Magnesium electrodes

When the potential of the magnesium electrode is made more positive, the rate of Mg+ ion formation increases, and with it that of reaction (16.5). Therefore, the rate of hydrogen evolution increases instead of falling off, with increasing anodic polarization of the magnesium (see Section 13.7). This phenomenon has become known as the negative difference effect. 

On the other hand, it has been found that the electrochemical reduction is a very unique and useful tool in synthetic organic chemistry when magnesium is used as the material of the electrode. The cathodic reduction of 1,3-dienes with magnesium electrode gives very unique products, i.e. 3-cyclopentenol derivatives when it is carried out in the presence of a carboxylic acid ester (equation 23)17. 

This novel electroreductive cyclocoupling corresponds to a 1,4-addition of a one-carbon unit to the 1,3-diene, and does not take place without using magnesium electrode. The first step in this coupling reaction is the cathodic reduction of 1,3-diene to an anion radical, and the second step is the formation of a Mg-diene complex, which thereafter reacts with the ester to yield the coupling product as shown in equation 23b. 

The intermediary formation of the Mg-diene complex is confirmed by a two-step reaction method, namely in the first step a solution of 1,3-diene is electrochemically reduced with magnesium electrode in the absence of the ester. After a sufficient amount of electricity is passed, the current is terminated and the ester is added to the solution. The fact that the coupling product is also formed by this two-step method strongly supports the formation of the intermediate Mg-diene complex. 

Although styrene is not a 1,3-diene, the cathodic reduction of a solution containing styrene and an ester with magnesium electrode interestingly affords a single stereoisomer of 2-phenylcyclopropanol derivative in which the phenyl and the alkyl (R2) groups are stereoselectively located in a cis relationship on the cyclopropane ring (equation 25). 

The electrochemical reduction of azulene with carbon, platinum, lead or zinc cathode does not give any product, whereas that with magnesium electrode yields a dimeric compound as the only reduction product, though the dimeric compound is easily transformed to the corresponding monomeric compound by a mild oxidation as shown in equation 2825. 

Magnesium electrodes are also effective for the formation of Si-Ge bonds... 

Cathodic protection systems have started a number of hydrocarbon fires, either by direct current sparking to earth, or from thermite reactions causing sparks when rusty lumps of iron fall on magnesium electrodes. 

In a galvanic cell, one half-cell has a cadmium electrode in a 1 mol/L solution of cadmium nitrate. The other half-cell has a magnesium electrode in a 1 mol/L solution of magnesium nitrate. Write the shorthand representation. 

More recently Kashimura et al. [96] has investigated the effect of ultrasound on the electroreductive synthesis of polysilanes, polymer germanes and related polymers using magnesium electrodes. They found that the presence of ultrasound greatly facilitated the reactions. 

The synthesis of polyaniline and copolymers of aniline with o-nitroaniline is aimed at obtaining an electroactive material. This material can be used, for example, as an electrode in conjunction with the magnesium electrode to construct chemical power sources. The polymers were prepared by oxidation of aniline or its mixture with nitroaniline ammonium persulfate in aqueous hydrochloric... 

The rate of the reoxidation of Mg deposits is controlled by their morphology, which in turn depends on the substrate material. Smooth and compact deposits were obtained using silver or gold, but not nickel or copper. It was also established that the open circuit potential (OCP) of magnesium electrode (a fresh deposit on a Pt) in concentrated solutions of 5 depends strongly on the solvent used. In THE solutions with c around 1 M at 22 °C under argon atmosphere, the values of OCP for 5a, 5b and 5f were equal to —2.8, —2.73 and —2.77 V vs. Ag+/Ag, respectively-. ... 

The cathodic cyclocoupling reaction (equation 38) with a diene under similar conditions was also observed for 1-vinylcyclohexene (67a) and 1-vinylcycloheptene (67b), giving 68 as the final product with satisfactory yields. Thus, similar intermediate metallocycles can be expected for this reaction. On the other hand, for the cathodic coupling of styrenes the formation of a Mg compound was not proved, although a magnesium electrode was necessary in order to obtain a 2-phenylcyclopropanol-type product. 

A magnesium electrode is immersed into a IM solution of MgS04, and a nickel electrode is dipped into an unknown Ni solution. If the potential of the cell is 1.98 V, what will be the nickel ion concentration ... 

Polymerization by an electron supplied at the cathode in electrolysis was first suggested by Yang, McEwer and Kleinberg (18). They employed anhydrous pyridine, magnesium electrodes, sodium iodide and styrene monomer. Two samples of polystyrene of molecular weight 1800 and 1600 were formed the former was made in the usual manner and did not contain nitrogen, and the latter was obtained when styrene was... 

The alternating magnesium electrode system is also applicable to the Si—Ge92 or Si—Sn93 bond formation (equations 62-64). 

The most common active metal electrodes in nonaqueous systems are indeed the surface film controlled ones described above. However, there are some exceptional cases in which surface film free active electrodes in nonaqueous solutions can be obtained. Three types of such systems are blue solutions, aluminum nonaqueous-aprotic plating systems, and magnesium electrodes in Grignard reagent solutions. 

C. Magnesium Electrodes in Grignard Reagent Solutions (see also Section VI)... 

Magnesium electrodes are passivated in most of the commonly used polar aprotic solvents by surface films. The origin of these surface films may be from reactions of atmospheric components (02, H20) on the active metal surface before introduction into the solution or from a result of the reduction of solution species. 

In contrast to the case of lithium electrodes, where the surface films deposited on them are usually good Li+ ion conductors, the Mg2+ ion conductivity of the surface films formed on magnesium electrodes is very poor. 

In contrast to the case of lithium, where extensive efforts have been devoted to the study of interfacial phenomena, surface film composition, etc., very little is yet known about the composition and structure of the surface films covering magnesium electrodes in nonaqueous media. In addition, the mechanisms of Mg dissolution and plating in the Grignard solutions are unclear. 

Two emf studies (7, 8 ) on the potential of the magnesium electrode in ether solution give results in disagreement with the... 

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