Polarograph aprotic solvents

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... 

Nitrobenzene radical-anion is more stable in aprotic solvents than its aliphatic counterparts. Nitrobenzene shows two one-electron polarographic waves in acetonitrile with By, -1,15 and -1.93 V vj. see, Tire first wave is due to tlie formation of the radical-anion and this species has been characterised by esr spectroscopy [6]. Nitrobenzene radical-anion can also be generated in steady-state concentration by electrochemical reduction in aqueous solutions at pH 13 [7] and in dimethyl-formamide [8]. It is yellow-brown in. solution with A., ax 435 nm. Protonation initiates a series of reactions in which niti osobenzene is formed as an intermediate and... 

In water, the ions of rare earth metals, aluminum and zirconium are hydrolyzed and do not give well-defined polarographic reduction waves. In aprotic solvents, however, the solvolysis reactions do not occur as easily as the hydrolysis reactions in water. Thus, in solvents like DMSO, DMF and AN, these ions give... 

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],... 

Bransted acids (proton donors, HA) have significant influences on the reduction of organic compounds in aprotic solvents. If a weak Bronsted acid like water is added step-wise to the electrolytic solution, the height of the first polarographic wave increases at the expense of that of the second wave (Fig. 8.12). By the addition of a weak acid, the following reactions occur at or near the electrode ... 

The most common electrochemical effects exerted in bulk solution are related to association (solvation, ion-pairing, complex formation, etc.) with the electroactive substance or electrochemically generated intermediates [4,19]. The importance of solvation can be gauged by comparing calculated and measured values of the parameter AE1/2 (defined as the difference, in volts between the half-wave potentials of the first and second polarographic waves) exhibited by polycyclic aromatic hydrocarbons (PAH) in dipolar aprotic solvents [46,47], It can be shown that AE1/2 is related to the equilibrium constant for disproportionation of the aromatic radical anion into neutral species and dianion, that is,... 

An investigation of the dc and ac polarographic responses of 24 pharmaceuticals in an aprotic solvent system has been described [82]. Barbiturates, salicylates, corticosteroids, and alkaloids were examined in acetonitrile-tetra-butylammonium perchlorate. The majority of the compounds yielded one-electron reversible waves in this solvent they were suitable for analytical pur-... 

Chemical reduction can also be used, but a very fruitful and informative method seems to be controlled polarographic reduction (Maki and Geske, 1960), which can be carried out with the solution in the resonance cavity. Again, it is best to use an aprotic solvent, and methyl cyanide appears to be very suitable. It has the advantage over ethereal solvents that the formation of contact ion pairs (Griffiths and Symons, 1960) is avoided (Maki and Geske, 1960). 

For reduction, relevant data from polarographic and cyclic voltammetric experiments are summarized in Tables 1 and 2, respectively. For the results in Table 1 the variety of solvents and reference electrodes used makes comparisons difficult. It is clear, however, that even with the activation of a phenyl substituent (entries 6,7,9-14) reduction occurs at very cathodic potentials. In this context it is worth noting that in aprotic solvents at ca. — 3 V vs. S.C.E.) it becomes difficult to distinguish between direct electron transfer to the alkyne and the production of the cathode of solvated electrons. Under the latter conditions the indirect electroreductions show many of the characteristics of dissolving metal reductions (see Section II.B). Even at extreme cathodic potentials it is not clear that an electron is added to the triple bond the e.s.r. spectra of the radical anions of dimesitylacetylene and (2,4,6,2, 4, 6 -hexa-r-butyldiphenyl)acetylene have been interpreted in terms of equal distribution of the odd electron in the aromatic rings . 

Electrochemical studies of quaternary arsonium and stibonium salts have mostly been carried out in aqueous or bufiered aqueous solution using polarographic methods, i.e. reduction at a dropping mercury electrode . A few studies in polar aprotic solvents have been carried out. 

This book is based on the study of reactions of thermal electrons with molecules using the ECD, negative ion chemical ionization (NICI) mass spectrometry in the gas phase and polarographic reduction in aprotic solvents [18]. Only the complementary studies related to our research are considered here. 

Polarographic half-wave reduction potential measured in aprotic solvents. 

Interpretation of data given in Table 2 is more difficiilt than in the case of polarographic reduction of metal cations, since complex formation is also influenced by anion solvation. Different solvents can differ considerably in their solvating ability towards anions, and this is especially true for protic and aprotic solvents (10, 43, 49, 50). 

DN = 38.8) is a stronger EPD than DMSO (DN=29.8) but its ability to solvate anions is much smaller 43) compared to other aprotic solvents, and coordination of HMPA to Co2+ (as wdl as to several other metal cations) is definitely sterically hindered (see preceding section). The role of solvent donicity in complex formation is supported by polarographic studies of the reduction of Eu + to Eu2+ in different solvents using supporting electrolytes with anions of different EPD properties (Fig. 8)... 

The ease of reduction of Q may be quantified by the reduction potential of the couple Q/Q , usually denoted by E and expressed in volts relative to the Normal Hydrogen Electrode (NHE). Only if both oxidant Q and reductant Q are in their thermodynamic standard states will the potential E equate to the standard reduction potential of the couple and symbolized as or E . In the present context, interest is focused on effective potentials under physiological conditions, i.e. in water at pH values close to 7. Reduction potentials such as polarographic half-wave potentials obtained in non-aqueous media, especially aprotic solvents such as dimethylformamide or acetonitrile and relative to a standard calomel electrode (for example), will be numerically quite different. should not be confused with a formal potential Eq, which is usually defined as the potential when the ratio of the total concentrations of oxidized and reduced... 

As observed before the first polarographic half-wave potentials determined in aprotic solvents can be correlated with Hammett a values. This was done for the 4-nitro-, chloro-, methyl-, methoxy- and a series of ring chlorinated dimethylamino-benzene-diazonium salts. A linear correlation was observed when corrected constants were used... 

The addition of only a very small quantity of water renders the intermediate radical unstable and the two waves coalesce. Frequently polarographic behaviour in a completely dry aprotic solvent is quite different to the same solvent slightly wet with traces of water. 

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