Acids, half-wave potentials

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

Using these assumptions and conventions, Imoto and co-workers have correlated a number of series of reactions of thiophenes and furans. The reactions studied are the acid-base equilibria pK values) and the acid catalyzed methylations (thiophenes only) of thiophene-and furan-carboxylic acids and the alkaline hydrolyses of their ethyl esters the side-chain bromination of the a-acetylthiophenes, and the a-mercuration of thiophenes and the polarographic half-wave potentials of the methyl esters of thiophene- and furan-carboxylic acids and of nitrothiophenes. The pK values were determined and the ester hydrolyses studied for all three substitution orientations in the thiophene series. For the 4-R-2-Y and 5-R-2-Y series, the p-values do not appear significantly different and the data could probably be combined into a single series unfortunately, however, no limits of accuracy are reported for the p-values, and some of the raw data are not readily available so recalculation is not easily possible. For the 5-R-3-Y series the p-values deviate considerably from the other values however, whereas they are higher for the pK values, they are lower for the ester hydrolyses, and it is possible that the differences are neither systematic nor significant. 

FIGURE 3-2 Polarograms for 1 M hydrochloric acid (Curve A) and 4 x 10 4 M Cd+2 in 1 M hydrochloric acid B. id represents the limiting current is the half-wave potential. 

The role of the pH of the medium in the electrode reactions of organic compounds in aqueous solutions is well understood and has been recently reviewed in detail (Zuman, 1969). In particular, our understanding of this parameter is due to the large number of polarographic investigations where it has been found that the half-wave potential, the limiting current and the shape of the wave for an oxidation or reduction process may all be dependent on the acidity of the medium. 

Fig. 7. Effect of pH on the half-wave potential of methyl butyl phonacyl sulphonium perchlorate [2 x m] in water-H 0-2% ethanol. Britton-Robinsonbuffers, ) sulphuric acid, 3 sodium hydroxide. (Taken from Zuman and Tang, 1963.)...

In both cases, the half-wave potential shifts by RT/ ziF)vaN per pH unit, and a typical example of such a behavior is given in Fig. 9 for the transfer of two acidic fi-diketones at the water-nitrobenzene interface. These results were unexpected, since a current wave is measured at a pH where the compound of interest is by a very large majority neutral, but they in fact represent the typical behavior of ionizable compounds at the ITIES and prove that the interfacial potential and the transfer of protons plays a key role for the distribution in biphasic systems. 

In the course of our polarographic studies on organic cations we determined the half-wave potentials, 1/2, for various arylmethylium ions [1-11]. The aim of the present work is to extract from these values some new information concerning the relative magnitude of their solvation enthalpies in three very different solvents. A comparison of our results [obtained in methanesulphonic acid (MSA) and dichloromethane (DCM)] with those of Volz and Lotsch [12] [obtained in cyanomethane (CM) solutions] yields some useful conclusions. 

FIGURE 2.48. Electrochemical reduction of the diphenylmethyl radical produced by the reaction of diphenylmethyl chloride by photo-injected electrons in dimethylformamide in the presence of increasing amounts of benzimidazole. Variations of the half-wave potential the concentrations of acid added, from bottom to top, 0, 0.018, 0.049, 0.11, 2.8, 6.7 mM. Solid lines, simulations for each acid concentration. Adapted from Figure 1 of reference 50b, with permission from the American Chemical Society. 

Pertechnetate in 4 M hydrochloric acid has been found to undergo reduction to the oxidation state +4 a double wave was obtained corresponding to a one-and a two-electron transfer. The waves are rather poorly defined by half-wave potentials of —0.52 V and —0.68 V vs. SCE. Both waves are irreversible. No reduction occurs in 4 M perchloric acid. 

In a 0.5 M KCl solution to which is added hydrochloric acid until pH 2, TcO gives three diffusion-controlled waves with half-wave potentials of —0.14,... 

Benzene mono- di- and tri-sulphonic acids are stable towards electrochemical reduction. More highly sulphonated compounds undergo a two-electron cleavage of a carbon-sulphur bond with elimination of sulphurous acid [82]. Thus benzene-hexa-sulphonic acid 14, in aqueous solution at pH 8, shows three two-electron po-larographic waves with half-wave potentials -1.00, -1.45 and 1.65 V vs. see, due to the sequential loss of substituents. The half-wave potentials are independent of pH. 

In acid solution the half-wave potentials for these processes are pH dependent. The overall reaction involves two electrons and is irreversible. Bond cleavage is believed to lead to the enol as shown in Scheme 5.4. Where, as with acetophenone, the ketone product is electroactive at more negative potentials, the wave height for ketone reduction is less than expected and is limited by the rate of enol to ketone tautomerism. This is because the enol is not electroactive. 

Table 6.8. Half-wave potentials for the oxidation of 5-membered heterocyclic rings at a rotating platinum anode in acetic acid, sodium acetate...

Kihara et al. employed flow coulometry to study the electrode reactions for Np ions in various acidic media [49]. Flow coulometry has an inherent advantage over the conventional hulk coulometry methods in that the electrolysis can be achieved rapidly to aid in the characterization of unstable electrode products. The resulting coulopo-tentiograms for the Np02 /Np02 and Np /Np " " couples indicate reversible processes in nitric, perchloric, and sulfuric acids. The differences in potentials between the various acids are attributed to the associated stability constants of the electrode products with the anion of the acid in each case. Table 2 contains the half-wave potentials for each couple in the various acids. 

Tab. 2 Half-wave potentials (V versus Ag/AgCl) in acidic solution from flow coulometry studies [49]...

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

According to the hard and soft acids and bases (HSAB) concept, hard acids tend to interact strongly with hard bases, while soft acids tend to interact strongly with soft bases. The HSAB concept applies also to solute-solvent interactions. Figure 2.5 shows the polarographic half-wave potentials of metal ions in N-methyl-2-pyrrolidinone (NMP) and N-methyl-2-thiopyrrolidinone (NMTP) [13]. Here, we can compare the half-wave potentials in the two solvents, because they are referred to the half-wave potential of the bis(biphenyl)chromium(l)/(0) couple,... 

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

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