Reduction reaction alkyl carbonate solutions

The most expected reaction of alkyl carbonate solvents with Li (or on non-active electrodes polarized to low potentials in the presence of Li-ions) is their two-electron reduction to Li2C03 and alkanes or alkenes as by-products (e.g., PC + 2e + 2Li -> Li2C03 + CH3CH=CH2). However, FTIR spectra of Li or noble metal electrodes treated in alkyl carbonate solutions show a different picture. ... 

Ion batteries. In the commonly used LiPFe-alkyl carbonate solutions, the surface films formed on the an( es may be dominated by solvent reduction, while the surface films formed on the cathodes may be dominated by LiF formation due to reactions of I MOy with HF. Thereby, the overall impedance of Li-ion batteries may be determined mostly by the cathode s surface resistance (due to its coverage by highly resistive LiF films). Upon cycling, the impedance of both anodes and cathodes may change ... 

Triorganyl-sulfonium, -selenonium and -telluronium salts are reduced by carbon dioxide radical anions/solvated electrons produced in aqueous solution by radiolysis. The radical expulsion accompanying reduction occurred with the expected leaving group propensities, i.e. benzyl > secondary alkyl > primary alkyl > methyl > phenyl. Much higher product yields in the reduction of selenonium and telluronium compounds have been accounted for in terms of a chain reaction with carbon-centred radicals, with formate serving as the chain transfer agent.282... 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

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... 

Li reacts readily with N2 to form Li3N. We have had an indication that other Li-N compounds of different stoichiometry can also be formed [15,18], However, these reactions only take place in an 02-free atmosphere. Hence, in a usual glove box atmosphere from which 02 is continuously removed, while N2 may exist in a concentration of hundreds of ppm, freshly prepared Li surfaces may always be covered by Li3N film. This film has a typical absorption in the IR, appearing as a broad peak around 680 cm1 [18]. Li3N is a strong base and nucleophile which can further react with alkyl carbonate and ester solvents [18]. In solutions, the reaction between Li and N2 is much less important, due to competition with other reduction processes (of solution species). 

The choice of organic solvents permits the occurrence of certain reduction reactions of alkyl halides which, if attempted in aqueous solutions, would rapidly be inhibited by the precipitation of insoluble products on the electrode. Under mildly cathodic conditions, the formation of metal alkyls occurs in high yields when alkyl halides are reduced at cathodes of low boiling metals in acetonitrile, DMF or propylene carbonate containing quaternary ammonium supporting electrolytes. " Whilst the formation of lead tetraethyl from ethyl bromide in DMF proceeds irrespective of the substitution of a sodium salt for the quaternary ammonium salt, the same change of electrolyte in propylene carbonate no longer leads to metal alkyl products. ... 

In this chapter we deal with four major electrode surfaces active metals, carbons, non-active metals (e.g., noble metals), and composite electrodes comprising lithiated transition metal oxide powders as the active mass, plus polymeric binder and conductive additives (usually carbon black or graphite powders at low percentage). In terms of general surface chemistry, we find that the surface reactions on lithium, lithiated carbons, carbon, and noble metals polarized to low potentials in non-aqueous Li salt solutions are very similar. All of these electrodes are covered by surface films comprising insoluble Li salts, which are formed by reduction of solution species. Upon anodic polarization of carbon or noble metal electrodes in non-aqueous solutions, solution species are oxidized. Here, the impact of the cations is negligible. It seems that the species that determine the anodic stability of non-aqueous solutions are the solvents. For instance, ether may be oxidized at potentials below 4 V, while alkyl carbonates may apparently be stable up to 5 V (Li/Li ). However, it should be noted that some minor oxidation reactions of alkyl carbonate solvents on noble metal electrodes (e.g., Pt, Au) can be detected even at a potential below 4 V. The... 

Active metals (Li, Mg, Ca, etc.) react spontaneously with the main atmospheric gases (N2, O2, H2O, CO2) and with most relevant polar aprotic solvents and salt anions. All active metals are covered initially by native surface films formed during their production by their reaction with atmospheric gases. It should be noted that even a usual glove box atmosphere that officially contains less than 1 ppm of H2O and O2 (but may contain hundreds of ppm of CO2 and N2) should be considered as reactive towards lithium or magnesium surfaces prepared freshly in the glove box. Active metals are usually covered by bilayer surface films. The inner layer is comprised of metal oxide, while the outer layer contains mostly carbonates and hydroxides. When an active metal is introduced into a polar aprotic electrolyte solution, several processes take place in parallel. These include dissolution of part of the initial surface species, nucleophilic reactions between metal oxide and hydroxide and electrophilic solvents such as esters and alkyl carbonates, and diffusion of solvent molecules towards the active metal-native film interference and their reduction by the active metal. 

The intensive spectral studies of Li and noble metal electrodes in these solutions converged to the reduction paths of alkyl carbonate solvents, and then-secondary reactions (due to the presence of contaminants) are presented in Scheme 4. ... 

To these sets of primary and secondary reactions related to solvents, one has to add the eontributions of salt anion reduction, which usually forms metal halides and M AXy species (A is the main high oxidation-state element in the salt anion and X is a halide, such as chloride or fluoride). Most of the produets of aetive metal surface reactions are ionic compounds that are insoluble in the mother solution, and therefore, precipitate as surface films. It should be added to this picture that possible polymeric species can be formed, espeeially in alkyl carbonate solvents, whose reduction forms polymerizable species sueh as ethylene or propylene. Hence, the surface films formed on active metal electrodes are very complicated. They have a multilayer structure perpendicular to the metal surface, and a lateral, mosaic-type composition and morphology (i.e. containing mixtures and islands of different compounds and grains). Such a structure may induce very non-uniform current distribution upon metal deposition or dissolution processes, which leads to dendrite formation, a breakdown of the surface films, etc. These situations are demonstrated in Fig. 13.6 active metal dissolution leads to the break-and-repair of the surface films, thus forming mosaic-type structures. 

In a synthesis of minocycline, interesting use was made of a reductive alkylation of a nitro function, accompanied by loss of a diazonium group. The sequence provides a clever way of utilizing the unwanted 9-nitro isomer that arises from nitration of 6-demethyl-6-deoxytetracycline (//). When di-azotization was complete, urea and 40% aqueous formaldehyde were added, and the entire solution was mixed with 10% palladium-on-carbon and reduced under hydrogen. No further use of this combined reaction seems to have been made. 

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