Alkali metals electrochemical potential

Whereas the electrochemical decomposition of propylene carbonate (PC) on graphite electrodes at potentials between 1 and 0.8 V vs. Li/Li was already reported in 1970 [140], it took about four years to find out that this reaction is accompanied by a partially reversible electrochemical intercalation of solvated lithium ions, Li (solv)y, into the graphite host [64], In general, the intercalation of Li (and other alkali-metal) ions from electrolytes with organic donor solvents into fairly crystalline graphitic carbons quite often yields solvated (ternary) lithiated graphites, Li r(solv)yC 1 (Fig. 8) [7,24,26,65,66,141-146],... 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

Other coordination modes of trans-diammac have been identified where one (154) or both (155) primary amines are free from the metal.721 725 An extension of this concept involves attachment of active functional groups such as crown ethers selectively at one primary amine to generate ditopic ligands capable of electrochemically sensing alkali metal ions through their inductive effect on the Co11111 redox potential. One example is provided by (156) further, the 15-crown-5 and 18-crown-6 analogs were also prepared.726... 

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

Compare the chemical activity of the studied alkali metals. What does it depend on Why does lithium head the electrochemical series of the metals Find the values of the standard electrode potentials of the alkali metals (see Appendix 1, Table 21). 

Attempts to reduce anthracene with an alkali metal in acetonitrile causes solvent decomposition, whereas controlled-potential electrolysis produces stable anion radicals. Thus the working electrode of a coulometric cell can be considered as a continuously adjustable reagent, capable of producing a wide variety of radical species in diverse solvent systems. The versatility of electrochemical EPR methods is best illustrated by citing a few specific examples from the extensive literature. More complete compilations appear in the reviews listed in Appendix I, but the studies mentioned next provide some appreciation for the techniques. 

The number of solvents that have been used in SrnI reactions is somewhat limited in scope, but this causes no practical difficulties. Characteristics that are required of a solvent for use in SrnI reactions are that it should dissolve both the organic substrate and the ionic alkali metal salt (M+Nu ), not have hydrogen atoms that can be readily abstracted by aryl radicals (c/. equation 13), not have protons which can be ionized by the bases (e.g. Nth- or Bu O" ions), or the basic nucleophiles (Nu ) and radical ions (RX -or RNu- ) involved in the reaction, and not undergo electron transfer reactions with the various intermediates in the reaction. In addition to these characteristics, the solvent should not absorb significantly in the wavelength range normally used in photostimulated processes (300-400 nm), should not react with solvated electrons and/or alkali metals in reactions stimulated by these species, and should not undergo reduction at the potentials employed in electrochemically promoted reactions, but should be sufficiently polar to facilitate electron transfer processes. 

These studies of reduction of benzenoid aromatics reveal that the solvent, the electrolyte cation, the current density and the water content are all important variables. In general it is important to have a rather negative potential (large TAA+) and a proton source (water) present under conditions where hydrogen evolution or attack on the solvent does not occur. Under such conditions difunctional molecules can be selectively reduced by control over the number of Faradays/mole which are passed. This kind of predictable selectivity should give the electrochemical method real advantage over alkali metal reductions and the possibility to use materials other than liquid ammonia and alkali metal is quite attractive. 

Although a selection of ferrocene crown ethers (Scheme 4) were initially reported by Biernat and Wilczewski (44), Saji (45) described the first evidence of anodic shifts in the oxidation potential of pentaoxa[13]-ferrocenophane (6) resulting from the addition of alkali metal salts. Two distinct electrochemical CV waves corresponding to complexed and uncomplexed (6) were observed for both Na+ and Li+ guest cations (Fig. 4). The respective anodic shifts correspond to a decrease of the... 

The electrochemical properties of (40)-(47) in the presence and absence of stoichiometric amounts of Na+ and K+ guest cations were investigated in acetonitile solution by cyclic voltammetry. Table VI shows that addition of alkali metal salt in 1 1 molar ratio produces anodic shifts (AE) in the original redox couple of 40-320 mV in the reduction potentials of the respective host s molybdenum redox center. Comparing (45)-(47) with the organic redox-active quinone systems described earlier (see Table I), in the case of Na+ guest cation these AE... 

