Reduction potentials mercury

Electrode reduction potentials of mercury are given in Table 4. 

In order to understand the formation and stability of mercury(I) compounds it is helpful to consider the relevant reduction potentials ... 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

In the course of the synthesis and the study of several group 14 metalloles (silacyclopentadienes and germacyclopentadienes) their reduction potentials were recorded (vs SCE), with 0.1 M TBAP in dimethoxyethane on DME by polarography and by hanging mercury electrode voltammetry176. In (r 4-gcrmacyclopcnladicnc)C.o(C.O)212... 

Reduction potentials also depend greatly on the nature of the cathode material as shown in Table 2 [8], For example, primary perfluoroalkyl iodides are ca. 0.3 V easier to reduce at mercury than at platinum due to the strong interaction of Rfl with mercury. In fact, cathodic reduction of Rfl at a mercury cathode provides RfHgl [9]. 

The direct reduction of haloalkynes using either mercury or vitreous carbon as the cathode has been examined in considerable detail [80-84] one example is portrayed in Eq (77). The influence of reduction potential, current consumption, proton donor, electrode, and substrate concentration on the course of the process has been examined. Vitreous carbon electrodes are preferred, though mercury has been used in many instances. Unfortunately, these reactions suffer from the formation of diorganomercurials. While both alkyl iodides and bromides can be used, the former is generally preferred. Because of their higher reduction potential, alkyl chlorides react via a different mechanism, one involving isomerization to an allene followed by cyclization [83]. 

The photoreaction of polysilanes with Ceo has also been investigated [35]. Reaction (8.16) shows an example in which the irradiation in benzene with a low-pressure mercury-arc lamp afforded a product that contains 14wt% of Ceo into the polysilane chain. The incorporation of Ceo into the polysilane backbone has not been observed upon irradiation with X > 300 nm, when the cleavage of Si—Si bond does not take place. The adduct obtained from Reaction (8.16) has a lower oxidation potential than C6o( + 0-77 vs + 1.21 V) and a lower reduction potential than polysilane (—1.24 vs — 2 V). 

Formation of the dihydrocoumarin by reduction of 4-methylcoumarin in the presence of alkaloids in aqueous methanol pH 5-6, mercury cathode potential -1.8 V vs. see. Ref [137],... 

Benzyl radicals have a reduction potential only slightly more negative than the reduction potential of the corresponding benzyl bromide. At a mercury cathode in... 

Polarographic reductions of (21) are easier than those of ketones (138) and (139). The size of the bridging ring influences both the absorption of the ketone on the dropping mercury electrode (139 > 138) and the reduction potential (70MI52200). 

Of the Group 12 elements, zinc, cadmium and mercury, only Hg has a water-stable -I-1 state, and all three elements have + 2 states that are water-stable. Their reduction potentials are summarized in the Latimer diagram ... 

Gold(III) was identified as the most active catalyst for that process in 1985, when Hutchings recognized that the efficiency in catalyzing the hydrochlorination of ethyne to vinyl chloride (a very important industrial process that previously used mercury salts as catalysts) correlated with the standard reduction potential of the supported metal cation. That meant that the metal could be found as a transient species in the reaction [10]. 

C. E. Ophardt, "Redox Demonstrations and Descriptive Chemistry Part I. Metals/ ]. Chem. Educ., Vol. 64,1987, 716. Redox reactions of iron(III) with thiosulfate, iron(II) with permanganate, and tin(II) with mercury(I) are used to show how an abbreviated table of standard reduction potentials is used to predict the products of these reactions from the relative positions of the oxidizing agents and reducing agents in the table. 

Nitrobenzyl halides are reduced in a 1 e-process to radical anions, which rapidly lose halide ion to form the neutral nitrobenzyl radical. The rates for this dissociation were calculated from cyclic voltammetry data to be k = 2,5 for m-nitrobenzyl chloride and k = 2 109 secfor o-nitrobenzyl bromide. The nitrobenzyl radicals predominantly dimerize (90%), whereas a small amount yields nitrotoluenes (< 10%) by hydrogen abstraction 4541. From a series of substituted benzyl bromides those with the more positive reduction potential form bibenzyl in 25—74% yield, whereas from the less easily reducible ones dibenzyl-mercury derivatives are obtained (50—60%) 485). Reduction of benzyl chloride at the plateau of the first wave yields dibenzylmercury 4 By reduction of diphenyliodonium hydroxide at -1,6 V 51% diphenylmercury is obtained 488 ... 

In both polarographic and preparative electrochemistry in aptotic solvents the custom is to use tetraalkylammonium salts as supporting electrolytes. In such solvent-supporting electrolyte systems electrochemical reductions at a mercury cathode can be performed at —2.5 to —2.9 V versus SCE. The reduction potential ultimately is limited by the reduction of the quaternary ammonium cation to form an amalgam, (/ 4N )Hg , n = 12-13. The tetra-n-butyl salts are more difficult to reduce than are the tetraethylammonium salts and are preferred when the maximum cathodic range is needed. On the anodic side the oxidation of mercury occurs at about +0.4 V versus SCE in a supporting electrolyte that does not complex or form a precipitate with the Hg(I) or Hg(II) ions that are formed. 

Polarographic studies on pyrazine and methylpyrazines indicate that 1 4-dihydropyrazines are the products of reduction. The reduction of pyrazine itself at the dropping mercury electrode proceeds reversibly. The substitution of methyl groups makes the reduction more difficult with an increased number of methyl groups an increased tendency toward irreversible reduction is noted.102-104 The half-wave reduction potentials for pyrazine, methylpyrazine, 2,6-dimethyl-pyrazine, and tetramethylpyrazine are 2.17, 2.23, 2.28, and 2.50 eV, respectively. Pyrazine is thus more easily reduced than pyridine which has a half-wave potential of 2.76 eV, and less easily reduced than quinoxaline which has a half-wave potential of 1.80 eV.105... 

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