Metals amalgams

Metal acetylides Metal-air cells Metal alcoholates Metal alkoxides Metal alloys Metal amalgams... 

The speed of the reaction depends both on the metal and on the alcohol, increasing as electropositivity iacreases and decreasiag with length and branching of the chain. Thus sodium reacts strongly with ethanol, but slowly with tertiary butyl alcohol. The reaction with alkaU metals is sometimes carried out ia ether, ben2ene, or xylene. Some processes use the metal amalgam or hydride iastead of the free metal. Alkaline earth metals and aluminum are often covered with an oxide film which hinders the reaction. 

Metal Amalgams and Hydrides. Metal hydrides and amalgams are sometimes the preferred method of reducing various functional groups in the laboratory, especially when the necessary equipment for catalytic hydrogenations is unavailable. However, these reagents are usually too expensive to make their use on a large commercial scale feasible. 

Neta.1 Ama.lga.ms. Alkali metal amalgams function in a manner similar to a mercury cathode in an electrochemical reaction (63). However, it is more difficult to control the reducing power of an amalgam. In the reduction of nitro compounds with an NH4(Hg) amalgam, a variety of products are possible. Aliphatic nitro compounds are reduced to the hydroxylamines, whereas aromatic nitro compounds can give amino, hydra2o, a2o, or a2oxy compounds. 

Alternatively, BH3.thf can be reduced by alkali metal amalgams (M = K, Rb, Cs) to give good yields of solvent-free products ... 

The most important method for reduction of compounds to an oxidation state suitable for titration with one of the common oxidising titrants is based upon the use of metal amalgams, but there are various other methods which can be used, and these will be discussed in the following sections. 

Although as already stated the use of metal amalgams, and in particular use of the Jones reductor or of the related silver reductor, is the best method of reducing solutions in preparation for titration with an oxidant, it may happen that for occasional use there is no Jones reductor available, and a simpler procedure will commend itself. In practical terms, the need is most likely to arise in connection with the determination of iron, for which the reduction of iron(III) to iron(II) may be necessary. 

Perhaps of more significance is a detailed study132 into the reductive desulphonylation of 7-methyl-7-phenylsulphonylestratrienes. The goal was stereoselective removal of the sulphonyl group, and hydride reductions, alkali-metal-amalgam reductions and electrochemical reductions were explored. The latter proved to be the most effective and the best results are illustrated in Scheme 3. 

Liquid alloys of Hg with a variety of metals (amalgams) constitute particularly complex systems in view of the potential dependence of surface composition. A detailed study of In and T1 amalgams, with... 

For a metal amalgam electrode one cannot simplify the equation so that... 

In agreement with the theory of electrolysis, treated in Sections 3.1 and 3.2, the parts of the residual current and the limiting current are clearly shown by the nature of the polarographic waves because for the cathodic reduction of Cd2+ and Zn2+ at the dme we have to deal with rapid electron transfer and limited diffusion of the cations from the solution towards the electrode surface and of the metal amalgam formed thereon towards the inside of the Hg drop, we may conclude that the half-wave potential, Eh, is constant [cf., Fig. 3.13 (a ] and agrees with the redox potential of the amalgam, i.e., -0.3521V for Cd2+ + 2e - Cd(Hg) and -0.7628 V for Zn2+ + 2e -> Zn(Hg) (ref. 10). The Nernst equation is... 

Electrodes of the first kind. These include cationic electrodes (metal, amalgam and, of the gas electrodes, the hydrogen electrode), at which equilibrium is established between atoms or molecules of the substance and the corresponding cations in solution (see Eqs 3.1.21 and 3.1.65), and anionic electrodes, at which equilibrium is established between molecules and anions. 

Electrochemical corrosion processes also include a number of processes in organic chemistry, involving the reduction of various compounds by metals or metal amalgams. A typical example is the electrochemical carbonization of fluoropolymers mentioned on p. 316. These processes, that are often described as purely chemical reductions, can be explained relatively easily on the basis of diagrams of the anodic and cathodic polarization curves of the type shown in Fig. 5.54. 

Very few references are available on the determination of the rate constant for each step of electron charge transfer in the reaction M2+ + 2e -> M(s), i.e., M2+ + e -> M+, M+ + c" -> M(s). Earlier studies are mostly related to two-electron charge transfer reactions either at M2+/Hg(dme), M2+/metal amalgam, or redox couple/Pt interfaces. Even in these studies, the kinetic parameters have been determined assuming that one of the two steps of the reaction is much slower and is in overall control of the rate of reaction in both... 

Metal alkyl cocatalysts, 20 153 Metal-alkylidene catalysts, 26 948 Metal-alkylidene complexes, 26 927 discovery of, 26 925 Metal amalgams, amines by reduction, 2 492 93... 

