Electro-reductions

Watanabe M, Sei H, Stonehart P. 1989. The influence of platinum crystallite size on the electro-reduction of oxygen. J Electroanal Chem 261 375-387. 

Takasu Y, Ohashi N, Zhang XG, et al. 1996. Size effects of platinum particles on the electro-reduction of oxygen. Electrochim Acta 41 2595-2600. 

The shine from the aluminium is lost as atoms on the surface of the foil are oxidized to form Al3+ ions (Equation (7.1)), which diffuse into solution. Because the aluminium touches the silver, the electrons generated by Equation (7.1) enter the silver and cause electro-reduction of the surface layer of Ag20 (Equation (7.2)). 

Recently, Shono and Kise et al. have successfully carried out the electro-reductive coupling of halofluoro compounds with aldehydes in the presence of chlorotrimethylsilane (CTMS) as shown in Eqs 2-4 [12]. The yields are fairly good. 

Figure 6.27 Square-wave voltanunogram obtained for the electro-reduction of a ferric oxalate complex (5 x 10 mol dm ) in aqueous oxalate buffer r = 33.3 ms, sw = 30 mV and A = 5 mV. Reprinted with permission from Turner, J. A., Christie, J. H., Vukovic, M. and Osteryoung, R. A., Anal. Chem., 49, 1899-1903 (1977). Copyright (1977) American Chemical Society.

Figure1.2 Comparativecurrentdensity (j) vs potential (E) curves for H2 and EtOH electro-oxidation at different Pt-based catalytic anodes and oxygen electro-reduction at a Pt cathode.

Chlorpromazine was quantitatively oxidized to either a free radical by 12N H2SO4, or to a sulfoxide by IN H2SO4 [148]. Electro-reduction of the free-radicals occurred at -0.25 V vs. S.C.E. on platinum wire. 

This chapter does not attempt to cover electrochemical processes which involve the insertion of a group already containing carbon-fluorine bonds into an existing molecule (the building block approach), e.g., perfluoroalkyl group insertion via the Kolbe reaction, or fluoroacyloxylations or fluoroalkoxylations via electro-oxidation, or fluoroalkylation via electro-reduction. These have been reviewed elsewhere [1,2]. 

It was shown later that cyanodimethylsulphonium salts behave similarly upon electro-reduction, although a much more reactive ylid is formed in this case. Formation of relatively unreactive and isolable sulphur ylids has teen reported for the electrolysis of 9-fluorenylsulphonium salts . 

J. Tafel found that while nitric acid is reduced only to hydroxylamine q.v.) by mercury or well-amalgamated electrodes, a copper cathode reduces it to ammonia and at the same time has no action on hydroxylamine. A. Brochet and J. Petit studied the electro-reduction of nitric acid by an alternating current. T. H. Jeffery described the electrolysis of nitric acid with a gold anode, and obtained from the anode liquor crystals of aurinitric acid, HAu(N03)4.3H20. R. Ihle s observations on the oxidation-potential of nitric acid have been discussed in connection with nitrous acid (q.v.). He found that if the cone, of the nitric acid be expressed by... 

The radical anions of phosphafulvene (80) and dibenzophosphafillvene (81) were generated by electrochemical reduction and their EPR spectra recorded between 110 K and room temperature.213 Comparisons of calculated spin densities and charge distributions are presented along with experimental data. The radical anions of o-, m-, or p-thiobenzoate esters were generated by electro-reduction and found by their ESR spectra to be persistent.214... 

A distinguishing aspect in electrode kinetics is that the heterogeneous rate constants, kred and kox, can be controlled externally by the difference between the inner potential in the metal electrode (V/>M) and in solution (7/>so1) that is, through the interfacial potential difference E = electrode setup (typically, a three-electrode arrangement and a potentiostat), the E-value can be varied in order to distort the electrochemical equilibrium and favor the electro-oxidation or electro-reduction reactions. Thus, the molar electrochemical Gibbs energy of reaction Scheme (l.IV), as derived from the electrochemical potentials of the reactant and product species, can be written as (see Eqs. 1.32 and 1.33 with n = 1)... 

From the above expression, it is evident that when the applied potential is more negative than EEq, the interfacial electrochemical equilibrium is broken and the electro-reduction of the oxidized species is thermodynamically favorable. 

The overall rate of the resulting electro-reduction process, v, is given by... 

From the analytical equation (6.105) obtained for CV, the study of the current-potential response in these techniques can be performed along with the analysis of the influence of the key variables. First, the effect of the parameter co (Eq. (6.98)) is shown in Fig. 6.15 where the curves are plotted for a spherical electrode of 50 pm radius. Note that large upvalues relate to the situation where the complexes of the reactant species A are more stable than those of species B, whereas the opposite situation is found for small negative potentials when co increases on account of the hindering of the electro-reduction reaction due to the stabilitization of the oxidized species with respect to the reduced ones. According to Eq. (3.289), an apparent formal potential can be defined as follows ... 

In the above equations, kIt(i/l and k(txh are the first-order heterogeneous rate constants (in s ) at potential Eh of a pulse sequence, for the electro-reduction and electro-oxidation reactions, respectively. It will be assumed that the potential dependence of the rate constants is in agreement with the Butler-Volmer formalism (see Eq. (1.101)), i.e.,... 

In other cases, e.g. 22, electro-reduction involving replacement of fluorine by hydrogen (23) [24,30] or dimerisations (24, 25) [31], have been observed [30] (Scheme 9). 

Stimulated by this work, the electro reduction was tested on the sterically more congested menthoneoxime (7). Substrate 7 - which is derived from optically pure L-menthone - could provide, upon reduction, both epimeric products, (+)-neo-menthylamine (8) and (-)-menthylamine (9), respectively. For both diastereomers there exist unique applications [38, 42-53], The preparation was formerly conducted by treatment with 30 equiv. of sodium metal leading to safety concerns for a scale-up [54]. However, detailed investigations with BDD cathodes indicated that 7 is not a useful substrate. Almost no conversion is observed and only traces of both epimeric amines could be detected. However, the cathodic treatment of 7 using lead results in almost quantitative reduction and a splendid current efficiency (6 F/mol, 66%) [55] (Scheme 4). 

Ishifune M, Yamashita H, Matsuda M, Ishida H, Yamashita N, Kera Y, Kashimura S, Masuda H, Murase H (2001) Electro reduction of aliphatic esters using new paired electrolysis systems. Electrochim Acta 46 3259-3264... 

Crystalline 2-desoxy-D-glucitol was obtained through a crystalline pyridine complex, in approximately 5 % yield, from a commercial product rich in sorbitol, manufactured by the electro-reduction of D-glucose at pH 7-10. It was identified by comparison of melting points, mixed... 

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