindered substrate 23, coupling occurs by... 

Radical additions to alkenes and aromatic systems are well known reactions. The trapping in this manner of radicals obtained by reduction of the aliphatic carbonyl function has proved to be a versatile electrochemical route for the formation of carbon-carbon bonds. Such reactions are most frequently carried out in protic solvents so that the reactive species is a o-radical formed by protonation of the carbonyl radical-anion. Tlie cyclization step must be fast in order to compete with further reduction of the radical to a carbanion at the electrode surface followed by protonation. Cyclization can be favoured and further reduction disfavoured by a... 

Conversion of substituted nitrobenzenes to the arylhydroxylamine is easily achieved by reduction in neutral or slightly acid solution. In the first classical experiments, Haber [35] used a platinum cathode and ammonia ammonium chloride buffer and die process was improved by Brand [57] using either a nickel or silvered copper cathode in an acetate buffer. The hydroxylamine can also be obtained from reduction in dilute sulphuric acid provided tire temperature is kept below 15° C to suppress furtlier reduction [58]. This electrochemical route to arylhydroxylamines due to Brand is superior to the chemical reduction using zinc dust and ammonium chloride solution. The latter process is known to give variable yields depending on... 

The electrochemical route (2) can be applied only for the synthesis of Bi(OMe)3, which is insoluble in the parent alcohol (in the other alcohols it was the electrochemical refinement of metal that took place) [967], The glycolates and derivatives of polyatomic phenols stable to hydrolysis have been obtained on alcoholysis ofBi203 [1808, 1352, 684, 559]. 

An alternative electrochemical route to the thiazoline-azetidinone 36 also has been developed. Carboxylic acid 39 prepared from penicillin G can be converted to 36 via 40 by electrochemical decarboxylative acetoxylation followed by hydrolysis (Scheme 2-13)The electrolysis of 39 in an AcOH/DME—AcONa—(C) system at 0 °C... 

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