Electrochemical Elimination

Formally, the anodic and cathodic elimination reaction is simply the reverse of cathodic and anodic addition, respectively. As we shall see later, the same E and Nu do not always work in the two types of reactions. 

---

If small amounts of PhSSPh were deliberately added from the beginning, the more positive reduction potential could be applied from the start. Indirect electrochemical eliminations were also performed in the case of 1,2-dihalides Ringopening reactions by the redox catalytic method were possible in the case of methylene cyclopropanes and epoxides... 

Costentin, C., Robert, M. and Savcant, J.-M. (2003) Successive removal of chloride ions from organic polychloride pollutants. Mechanisms of reductive electrochemical elimination in aliphatic gem-polychlorides, a, fi-polychloroalkcncs, and a, fi-polych loro alkanes in mildly pro-tic medium. J. Am. Chem. Soc. 125, 10729-10739. 

The unsaturated nucleoside (53) has been prepared in good yield by electrochemical elimination from either the 2 -bromo-arabino- or the 3-bromo-xylo-nucleoside indicated in Scheme 14 however, the electrolyte can control the product obtained, for whereas 2-bromo-2 -deoxy-3, 5-di-O-propanoyl-uracil gave the 2, 3-unsaturated nucleoside in presence of tetraethylammonium toluene-p-sulphonate in methanol, with sodium acetate a mixture of products was obtained 3-deoxy-3-iodo-adenosine yielded 3-deoxy-adenosine (cordycepin) besides the 2, 3-unsaturated nucleoside.1-Acetyl-glycenose derivatives have been condensed with purine and pyrimidine derivatives in presence of antimony... 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

Many electroless coppers also have extended process Hves. Bailout, the process solution that is removed and periodically replaced by Hquid replenishment solution, must still be treated. Better waste treatment processes mean that removal of the copper from electroless copper complexes is easier. Methods have been developed to eliminate formaldehyde in wastewater, using hydrogen peroxide (qv) or other chemicals, or by electrochemical methods. Ion exchange (qv) and electro dialysis methods are available for bath life extension and waste minimi2ation of electroless nickel plating baths (see... 

Selectivity of propylene oxide from propylene has been reported as high as 97% (222). Use of a gas cathode where oxygen is the gas, reduces required voltage and eliminates the formation of hydrogen (223). Addition of carbonate and bicarbonate salts to the electrolyte enhances ceU performance and product selectivity (224). Reference 225 shows that use of alternating current results in reduced current efficiencies, especiaHy as the frequency is increased. Electrochemical epoxidation of propylene is also accompHshed by using anolyte-containing silver—pyridine complexes (226) or thallium acetate complexes (227,228). 

Other processes have been developed in which the impregnation is accompHshed in one or two steps the most promising is electro deposition directiy from nitrate solutions having pH controlled at 4—5. After electro deposition, the plaques are either cathodicaHy polarized in sodium hydroxide solution or electrochemically formed in sodium hydroxide to eliminate all traces of nitrate. The latter steps must proceed at low current densities to avoid blistering and shedding of the loaded plaques. 

In most cases, the impregnation process is followed by an electrochemical formation where the plaques are assembled into large temporary cells filled with 20—30% sodium hydroxide solution, subjected to 1—3 charge—discharge cycles, and subsequentiy washed and dried. This eliminates nitrates and poorly adherent particles. It also increases the effective surface area of the active materials. 

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). 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

The electrochemical effects of slowly and erratically thickening oxide films on iron cathodes are, of course, eliminated when the film is destroyed by reductive dissolution and the iron is maintained in the film-free condition. Such conditions are obtained when iron is coupled to uncontrolled magnesium anodes in high-conductivity electrolytes and when iron is coupled to aluminium in high-conductivity solutions of pH less than 4-0 or more than 12 0 . In these cases, the primary cathodic reaction (after reduction of the oxide film) is the evolution of hydrogen. 

---