Nonaqueous systems, reactions

Mann, C. and Barnes, K. (1970). Electrochemical Reactions in Nonaqueous Systems. Dekker, New York... 

For the application of membrane reactors it can be concluded that these are accepted as proven technology for many biotechnological apphcations. The membranes used in this area can operate under relatively mild conditions (low temperature and aqueous systems). However, there is a tremendous potential for membrane reactors in the chemical industry, which often requires apphcation in nonaqueous systems. Long term stability of the membrane materials in these systems will require an ongoing development from the side of materials scientists. As reaction selectivity is of major importance in the production of fine chemicals and pharmaceutical products, it seems plausible to expect that membrane reactors will find their way in the production of chemicals through applications in these areas. 

Another problem is the very high concentrations of reactants present in the low-conversion region. The correct derivation of any rate expression such as Eqs. 2-20 and 2-22 requires the use of activities instead of concentrations. The use of concentrations instead of activities assumes a direct proportionality between concentration and activity. This assumption is usually valid at the dilute and moderate concentrations where kinetic studies on small molecules are typically performed. However, the assumption often fails at high concentrations and those are the reaction conditions for the typical step polymerization that proceeds with neat reactants. A related problem is that neither concentration nor activity may be the appropriate measure of the ability of the reaction system to donate a proton to the carboxyl group. The acidity function ho is often the more appropriate measure of acidity for nonaqueous systems or systems containing high acid concentrations [Ritchie, 1990]. Unfortunately, the appropriate ho values are not available for polymerization systems. 

On the general question of the anation reaction of the aquo complex by various anions, there is a little more information. In particular, of course, there are a few cases where one may identify rates of the same sort that are discussed in the paper on nonaqueous systems-i.e., rates of interchange between outer and inner sphere ligands. 

Among the solvents suggested for azo coupling reactions, aside from aqueous and specialized nonaqueous systems as mentioned above, are mixtures of water with water-soluble alcohols (e.g., methanol, ethanol, propanol), other water-soluble solvents such as tetrahydrofuran, N, A-dimethylacetamide, iV,iV-dimethylformamide, and organic acids such as formic, acetic, and propionic acids [11 ]. 

Another striking difference between aqueous and anhydrous, nonaqueous systems is the size of the aggregates that are first formed. As we have seen, n is about 50 or larger for aqueous micelles, while for many reverse micelles n is about 10 or smaller. A corollary of the small size of nonaqueous micelles and closely related to the matter of size is the blurring of the CMC and the breakdown of the phase model for micellization. Instead, the stepwise buildup of small clusters as suggested by Reaction (D) is probably a better way of describing micellization in anhydrous systems. When the clusters are extremely small, the whole picture of a polar core shielded from a nonaqueous medium by a mantle of tail groups breaks down. 

Systematic study of the stability of metal complexes in nonaqueous systems and their reactivity with thiols, free radicals, etc., is required for a fuller understanding of their complex effects on the co-oxidation reaction. 

Versus a normal hydrogen electrode. Various ionic media are used see references for details. b Corrected for normal hydrogen electrode from Ag/Ag+ by the addition of 0.578 V (Mann, C., and Barnes, K., Electrochemical Reactions in Nonaqueous Systems. Dekker, New York, 1970. c Generally with weakly coordinating media. 

Mann, C.K. Barnes, K.K. in "Electrochemical Reactions in Nonaqueous Systems" Marcel Dekker New York, 1970 chpt. 3. 

Noble metal electrodes include metals whose redox couple M/Mz+ is not involved in direct electrochemical reactions in all nonaqueous systems of interest. Typical examples that are the most important practically are gold and platinum. It should be emphasized, however, that there are some electrochemical reactions which are specific to these metals, such as underpotential deposition of lithium (which depends on the host metal) [45], Metal oxide/hydroxide formation can occur, but, in any event, these are surface reactions on a small scale (submonolayer -> a few monolayers at the most [6]). 

