Undivided or Divided Cells

Because of the low-cost construction and simple operation, an undivided cell is always desired but it cannot be realized in all cases. A precondition for electrolysis in an undivided cell is that disadvantageous reactions and reaction products at the counter electrode can be avoided, for example, by selection of the electrode material and/or of the electrolyte composition. 

A typical counter electrode reaction is the electrolysis of water. Here the cathodic evolution of hydrogen is coupled with the formation of base, the anodic development of oxygen produces acid additionally. Frequently, acid and base formation at both electrodes will be balanced. Otherwise, a buffer solution or a (continuous) base/acid addition, for example, by a pH-controlling system, can enable the application of an undivided cell. 

In many cases, it will be impossible to prevent unwanted reactions at the counter electrode. Then a separation of the anolyte and catholyte is needed. An optimal compromise has to be found for the separator between separation effectiveness and ion conductivity, that is, minimized electrical resistance and low energy consumption. Moreover, chemical, thermal, and mechanical stability and price of the separator have to be considered. Naturally, a complete separation is impossible, because a slight diffusion rate is inevitable. In laboratory scale experiments, probably a high cell voltage is acceptable in order to realize a maximal separation. 

Two basically different types of cell separators are available porous separators 

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In preparative laboratory scale electrolysis, in general between 1 and 100 mmol of substrate is converted. For that purpose the undivided or divided cell shown in Figs. 2 and 3, respectively, are suitable. 

The cylindrical cell is a more convenient cell because the potential or current distribution is nearly perfect. It can be used undivided or divided with a cylindrical diaphragm. This cell type is used with a sacrificial electrode cell, generally with the undivided configuration. 

Generally, direct electrolysis is carried out at a controlled potential (CPE) or constant current (CCE) using both undivided and divided cells. In contrast, an indirect method using a mediator is effective for substrates with higher oxidation potentials beyond the achievable region. 

The examples illustrate the diversity as well as the common features of a paired electrosynthesis. One can start with one or two substrates to generate one or two products. Electrode processes can be mediated or direct. Undivided and divided cells are employed in paired electrosyntheses. But as in the BASF phthahde example, it is crucial for the synthesis of glyoxylic acid, sorbitol, and methyl ethyl ketone that the cathodic process is the reduction of the substrate and not the reduction of protons because in these cases protons are generated at the anode and the electrolysis takes place in aprotic solvent. Therefore effects that minimize the overpotential of hydrogen have to be omitted. Reaction control is important in all described examples, and consequently the cell and the setup have to fit for each case. Work-up and product isolation are significant for a successful synthesis and can be even more challenging in a paired synthesis. 

The significance of the supporting electrolyte cation depends crucially on whether a divided or an undivided cell is used. In a divided cell, the choice of cation is of minor importance but in an undivided cell the cathode process should not lead to formation of base and thereby to buffering of the solution. Metal cations such as Li+, Na+ or Mg + are often the choice since in aprotic solvents the metal cation may be the most easily reduced component. This has been observed as deposits of metal on the surface of the cathode arising from... 

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

Different electrochemical reactors have been used for the anodic oxidation of organic pollutants. They are usually simple batch cylindrical, tank, or flow reactors that are designed as undivided cells or with separators between the anolyte (the solution contained in the anode compartment) and the catholyte (the solution filling the cathode compartment). Divided cells... 

The cell shown in Figure 1 (also called an H-cell because of its shape) is a divided cell, wherein a porous glass frit or polymer e.g. Nafion) prevents the anolyte and catholyte solutions from mixing with each other. In some cases, however, a simple beaker can be used (undivided cell). 

The Birch and Benkeser reactions of some unsaturated organic compounds [318 and references therein], which consist of a reduction by sodium or lithium in amines, can be mimicked electrochemically in the presence of an alkali salt electrolyte. The cathodic reaction is not the deposition of alkali metal on the solid electrode but the formation of solvated electrons. Most of the reactions described were performed in ethylenediamine [319] or methylamine [308,320]. A feature of these studies is variety introduced by the use of a divided or undivided cell. In a divided cell, the product distribution appears to be the same as that in the classic reduction by metal under similar conditions. In contrast, in an undivided cell the corresponding ammonium salt is formed at the anode it plays the role of an in situ generated proton donor. Under such conditions, the proton concentration... 

In the presence of a ligand L (L = N, P, O donor), co-adducts MLXm(n m) 1 are obtained by simple electrooxidation of the metal M. To prevent side reactions at the anode, a divided cell is recommended. On the contrary, an undivided cell is used to obtain metal thiolates or selenolates through the cathodic cleavage of S—S or Se—Sc bonds 51,52... 

The type of the electrochemical cell (divided or undivided) can influence the EOI values especially for the treatment of benzene derivatives containing a -NO2 substituent. A typical example is the electrochemical treatment of p-Nitro Toluene Sulfonic acid (p-NTS) low EOI values (- 0,1) were obtained in the divided cell contrary to the undivided cell where high EOI values (0,5) were obtained. The increase of EOI values in the undivided cell is due to the cathodic reduction of -NOg group to -NH2 group, this transformation promotes the electrochemical oxidation as the substituent constant (a) for -NH2 has negative value (favouring the electrophilic attack on the benzene ring) contrary to the -NO2 substituent which has positive value (see 4 i). 

Electrolytic cells are also used in a variety of devices - and appliances " to produce dilute solutions of electrolyzed water for cleaning and sanitization. Miniature, battery-powered cells are used to generate hypochlorite in handheld sprayers and small portable water disinfection devices. " " The brine may be saturated or have 1-5 g L of salt. An acidic solution of chlorine, hypochlorous acid, and possibly chlorine dioxide is obtained from the anode of a divided cell. It typically has 10-100 mg L of available chlorine and pH values of 2-4. Its stability is poor, and the volatile oxidants are rapidly lost from open solutions. A neutral solution of hypochlorous acid and sodium hypochlorite is dispensed from undivided cells or by combining effluents from the anode and cathode. It typically has 80-100 mg L of available chlorine and pH 5-8. Devices to make 500-1000 mg L" of available chlorine are also available. Neutral solutions made using larger versions of these devices are bottled and sold in some regions. 

Recent work has involved the production of organosilanes and germanes, as small molecules or polymeric systems by electroreduction of the appropriate halo species at a reactive metal cathode (magnesium, aluminum, sometimes copper) in aprotic media. The same metal is used as a sacrificial anode, and the cell is undivided. Thus, with lithium perchlorate as electrolyte in THE solvent, a dichlorosilane such as PhMeSiCl2 gives a polysilane of Mn -- 3000 in 22% yield. This contrasts with earlier work at Hg cathodes in divided cells, where Si-0-containing polymers and cyclotetrasilanes were obtained. Simultaneous... 

Many aspects like the optimal current density or the optimization of the anode process by lowering of the anode overpotential have been explored [25]. Together with the development of a special adapted cell, an optimized process in a large tonnage could be achieved. First, a divided cell with a membrane was used. Later on an undivided cell with a stack of vertical bipolar electrodes was employed [18, 25]. As in the BASF capillary gap cell, one achieves a large electrode area by electrode stacking. The vertical assembly has turned out to be advantageous for the reductive process. 

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