Potential half-wave, of organic compounds

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. 

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. 

Figure 42. Potential ranges for the polarographic and voltamelric half-wave and peak potentials of organic compounds...

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. 

Because the polarographic half-wave potentials of certain types of organic compounds depend strongly on the ionic... 

TABLE 8.31 Half-Wave Potentials (vs. Saturated Calomel Electrode) of Organic Compounds at 25°C The solvent systems in this table are listed below ... 

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. 

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