Nonaqueous solvents, oxidation

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

Metal organic decomposition (MOD) is a synthesis technique in which metal-containing organic chemicals react with water in a nonaqueous solvent to produce a metal hydroxide or hydrous oxide, or in special cases, an anhydrous metal oxide (7). MOD techniques can also be used to prepare nonoxide powders (8,9). Powders may require calcination to obtain the desired phase. A major advantage of the MOD method is the control over purity and stoichiometry that can be achieved. Two limitations are atmosphere control (if required) and expense of the chemicals. However, the cost of metal organic chemicals is decreasing with greater use of MOD techniques. 

A recently discovered (2) oxidizing system promises to become very important for the oxidation of acid-sensitive compounds. The reagent is chromium trioxide-pyridine complex, which may be isolated after preparation and employed in nonaqueous solvents (usually methylene chloride). A remarkable feature of the reagent is that good yields of aldehydes are obtained by direct oxidation of primary alcohols. The preparation of the reagent and its use are given. 

Most 2,5-unsubstituted pyrroles and thiophenes, and most anilines can be polymerized by electrochemical oxidation. For pyrroles, acetonitrile,54 or aqueous55 electrolyte solutions are normally used, while the polymerization of thiophenes is performed almost exclusively in nonaqueous solvents such as acetonitrile, propylene carbonate, and benzonitrile. 0 Polyanilines are generally prepared from a solution of aniline in aqueous acid.21 Platinum or carbon electrodes have been used in most work, although indium-tin oxide is routinely used for spectroelectrochemical experiments, and many other electrode materials have also been employed.20,21... 

Stable solutions of radon difluoride can be prepared in nonaqueous solvents, such as halogen fluorides and hydrogen fluoride (Stein, 1969, 1970). Radon reacts spontaneously at 25°C or at lower temperatures with each of the halogen fluorides except IF3. It also reacts with mixed solvent-oxidant pairs, such as HF-BrF, HF-BrF, and IF -BrF, and solutions of NiF in HF. 

Sulfuric acid undergoes many other reactions with metals as well as with metal oxides, carbonates, nitrates, sulfides, and so on. It is a versatile nonaqueous solvent. 

Oxidized Fe-TAML could be the iron(V)oxo complex 6, which as noted above can be produced from la and m-chloroperox-ybenzoic acid at low temperatures (—60°C) in a nonaqueous solvent (51). Presumably such an iron(V)oxo complex can behave in a substrate-dependent way as both a two-electron or one-electron oxidant. In the former case, it is returned in one step to the iron(III) state. In the latter, it must first pass through an iron(IV) intermediate. At pH>12, the likely iron(IV) species would be the same compound as is formed from la and H202, (48) namely the iron(IV)-oxo complex 7, which has similar features with [(H20)sFeIV = 0]2 +, (54) or its water adduct 7". At other pHs, other iron(IV) compounds are known to be formed (48). Both the iron(V)-oxo and iron(IV)-oxo complexes as well as the other iron(IV) species could be involved in catalysis by 1 (see Section V.B). The possible involvement of complexes that are in a higher oxidation state than 6 cannot be ruled out. 

The versatility of permanganate as an oxidant has been greatly enhanced in the past decade by the observation that it can be solubilized in nonaqueous solvents with the aid of phase transfer agents (1). The literature contains descriptions for the use of this procedure for the oxidation of alkenes (2-13), alkynes (13-18), aldehydes (19), alcohols (20), phenols (21,22), ethers (23), sulfides (24,25), and amines (20,26). The dehydrogenation of triazolines has also been achieved by the use of permanganate and a phase transfer agent (27). ... 

From observations of widely different products in nonaqueous solvents, it is concluded that higher valence iron complexes are primary oxidants (as in biochemical oxidations... 

Because of the small ionic radius of lithium ion, most simple salts of lithium fail to meet the minimum solubility requirement in low dielectric media. Examples are halides, LiX (where X = Cl and F), or the oxides Li20. Although solubility in nonaqueous solvents would increase if the anion is replaced by a so-called soft Lewis base such as Br , I , S , or carboxylates (R—C02 ), the improvement is usually realized at the expense of the anodic stability of the salt because these anions are readily oxidized on the charged surfaces of cathode materials at <4.0 V vs Li. 

The attempt to use these salts originated from the hope that their dissociation constants would be high even in low dielectric media, and the organic nature of perfluorinated alkyls would always assist the solubility of the salts in nonaqueous solvents. Because of the requirement for electrochemical stability, lithium carboxylates (RF-C02Li, where Rp- = perfluorinated alkyls) are excluded from consideration, because their oxidation still occurs at - 3.5 V vs lithium, which is similar to the cases of their non-fluorinated counterparts. Obviously, the electron-withdrawing groups do not stabilize the carboxylate anions sufficiently to alter their oxidative stability. 

In some nonaqueous solvents, it is necessary to use a standard reaction other than the oxidation of molecular hydrogen. At present, there is no general choice of a standard reaction (reference electrode). Although in some cases... 

It is possible to observe the reversible one-electron oxidation of cobalt(II) salen to cobalt(III) salen in a nonaqueous solvent such as acetonitrile [137] ... 

Electrochemical oxidation of cadmium in a solution of ](4-methylphenyl)sulfonyl]-2-pyridylamine] (HL) in acetonitrile/di chloromethane mixtures resulted in CdL2 complex formation [149]. The electrochemical oxidation of cadmium amalgam in nonaqueous solvents CH2CI2, 1,2-C2H4CI2, and PC was also used for the preparation of cadmium complexes with 18-membered macromonocyclic ligands, 18-06, 18-S6, I8-N2O4, and 18-N6 [150]. The stoichiometry and stability of resulted complexes were determined. The same method was used to examine the complexation of Cd(II) cation with 12-crown-4 ether, azacrown ether 1,4,8,11-tetra-azacyclotetradecane, and thiaazacrown... 

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