Fig. 2.4-1. A schematic view of the gradual change of the density of states (DOS) with increasing mercury contents (from left to right) for selected alkali metal amalgams. The participation of the respective valence states is marked approximately.

Metal amalgams may be used for reduction of the keto groups in keto esters provided the medium does not cause hydrolysis of the ester. Because of that aluminum amalgam in ether is preferable to sodium amalgam in aqueous solutions. Diethyl oxalacetate was reduced to diethyl malate by sodium amalgam in 50% yield and with aluminum amalgam in 80% yield [148], Stereospecific reduction of a- and fi-keto esters to optically pure hydroxy esters was achieved by biochemical reduction in moderate to good yields. Saccharomyces cerevisiae converted methyl 2-keto-2-phenylacetate to methyl... 

The Kolbe reaction is earned out in an undivided cell with closely spaced platinum electrodes. Early examples used a concentrated, up to 50 %, aqueous solution of an alkali metal salt of the carboxylic acid and the solution became strongly alkaline due to hydrogen evolution at the cathode. Ingenious cells were devised with a renewing mercury cathode, which allowed removal of alkali metal amalgam. These experimental conditions have been replaced by the use of a solution of the carboxylic acid in methanol partially neutralised by sodium methoxide or trieth-... 

NIST) [1, 2]. These values are in close agreement with those reported in the classic work of Latimer [18]. The second column of standard potentials shows values measured by Lewis et al. [12]. These measurements, which were performed using dilute alkali metal amalgams that were stable enough to be used in aqueous solutions, have long served as the benchmarks for subsequent refined measurements. The final column shows more refined measurements of standard potentials for alkali metal couples in water [5-8]. 

A number of detailed thermodynamic comparisons of half-cells containing alkali metal and alkali metal amalgams are available. For example, Cogley and Butler examined cell potentials as a function of amalgam concentration for the cell shown below [22]. 

Tab. 6 Standard potentials (in V versus SHE) and free energies of amalgamation (in kj mol ) for alkali metal amalgams and their monovalent cations in water...

Although the properties of metal amalgams and intermetallic compounds in mercury have been investigating for many years and are already well recognized, the nature of ammonium amalgam, as well as quaternary ammonium amalgams, still remains a subject of controversy. 

The other fulminates are of no practical value. They are prepared from mercury fulminate either by reacting the metal amalgam with a suspension of mercury fulminate in water (this is applicable to the majority of metals, including the alkali metals, or simply by the action of the metal itself (e.g. zinc or thallium) which displaces mercury from mercury fulminate (also in water). For example, chips of thallium, zinc, or copper are allowed to stand for some time in a suspension of mercury fulminate in water, the corresponding metal fulminate is gradually formed. 

Data on the standard potentials for inorganic redox systems in aqueous solutions have been compiled by IUPAC [1], The standard potentials for some M"+/M and Mn+/M(Hg) couples are shown in Table 4.1 [2]. For alkali metals, the standard potentials of M+/M(Hg) are about IV more positive than those of M+/M. This is because alkali metals have strong affinities to mercury and are stable in the amalgams. It is impossible to measure the potentials of alkali metal electrodes directly in aqueous solutions, because alkali metals react with water. In order to determine the potential of an alkali metal electrode in an aqueous solution, we measure the potential of the corresponding amalgam electrode in an aqueous solution and then the difference between the potentials of alkali metal and alkali metal amalgam electrodes using an appropriate non-aqueous solution [2].2 ... 

By running a potentiometric precipitation titration, we can determine both the compositions of the precipitate and its solubility product. Various cation- and anion-selective electrodes as well as metal (or metal amalgam) electrodes work as indicator electrodes. For example, Coetzee and Martin [23] determined the solubility products of metal fluorides in AN, using a fluoride ion-selective LaF3 single-crystal membrane electrode. Nakamura et al. [2] also determined the solubility product of sodium fluoride in AN and PC, using a fluoride ion-sensitive polymer membrane electrode, which was prepared by chemically bonding the phthalocyanin cobalt complex to polyacrylamide (PAA). The polymer membrane electrode was durable and responded in Nernstian ways to F and CN in solvents like AN and PC. 

Many potentiometric studies have been carried out to obtain information about the compositions and formation constants of metal complexes in non-aqueous solutions. Ion-selective electrodes and metal (or metal amalgam) electrodes are used as indicator electrodes. 

Various potentiometric indicator electrodes work as sensors for ion solvation. Metal and metal amalgam electrodes, in principle, respond in a thermodynamic way to the solvation energy of the relevant metal ions. Some ion-selective electrodes can also respond almost thermodynamically to the solvation energies of the ions to which they are sensitive. Thus, the main difficulty in the potentiometric study of ion solvation arises from having to compare the potentials in different solvents, even though there is no thermodynamic way of doing it. In order to overcome this difficulty, we have to employ a method based on an extra-thermodynamic assumption. For example, we can use (1) or (2) below ... 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

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