Polarographic half-wave

Tirouflet et al. measured polarographic half-waves for a series of substituted thiazoles (143) in order to determine the ability of polarog-raphy for the selective determination of thiazoles mixtures. The values found for 2-amino-5-bromothiazole agree with the +M effect ot the NHj... 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

The use of the Hammett equation has also been extended to several new types of applications. Since these are not germane to the subject matter of the present chapter, we wiU simply mention work on applications to ethylenic and acetylenic compounds the various applications to physical properties, such as infrared frequencies and intensities, ultraviolet spectra, polarographic half-wave potentials, dipole moments,NMR and NQR spectra,and solubility data and applications to preparative data and biological activity. 

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. 

In stripping voltammetry the stripping potential of a given ion is generally close to the polarographic half-wave potential of that ion in solutions with similar supporting electrolytes. Thus, typical stripping potentials in a 0.05M potassium chloride base solution are as follows Zn, — 1.00 V Cd, -0.07 V Pb, -0.45 V Bi, -0.10 V Cu(II), -0.05 V. 

Thus, the peak separation can be used to determine the number of electrons transferred, and as a criterion for a Nemstian behavior. Accordingly, a fast one-electron process exhibits a AEp of about 59 mV Both the cathodic and anodic peak potentials are independent of die scan rate. It is possible to relate the half-peak potential (Ep/2. where the current is half of the peak current) to the polarographic half-wave potential, El/2 ... 

The peak potential (Ep) can be used to identify the species, as it occurs near the polarographic half-wave potential ... 

Ev depends on both v and L to such an extent that its position can be even 40 mV before (more negative than) the polarographic half-wave potential E,. 

From measurements of this type Thomaz and Stevens found a linear relationship for a graph of log(kJrPC,2) vs. where n is the number of halogen atoms in the molecule, is the spin-orbit coupling constant, and Em is the polarographic half-wave reduction potential of the heavy-atom quencher (Figure 5.16). This correlation suggests that an exciplex is formed by partial... 

The synthetic chemistry of these complexes and their stability to various redox reagents may be considerably systematized by the use of polarographic half-wave potentials for the one-electron... 

TABLE III Polarographic Half-wave Potentials of Complexes in Acetonitrile Solution... 

Quantitative structure-chemical reactivity relationships (QSRR). Chemical reactivities involve the formation and/or cleavage of chemical bonds. Examples of chemical reactivity data are equilibrium constants, rate constants, polarographic half wave potentials and oxidation-reduction potentials. 

For metals which are soluble in mercury, such as the alkali and alkaline earth metals, the polarographic half-wave potential is a function of the... 

Thermodynamic reduction potentials of numerous aromatics were first measured by Hoijtink and van Schooten in 96% aqueous dioxane, using polarography [15, 16]. These fundamental works were decisive tests of the HMO theory, showing that the polarographic half-wave potentials vary linearly with the HMO energies of the lowest unoccupied molecular orbitals (LUMO) of the hydrocarbons [1]. Hoijtink etal. had already noticed that most aromatics can be further reduced to their respective dianions [17]. They proposed a... 

To learn that numerical values of the polarographic half-wave potential 1/2 are a characteristic of the analyte. 

Alkenes Influence of conjugation on the polarographic half-wave potential for... 

Conjugation with an electron-withdrawing group substantially lowers the energy of the lowest unoccupied molecular n-orbital, which results in less negative reduction potentials for the alkene system. The class of compounds is referred to as activated alkenes, Polarographic half-wave potentials for some activated alkenes in aprotic solvents are listed in Table 3.3... 

Polarographic half-wave potentials for activated alkenes in aprotic solvents... 

Polarographic half-wave potentials for the reduction of vjc-dibromides at mercury in dimethylformamide with a tetraalkylammonium salt electrolyte. 

Polarographic half-wave potentials Table 4.8) for the reduction of 1,2-dibromides are sensitive to the relative stereochemistry of the halide substituents... 

Polarographic half-wave potentials for onium salts in aqueous solution... 

Tetrasubstituted onium salts are cleaved by reduction and lose the substituent which gives the most stable radical. Polarographic half-wave potentials for some of... 

Polarographic half-wave potentials for tetraphenyl and triphenyl onium salts in water. Ref [47]. 

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