Electrolysis preparative

The correct potential for a preparative electrolysis is normally chosen by inspection of a steady state current-potential (i-F) curve. Figure 1 shows a typical i-E curve for the reduction of anthracene at a mercury cathode in dimethylformamide (Peover et al., 1963) the curve shows two reduction waves. In the potential range where the current rises with variation of the potential, the rate of an electron transfer process is increasing while in the plateau regions the rate of the electron transfer... 

Hence, it is important to remember that the products, reaction mechanism and the rate of the process may depend on the history and pretreatment of the electrode and that, indeed, the activity of the electrode may change during the timescale of a preparative electrolysis. Certainly, the mechanism and products may depend on the solution conditions and the electrode potential, purely because of the effect of these parameters on the state of the electrode surface. 

Fig. 16. Small-scalo laboratory cell for preparative electrolysis. A, Pt gauze working electrode. B, Pt sheet secondary electrode. C, Reference electrode. D, Luggin capillary on a syringe barrel so that the position of the tip of the Luggin probe relative to the working electrode is readily adjustable. E, Glass sinter to separate anode and cathode compartments. F, Gas inlet to allow stirring with inert gas or the continuous introduction of reactant. G, Three-way tap where a boundary between the reference electrode and the working solutions may be formed.

FIGURE 2.36. Oxalate (as opposed to CO and carbonate) yield in the preparative electrolysis of C02 in DMF at a current density of 1.6 mA/cm2 at 0°C. The fitting with the theoretical curve implies that ferr/fej/2 = 8.5 x 105M1/2s-1/2 and D = 10 5 cm2 s-1. Adapted from Figure 5 in reference 38, with permission from the Royal Society of Chemistry. 

Prior to beginning it is necessary to evaluate the aim and the scale of the planned investigations because many particular aspects, discussed in this chapter, are dependent on this decision. There may be a wide range of intentions for preparative electrolysis investigations, demonstrated here by two borderline cases ... 

Slightly more sophisticated preparative electrolysis setups can be obtained from The Electrosynthesis Co., Inc, 72 Ward Road, Lancaster, NY 14086-9779. 

The electrode processes on the voltammetric and the preparative electrolysis time scales may be quite different. The oxidation of enaminone 1 with the hydroxy group in the ortho position under the controlled potential electrolysis gave bichromone 2 in 68% yield (Scheme 4.) with the consumption of 2.4 F/mol [21], The RDE voltammogram of the solution of 1 in CH3CN-O.I mol/1 Et4C104 showed one wave whose current function, ii/co C, was constant with rotation rates in the range from 1(X) to 2700 rpm and showed one-electron behavior by comparison to the values of the current function with that obtained for ferrocene. The LSV analysis was undertaken in order to explain the mechanism of the reaction which involves several steps (e-c-dimerization-p-deamina-tion). The variation of Ep/2 with log v was 30.1 1.8 mV and variation of Ep/2 with logC was zero. Thus, our kinetic data obtained from LSV compare favorably with the theoretical value, 29.6 mV at 298 K, for a first order rate low [15]. This observation ruled out the dimerization of radical cation, for... 

M (CO)6 complexes all undergo irreversible electrochemical reduction in nonaqueous electrolytes at peak potentials close to —2.7 V versus SCE in tetrahydrofu-ran (THF) containing [NBu4][Bp4]. The product of the reductions are the din-uclear dianions [M2(CO)io] although under some conditions polynuclear products can also been obtained, Sch. 3 [2]. It was initially proposed that the primary step involved a single-electron transfer with fast CO loss and subsequent dimerization of the 17-electron radical anion [M(C0)5] [34]. A subsequent study showed that a common intermediate detected on the voltammetric timescale was the 18-electron species [M(CO)5] and that the overall one-electron process observed in preparative electrolysis arises by attack of the dianion on the parent material in the bulk solution, Sch. 2 [35]. 

Diols are faster oxidized than the corresponding monoalcohols as the voltammetric-ally determined relative rates have already indicated. In preparative electrolysis 1,10-decane diol is about five times faster oxidized than 1-decanol. 

As can be seen from current-voltage curves the lower amines are about 10 times faster oxidized than the corresponsing alcohols In a competitive preparative electrolysis (0.3 M potassium hydroxide in 50% t-butanol 50 % water, 40 °Q 1-decylamine is 5.3 times faster oxidized than 1-decanol The electrochemical kinetics have been investigated . ss.es.es) following mechanism proposed... 

Dialkylaziridinium salts are reducible both in aqueous and aprotic media. In aqueous solution the data are consistent with one two-electron reaction. The products obtained from a preparative electrolysis stem in part from the electrode reaction, but side reactions between primary products and starting material complicate the product mixture.181... 

An exhaustive coulometric technique can be used both as an analytical tool and as a preparative tool. These two applications often require different cell designs. The present discussion is restricted to analytical coulometric cells. The design of electrochemical cells for preparative electrolysis has been treated in Chapter 22 and elsewhere [10]. 

Electrochemical methods are very useful in structural studies but are barely applicable for preparative aims. The cause is the limited stability of cation radicals. It is difficult to do low-temperature preparative electrolysis, and the main problem is to dispose of the large amount of heat generated during the electrode work. That is, not much current can be passed through an ordinary-sized electrode without generating too much heat. When potential and temperature control are necessary, only small quantities of a material can be obtained in a reasonable period of time. When potential and temperature control are not necessary, as in Kolbe electrolysis, anodic oxidation is indeed useful as a preparative method. 

The polarographic reduction of tetraphenylethylene in dimethylformamide with n-BU4NCIO4 as a supporting electrolyte is an irreversible two-electron process. Preparative electrolysis in the same solution resulted in the formation of the dianion. The exocyclic bond of this dianion can be disrupted homolytically. Therefore, water treatment of the electrolyzed products leads to diphenylmethane (Wawzonek et al. 1965). 

The number of electrons exchanged on a time scale similar to that of a preparative electrolysis is determined by coulometry. A coulometry experiment involves the complete conversion of the substrate to product(s) and, accordingly, C 0 decreases with time, in principle to zero. This is in contrast to the electro analytical methods where C 0 stays essentially constant during the experiments. Coulometry is carried out at either constant potential or constant current and, usually, the solution is stirred magnetically. 

A.4 Preparative or semi-preparative electrolysis, identification of products... 

Preparative electrolysis of cyclohexanone17 in solutions containing 0.1 M (C4H9)4NBF4 as the electrolyte were carried out at —2.95 V(SCE), more positive potentials resulted in negligible current. When 0.01 M (DMP)BF4 was added to the solution, electrolysis of cyclohexanone was possible at —2.70 V(SCE). Thus, DMP+ caused a 0.25 V positive shift in the preparative reduction potential of cyclohexanone. DMP + also altered the nature of the product. In the presence of DMP+, cyclohexanone formed only the corresponding pinacol, while in its absence cyclohexanol was the sole product. From this and experiments with other aliphatic ketones (that will be described later) it could be concluded that DMP+ catalyzes the reduction and redirects the... 

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