Half-wave potential for reduction

For quantitative observations, cyclic voltammetric (CV) measurements were performed over the triad of oxidation states, that is, anion, radical, and cation, available to these systems. The half-wave potentials for reduction and oxidation are summarized in Table 1. For all DTA radicals, oxidation is essentially reversible. Electrochemical reduction of radicals 10 and 12 is, however, almost irreversible, as in the case of radicals 3, 7, 10, and 12. Only for dithiazoles 13 and 33 <2004JA8256> are both the oxidation and reduction steps reversible. 

The half-wave potential for reduction or oxidation varies with kd, since there is not equilibrium on the electrode surface. For cathodic and anodic processes respectively we may write... 

Conjugation between the triple bond and the carbonyl function lowers the reduction potential considerably whereas alkyl substitution makes reduction more difficult (entries 1-5). A comparison between the half-wave potentials for reduction of PhC=CPh (1-69 V, vs. Hg pool) and // <7/t -PhCH=CHPh (1-65 V) substantiates the fact that, at least for this case, a likely product of reduction is more vulnerable to electroreduction than the starting material. In practice electrolyses in protic media aimed at producing alkene from alkyne usually proceed to give alkane. 

TABLE 1. Half-wave potentials, for reduction of arsonium, stibonium and... 

It is a general observation that reduction of arsonic acids in aqueous solution only takes place under acidic conditions (otherwise, the reduction of water is the first electrochemical process taking place when the potential is scanned in the negative direction), and that the value of the half-wave potential is pH-dependent. In Table 3 the half-wave potentials for reduction of a number of arylarsonic adds are given. For several of the adds, values of are given at more than one pH value in order to facilitate direct comparison within as large a number of substituted adds as possible. Data from older studies obtained at large concentrations of the substrate (> 10 mM) and under unbuffered conditions are not included. 

Dimethylarsinic acid appears to be the only arsinic acid studied in non-aqueous solvents. In agreement with the findings for methylarsonic acid, reduction only takes place in MeOH or in MeCN in the presence of an acidic supporting electrolyte (guanidin-ium perchlorate). In contrast to the results obtained in buffered aqueous solution, the half-wave potential for reduction of MejAsO(OH) in MeOH or in MeCN is lower than the half-wave potential for reduction of MeAsO(OH)2 under the same conditions. ... 

In the absence of benzoic acid the half-wave potential for reduction of PhgAsO in MeOH is very close to the background reduction (Eij2 — 1.9 V vs In polar... 

Voltammetric data for ester reductions are available for several aromatic esters [51-54], and in particular cyclic voltammetry shows clearly that in the absence of proton donors reversible formation of anion radical occurs [51]. In dimethylfonnamide (DMF) solution the peak potential for reduction of methyl benzoate is —2.29 V (versus SCE) for comparison dimethyl terephthalate reduces at —1.68 V and phthalic anhydride at —1.25 V [4]. Half-wave potentials for reduction of aromatic carboxylate esters in an unbuffered solution of pH 7.2 are linearly correlated with cr values [51] electron-withdrawing substituents in the ring or alkoxy group shift Ei/o toward less negative potentials. Generally, esters seem to be more easily reducible than the parent carboxylic acids. Anion radicals of methyl, ethyl, and isopropyl benzoate have been detected by electron paramagnetic resonance (epr) spectroscopy upon cathodic reduction of these esters in acetonitrile-tetrapro-pylammonium perchlorate [52]. The anion radicals of several anhydrides, including phthalic anhydride, have similarly been studied [55]. 

To understand why the waveform is a peak, it is necessary to consider what is being measured. The output is a current difference arising from the same potential difference, that is, AilAE. If AE becomes very small (AE - dE), we should obtain the first derivative of the DC polarogram, and the peak and Ey, would coincide. However, because the modulation amplitude is finite and, in fact, has to be reasonably large in order to produce an adequate current signal, the Fpeak value of the differential pulse polarogram is shifted positive at the half-wave potential for reduction processes ... 

Figure 3. Plot of half-wave potentials for reduction of Cso as a function of temperature in benzonitrile, 0.1 M TBACIO4 (solid points) and dichlOTomethane, 0.1 M TBACIO4 (open points).

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