Polarography anions

Polarography is used extensively for the analysis of metal ions and inorganic anions, such as lOg and NOg. Organic compounds containing easily reducible or oxidizable functional groups also can be studied polarographically. Functional groups that have been used include carbonyls, carboxylic acids, and carbon-carbon double bonds. 

An analogy with reductive dimerization of diphenyl cyclopropenone (p. 58) was found on polarography of l,2-diphenyl-4,4-dicyano triafulvene (64)289. In a one-electron reduction step the cyclopropenyl radical anion 469 is likely to be generated and dimerized to the dianion of tetraphenyl-l,4-dicyanomethyl benzene (470) the dianion 471 could be successively oxidized via the anion radical 472 to the 1,4-quinodimethane derivative 473. 

The electrochemical reduction of aryllead triacetates was smdied by Chobert and Devaud82, as a re-examination of some previous work83 to detect the role of intermediates such as [ArPb(OAc)2]V The reductions were carried out by polarography in acetic acid or acidic alcohol solutions and show three diffusion controlled waves. The first step involves a single electron transfer to produce a radical anion which dimerizes, arylates the electrode or hydrolyzes to phenol ... 

Studies on the reduction of Ph3GeX expose more problems related to interpretation of polarography. Whereas Corriu and coworkers stress the description by which the first ET creates a radical anion that reacts further (vide infra), Fleet and Fouzder103 emphasize the formation of a radical in the first step ... 

Reduction of triphenyltin piperidyldithiocarbamate in acetone was shown by polarography and voltammetry to consist of two diffusion-controlled peaks and two peaks which seem to reflect adsorption142. Apparently, a dithiocarbamate group dissociates and triphenyltin radical forms by reduction. The latter partly dimerizes and partly reduces to triphenyltin anion. 

The initial electron transfer to form the anion radical species seems to be reversible. For example, Allred et al. investigated the ac polarography of bis(trimethylsilyl)benzene and its derivatives which showed two waves in di-methylformamide solutions [71] the first one is a reversible one-electron wave, and the second one corresponds to a two-electron reduction. Anion radicals generated by electrochemical reduction of arylsilanes have been detected by ESR. The cathodic reduction of phenylsilane derivatives in THF or DME at — 16° C gives ESR signals due to the corresponding anion radicals [5] (See Sect. 2.2.1). 

The direct access to the electrical-energetic properties of an ion-in-solution which polarography and related electro-analytical techniques seem to offer, has invited many attempts to interpret the results in terms of fundamental energetic quantities, such as ionization potentials and solvation enthalpies. An early and seminal analysis by Case etal., [16] was followed up by an extension of the theory to various aromatic cations by Kothe et al. [17]. They attempted the absolute calculation of the solvation enthalpies of cations, molecules, and anions of the triphenylmethyl series, and our Equations (4) and (6) are derived by implicit arguments closely related to theirs, but we have preferred not to follow their attempts at absolute calculations. Such calculations are inevitably beset by a lack of data (in this instance especially the ionization energies of the radicals) and by the need for approximations of various kinds. For example, Kothe et al., attempted to calculate the electrical contribution to the solvation enthalpy by Born s equation, applicable to an isolated spherical ion, uninhibited by the fact that they then combined it with half-wave potentials obtained for planar ions at high ionic strength. 

Other electroanalytical methods The use of h.v.t. in conjunction with electroanalytical techniques of the potentiometry-polarography type has been described in detail (Kesztelyi, 1984), so that it need not be discussed here. That author, however, ignores a very useful cell for electrosynthesis under vacuum (Schmulbach and Oommen, 1973) and the electrochemical techniques developed by Szwarc and his co-workers and others in the context of anionic polymerisation, which we have mentioned above. 

On the other hand, for anthraquinone and benzophenone, the ESR results showed that the concentration of the radical anion was almost time-independent, although, in polarography, the height of the first wave increased, consuming the height of the second wave as in Fig. 8.12. This inconsistent result was explained as the establishment of the following disproportionation equilibrium in the solution ... 

Although there is a wealth of data in the literature on acid-base behavior in aprotic solvents,60 61 there are few examples of the use of buffers for polarography and voltammetry in aprotic solvents. This has occurred because most investigators have sought to keep all potential proton donors out of the system and thereby stabilize anion radicals. Although the picric acid-picrate system has been used as a buffer in a number of studies in aprotic solvents, its use in voltammetric work is limited because of the ease of reduction of picric acid. 

The oxidation-reduction behavior, ascertained by direct current (conventional) polarography, and its dependence on pH in aqueous solution of the [P2Mo18062] 6, [As2Moi8062]-6, and [P2Wi8062]-6 anions may be found in Refs.1S9 161>. The polarographic behavior of the dimeric 9-molybdophosphate anion of (NH4)6 [P2Moi 8062] was examined by cyclic voltammetry and alternating current... 

Its HFS character is caused by interaction of the unpaired electron with five nitrogen atoms and three equivalent protons. It is impossible to fix an ESR signal of its radical anion in the experimental conditions. Actually, the polarography data prove the fast passing reactions of primary RA dimerization [853], The assignment of the HFS constants in radical ions of the nitroazoles was made with the help of quantum chemistry calculation [855],... 

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