Half-wave potentials of organic

Section 8 now combines all the material on electrolytes, electromotive force, and chemical equilibrium, some of which had formerly been included in the old Analytical Chemistry section of earlier editions. Material on the half-wave potentials of inorganic and organic materials has been thoroughly revised. The tabulation of the potentials of the elements and their compounds reflects recent lUPAC (1985) recommendations. 

Another analytically useful phenomenon in electrolysis at ITIES is ion transfer faciUtated by ionophores present in the non-aqueous phase [8]. If the ionophore is present at a low concentration in the non-aqueous phase and the aqueous phase contains a large concentration of the cation that is bound in a complex with the ionophore (for example as a component of the base electrolyte), then a voltammetric wave controlled by diffusion of the ionophore toward the ITIES or by diffusion of the complex formed away from the ITIES into the bulk of the organic phase appears at a potential lower than the potential of simple cation transfer. The peak height of this wave is proportional to the ionophore concentration in the solution and can be used for the determination (fig. 9.8). This effect has been observed with valinomycin, nonactin, cycUc polyethers and other substances [2,3,23]. The half-wave potential of these waves is... 

Fig. 4.4 Relation between the half-wave potentials of K+ (V vs BCr+/BCr) in various organic solvents and the donor number of solvents [3]. For solvents, see Fig. 4.2.

If a tetraalkylammonium salt is used as supporting electrolyte, this process is either reversible or quasi-reversible and occurs at around -0.8 V vs aqueous SCE in various aprotic solvents and with various electrode materials (Hg, Pt, GC). If a Bmisted acid is added to the solution, the first step is converted to a two-electron process 0 produced in the first step is protonated to form 02H, which is more reducible than 02. Thus, 02H is further reduced to 02H at the potential of the first step. According to detailed polarographic studies in H20-DMS0 mixtures, about 30% v/v water is needed to convert the one-electron process to the two-electron process [41]. A metal ion, M+, interacts with 02 to fonn an ion-pair M+-02 (often insoluble) and shifts the half-wave potential of the first wave in a positive direction [42]. Electrogenerated superoxide 02 can act either as a nucleophile or as an electron donor and has been used in organic syntheses [43],... 

Relation between the LUMO and the half-wave potential of the first reduction wave When an organic compound, Q, is reduced, it accepts an electron from the electrode to its lowest unoccupied molecular orbital (LUMO). Here, the energy of the LUMO of Q corresponds to its electron affinity (EA). If the energies of LUMO ( lu) for a series of analogous compounds are obtained by the molecular orbital method, there should be a relationship ... 

The polarographic and potentiometric methods are not HPLC-run. The polarographic method relies upon the measurement of half-wave potentials of various sulfur compounds reacting with a mercury electrode. It is sensitive to submicromolar concentrations (Luther, pers. com.). While sulfide, thiosulfate, polysulfide and polythionates can be measured, the initial sample must be subdivided and pretreated in different ways. The disadvantages are that sample preparation ana analysis are time-consuming and there is no way to preserve samples for later analysis nor to study organic thiols with precision., ... 

Observed Half-Wave Potentials of Some Organic Compounds In 2N Aqueous Sodium Ifydroxlde With and Without Ifyamlne Micelles... 

Figure 4. Comparison of anodic half-wave potentials of different organic compounds obtained in aqueous sodium hydroxide containing A, nothing B, acetonitrile + Hyamine 2389 (emulsion) and C, Hyamine (micelle). Key 5, anodic half-wave potentials of Hyamine in the micelle system and J, Anodic half-wave potentials of Hyamine in the emulsion system.

The situation in which the supporting electrolyte and/or the solvent is oxidized at a lower potential than the organic substrate is frequently encountered. Anodic methoxylation and cyanation are two typical cases for which homolytic processes involving methoxy and cyano radicals, respectively, have been invoked [125,126]. However, it can be shown that in cyanation [127-129] and at least some cases of methoxylation [130], one must work at an anode potential around or higher than the half-wave potential of the organic substrate in order to get any substitution product. Similar mechanism problems are apparent for the side-chain acetoxylation of alkylaromatic compounds in Ac0H-Me4NN03 [131,132]. 

Lambert, F.L. (1966). Polarography of Organic Halogen Compounds. III. Quantitative Correlation of the Half-Wave Potentials of Alkyl Bromides with Taft Polar and Steric Constants. J.Org.Chem., 31, 4184 188. 

For some substances the half-wave potential of the oxidized form differs substantially from that of the reduced form. The shape of these waves, as well as the shifts of the half-wave potentials, are such as we would expect for a reversible process. Sometimes, the slope of the wave and the dependence of half-wave potentials on pH correspond to a transfer of a smaller number of electrons than is deduced from the limiting current. In such instances we assume that a part of the electrode process is mobile, but usually we describe such total processes as irreversible. Examples of such behaviour are the reductions of carbonyl compounds. For irreversible systems the exchange of water for an organic solvent, e.g. dioxane or dimethylformamide, can disclose whether or not, in the electrode process in aqueous solution, there is interaction with a molecule of water. 

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