Electron-catalyzed (or electron-stimulated) processes constitute a relatively new class of reactions of great potential synthetic interest (Zelenin and Khidekel, 1970 Linck, 1971). Foremost among these ranks the SRN1 mechanism, which is an electron-initiated radical-chain mechanism of nucleophilic substitution (21-24 X- = halide ion) (for reviews, see Kornblum, 1975 Bunnett, 1978, 1982). The initiation step (21) can be performed photochemically, electrochemically, or by adding alkali metal (Pinson and Saveant, 1978 Amatore et al., 1979 van Tilborg et al., 1977, 1978 Saveant, 1980). 

Table 1 summarizes a few values of the dipole moment fx and of the electrosorption valency l of halide and alkali metal ions adsorbed on mercury. The experimental values of l are relative to low coverages near the potential of zero charge and are taken from Schultze and Koppitz,50 while the corresponding // values were calculated from Eq. (75). The theoretical values in the last column are from a hard sphere electrolyte model. Further data can be found in the article by Schmickler49 Note that, in the electrochemical environment, the dipole moments are much smaller than in vacuo, where they can reach values of the order of 7 D for the alkali metal ions. No doubt, this difference is caused by the screening of the adsorbate dipole by the solvent molecules. 

Intercalation reactions of the dichalcogenides with alkali metals are redox reactions in which the host lattice is reduced by electron transfer from the alkali metal. Lithium and sodium intercalation reactions, for example, have been studied using cells of the type Li/LiC104-dioxolane/MX2 andNa/Nal-propylene carbonate/MX2. The reactions proceed spontaneously to form the intercalation compound if the cell is short circuited alternatively, a reverse potential can be apphed to control the composition of the final product. Apart from their application in synthesis, such electrochemical cells can be used to obtain detailed thermodynamic information and to establish phase relations by measuring the dependence of the equilibrium cell voltage on composition (see Figure 4). 

Although these solvents should be electrochemically inactive over a wide potential range, this is not always the case since some solvents may be relatively easily reduced (e.g., nitrobenzene) or oxidized (e.g., amines). However, acetonitrile, which is frequently used in such studies, exhibits a very wide potential window from -t-3.4 to - 2.9 V vs. SCE [5] when tetrabutylammonium salts are used as background electrolytes. In acetonitrile and liquid SO2 as solvents, alkali metal cations could be oxidized [6] at an ultramicroelectrode, even to the divalent state. 

The most commonly used procedure is that established by Wooster and extensively developed by Birch, i.e. the reduction of a solution of the substrate in a mixture of liquid ammonia with an alcohol (usually ethanol or r-butyl alcohol) and an inert cosolvent (e.g. diethyl ether, tetrahydrofuran) with an alkali metal (lithium, sodium or potassium). Low molecular weight amines have been utilized in place of the ammonia, although the procedure then leads to more extensive reduction. Hexamethylphosphoramide may also serve in place of the ammonia, but there is no apparent advantage to offset its higher cost, toxicity and carcinogenicity. Of rather more interest is the potential of electrochemical and photochemical approaches, which may give complementary outcomes. 

The electrochemical potential for redox reaction controls the situation where atoms of one element are available to be sorbed by a zeolite containing exchangeable cations of another element. Within the zeolite and even in the absence of water, aqueous reduction potentials are usually capable of deciding whether reaction will occur, with an error due to the difference between the zeolitic environment and aqueous solution of no more than 0.1 (or perhaps 0.2) V. Accordingly there is no question that alkali-metal vapors will reduce transition-metal ions within a zeolite, and that vapors of zinc, mercury, or sulfur will not reduce the cations of the alkali or alkaline-earth metals. 

Obtaining solvated electrons by dissolving alkali metals and by electrochemical generation is of special interest. Back in the last century, the dissolution of alkali metals in liquid ammonia gave the very first evidence of obtaining solvated electrons. Electrochemical (cathodic) generation of solvated electrons is a process in which electrons are transferred from the electrode into solution under the action of high cathode potentials. 

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