Alkali metal amalgams

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 ... 

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. 

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.

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 These experimental conditions have been replaced by the use of a solution of the carboxylic acid in methanol partially neutralised by sodium methoxide or trieth-...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...

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 ... 

Sodium-naphthalene reduction of organotrineopentoxyphosphonium salts led to the instantaneous loss of phosphonium ion phosphonates and phosphites were obtained748 (reaction 224). Alkali metal amalgams are efficient reagents for the reductive cleavage of both achiral and optically active phosphonium salts configuration is retained750 (Table 23). 

On the other hand, application of alkali metal amalgam permits the slowing down of the reaction of metals with alcohols, which is used in the industrial production of alkali metals alkoxides. Production of NaOR by Mathieson Alkali Works is based, for instance, on the reaction of sodium amalgam (formed as a result of the electrolysis of aqueous NaCl solution with the mercury cathode) with alcohol NaOR ROH is isolated from the solutions. Na residue in the amalgam is hydrolyzed, the obtained mixture is returned to the electrolyz-... 

Special success has followed the use of alkali metal amalgams in tetra-hydrofuran or other ethers (VII, 11, 44) as reducing agents in the syntheses of mono- and polynuclear metal carbonyls, e.g.,... 

Various attempts have been made to circumvent these problems and to eliminate junction potentials, including (1) extrapolation procedures designed to eliminate the difference between the compositions of the two solutions in the appropriate limit, (2) separation of the two solutions by means of a doublejunction salt bridge, (3) the use of double cells with dilute alkali metal amalgam connectors, and (4) the use of glass or other types of ion-specific electrodes as bridging reference electrodes. 

Electrochemical reductions in inorganic industry are limited to the large scale manufacture of alkali metal amalgams from which hydroxides, sodium sulphide, hydrosulphite etc. are produced. On the other hand in small scale production electrochemical reductions are frequently applied to the preparation of compounds which are difficult to prepare in a chemical way (metal salts with lower valence, e. g. Ti, Mo, V, U and Cr). 

The decomposition of alkali metal amalgams by water proceeds very slowly owing to the high hydrogen overvoltage on mercury. The rate of this chemical decomposition increases with temperature which reduces the hydrogen overvoltage, but this is still insufficient for technical practice. 

Yb 2 --r2 7 Reduction by alkali metal amalgam, FeCl3 added [96]... 

Amalgam Cells.—If the electrolyte in the concentration cell without transference is a salt of an alkali metal, e.g., potassium chloride, it is necessary to set up some form of reversible alkali metal electrode. This is achieved by dissolving the metal in mercury, thus forming a dilute alkali metal amalgam which is attacked much less vigorously by water than is the metal in the pure state." The amalgam nevertheless reactia with water to some extent, and also with traces of oxygen that may be... 

Metals such as Na or alkali metal amalgams can also be used in the cleavage of the C—P" bond. In the latter case, reductive cleavage of achiral and optically active quaternary phosphonium salts succeeds in high yields with retention of configuration. ... 

Another example of this type of reaction is the hydrodimerization of dialkylfulvenes to dicyclopentadienyl alkanes by alkali metal amalgams in an aqueous-alcoholic medium [49,50]. 

Reduction of an aromatic acid chloride by Li(Hg) or another alkali metal amalgam in diethyl ether or THF yields the enediol diester (XVI) [113] further reduction by Li(Hg) yields diarylacetylene (XVII) [114] ... 

Carbon dioxide may be reduced by alkali metal amalgams in alcohol to alkali metal formate [115] in a yield of 85-90% the rate increases with the CO2 pressure and is dependent on the alkali metal, Na(Hg) < Li(Hg) < K(Hg). 

In other cases a wide variety of products, ranging from simple organic acids such as formic and oxalic acid to totally reduced CO2 in the form of carbon black, have been obtained by reduction of CO2 by an appropriate alkali metal amalgam [117]. 

The reductive cleavage of achiral and optically active quaternary phosphonium and arsonium salts with alkali metal amalgams to form tertiary phosphines and arsines succeeds in high yield with retention of configuration [124]. The reduction with the amalgams was found to give better yields than the conventional cathodic cleavage. 

Alkyl arylsufonates and diaryl sulfones are cleaved by alkali metal amalgams to the corresponding sulfinate salts. Some sterically hindered arylsulfonates, however, undergo cleavage at the ring-S bond [128]. 

If a mercury cathode is expected to be necessary, the aprotic solvent-alkali-metal salt system appears to be inconvenient since many compounds are cathodically cleaved, reduced, or/and deprotected at potentials beyond that of alkali-metal amalgam formation. nevertheless, in certain cases the use of lithium salts as an electrolyte possessing strong electrophilic properties appears necessary in order to avoid the possibility of a Hofmann degradation of the tetraalkylammonium ion by electrogenerated bases. Under such experimental conditions, the cathodic synthesis of some aza and aza-oxa ligands [31] has been successfully achieved from the corresponding and readily obtained poly-... 

The generic process for electrochemical synthesis of sp-carbon chains was electrochemical reductive carbonization (corrosion) of poly(tetrafluoro-ethylene) (PTFE) by alkali metal amalgams, pioneered by Jansta and dousek [6 9] (for review see Reference 3). The reaction occurs at the interface of a dry contact between PTFE and alkali metal amalgams, hence, it does not seem to recall an electrochemical synthesis in its classical sense. The purely electrochemical carbonization of PTFE on a Pt electrode in aprotic electrolyte solution is also possible [3], but the amalgam-driven process is superior, presenting a clean and well-defined alternative to classical (wet) electrochemistry. 

The theoretical treatment presented (Eqs 4.1-4.5) is applicable also for direct wet electrochemistry on Pt cathode in aprotic electrolyte solution [12,13] (Table 4.1) and for some other chemical reductants, Rj, viz. benzoin dianion [14] and sodium dihydronaphthylide [15] (Table 4.1). Apparently, the decision between chemical and electrochemical carbonization may not be straightforward. The latter scenario requires a compact solid electrolyte with mixed electron/ion conductivity to be present at the interface. This occurs almost ideally in the reactions of solid fluoropolymers with diluted alkali metal amalgams [3]. If the interfacial layer is mechanically cracked, both electrochemical and chemical carbonization may take place, and the actual kinetics deviates from that predicted by Eq. 4.4 [10]. There is, however, another mechanism, leading to the perturbations of the Jansta and Dousek s electrochemical model (Eq. 4.4). This situation typically occurs if gaseous perfluorinated precursors react with Li-amalgam [4,5], and it will be theoretically treated in the next section. 

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