Reactive electrodes refer mostly to metals from the alkaline (e.g., lithium, sodium) and the alkaline earth (e.g., calcium, magnesium) groups. These metals may react spontaneously with most of the nonaqueous polar solvents, salt anions containing elements in a high oxidation state (e.g., C104 , AsF6 , PF6 , SO CF ) and atmospheric components (02, C02, H20, N2). Note that ah the polar solvents have groups that may contain C—O, C—S, C—N, C—Cl, C—F, S—O, S—Cl, etc. These bonds can be attacked by active metals to form ionic species, and thus the electrode-solution reactions may produce reduction products that are more stable thermodynamically than the mother solution components. Consequently, active metals in nonaqueous systems are always covered by surface films [46], When introduced to the solutions, active metals are usually already covered by native films (formed by reactions with atmospheric species), and then these initial layers are substituted by surface species formed by the reduction of solution components [47], In most of these cases, the open circuit potentials of these metals reflect the potential of the M/MX/MZ+ half-cell, where MX refers to the metal salts/oxide/hydroxide/carbonates which comprise the surface films. The potential of this half-cell may be close to that of the M/Mz+ couple [48],... 

Active metals such as lithium and sodium can be used as stable reference electrodes in nonaqueous solutions in which they are apparently stable. To a limited extent this may be true for the Mg/Mg2+, Ca/Ca2+ and A1/A13+ couples as well (though they must be checked separately for each specific solution). It is important to note that in most of the commonly used nonaqueous systems, the above active metals are thermodynamically unstable and react readily with the solvent, the salt anions and the unavoidably present atmospheric contaminants. However, the active metals are apparently stable in many systems because the above reaction products, which are usually insoluble (metal salts), precipitate as protective passivating surface films. These films prevent further corrosion of the active metals in solutions [21], Hence, the active metal covered by the surface films may... 

In this chapter we review the data accumulated over the years for various electrochemical reduction and oxidation reactions of nonaqueous systems, with... 

The first comprehensive review of electrochemical windows of nonaqueous systems, by Mann, appeared 20 years ago in the series Electroanalytical Chemistry [13], edited by A. J. Bard. In general, the picture provided by Mann is quite reliable. However, over the years, a vast amount of work has been done with nonaqueous systems, particularly within the framework of basic studies related to high energy density batteries and the application of novel spectroelectrochemi-cal tools. The accumulated data provide a more precise picture of the various reactions that limit the electrochemical window of commonly used systems. 

In the following sections we present data that was published from the time of Mann s review to date, on the behavior of a variety of nonaqueous systems with the so-called nonactive electrodes defined in Section I. The chapter is divided into two major sections reduction and oxidation reactions. 

In reviewing the intrinsic electrochemical behavior of nonaqueous systems, it is important to describe reactions of the most common and unavoidable contaminants. Some contaminants may be introduced by the salts (e.g., HF in solutions of the MFX salts M = P, B, As, etc.). Other possible examples are alcohols, which can contaminate esters, ethers, or alkyl carbonates. We examined the possible effect of alcoholic contaminants such as CH3OH in MF and 1,2-propylenegly-col at concentrations of hundreds of ppm in PC solutions. It appears that the commonly used ester or alkyl carbonate solvents are sufficiently reactive (as described above), and so their intrinsic reactivity dominates the surface chemistry if the concentration of the alcoholic contaminant is at the ppm level. We have no similar comprehensive data for ethereal solutions. However, the most important contaminants that should be dealt with in this section, and which are common to all of these solutions, are the atmospheric ones that include 02, H20, and C02. The reduction of these species depends on the electrode material, the solvent used, and their concentration, although the cation plays the most important role. When the electrolyte is a tetraalkyl ammonium salt, the reduction products of H20, 02 or C02 are soluble. As expected, reduction of water produces OH and... 

In the case of C02 contamination, we have strong evidence that its reduction on noble metal electrodes in nonaqueous systems in the presence of Li ions (and the absence of water) forms Li2C03 and CO [17], Figures 20 and 21 show typical FTIR spectra obtained from noble metal electrodes polarized to low potentials in C02-saturated nonaqueous Li salt solutions and provide clear evidence for Li2C03 formation as the major surface species that is precipitated [15,39], The C02 reduction mechanism for the reaction appears in the literature [43] and is described in the following equations ... 

With TAA salts of small alkyl groups (e.g., ethyl, methyl), cation reduction is usually the limiting cathodic reaction. The anodic limiting reaction for ammonium ions is their oxidation to nitrogen and protons. It should be emphasized that atmospheric contaminants are supposed to influence the above cathodic and anodic limits of liquid ammonia, as they do for the other nonaqueous systems discussed in the previous sections. 

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