Overall zinc/copper

Let us continue with the example of copper ions in contact with copper metal and zinc ions in contact with zinc metal. This combination is usually referred to as the Darnell cell or zinc/copper couple(Fig. 6.5a). For this electrochemical cell the reduction and oxidation processes responsible for the overall reaction are separated in space one half reaction taking place in one electrode compartment and the other takes place in the other compartment. 

In the chart of standard electrode potentials (Table 12-1), reactions are arranged in order of their tendency to occur. Reactions with a positive EMF occur more readily than those with a negative EMF. The zinc-copper cell has an overall EMF of + 1.10 volts, so the solution of zinc and deposition of copper can proceed. 

Three syntheses of 1,2-dicyanocyclobutene (2) have been previously described. The first, involving dehydration of cyclobutene-1,2-dicarboxamide does not specify the yield.9 The second procedure involves a concomitant chlorination and catalytic dehydrochlorination of 1,2-dicyanocyclobutane in the gas phase, yielding 1,2-dicyanocyclobutene (2) in a mixture of several other products.10 The third method consists of dechlorination of 1,2-dichloro-1,2-dicyanocyclobutane using metals, such as zinc copper couple,11 Raney nickel,11 and, especially, Raney cobalt.2 In comparison with this last-mentioned synthesis, the overall yield of the present procedure is 5-10% higher. Furthermore, the reaction is performed in less time and utilizes considerably cheaper reagents. 

The bifunctional cyclopropane 156 was also prepared by modified Simmons-Smith cyclopropanation 85) of 2-trimethylsilyl-2-propen-l-ol 157 84) followed by oxidation of the cyclopropylcarbinol 158 with activated manganese dioxide 88 >, in 72% overall yield, Eq. (50) 86,89). Coupling of the aldehyde 156 with 2,6-dimethylcyclohexenone 159 90) induced by the low valent titanium reagent from TiCl3 and zinc-copper couple (or lithium metal) provided the silylated cyclopropyldiene 160, in 50-60% yield, Eq. (51) 89 91>. 

The chloro nitroso adducts of the ( -protected glycals were converted, via the intermediate oximes, to 2-amino-2-deoxy sugars48,50,53,59 the overall oxyamination of glycals is hence accomplished. Amines of the opposite configuration are prepared depending on the method of reduction. The chloro nitroso adducts can be directly treated with zinc-copper couple in acetic acid, or can be first converted to fully acetylated oximes which are then catalytically hydrogenated. 

An HDS additive, zinc/copper hydroxy stearate, was melt-blended with low density poly(ethylene). X-ray diffraction analysis of the composite materials was similar to that found with copper hydroxy dodecyl sulfate combined with poly(vinyl ester), where nanocomposite formation was observed, but additional work is necessary for full characterization of the dispersion. The (nano) composites were found to have better thermal stability via TGA and improvement in PHRR in cone calorimetry. However, smoke production was observed to increase. The 5% loading had better overall performance than 10% in terms of thermal stability and most fire properties. 

Cross-coupling between functionalized zinc-copper reagents and l-iodoalkynes or l-bromoalkynes is very fast [83]. This smooth cross-coupling occurs at low temperatures (—55 "C) and offers hi stereoselectivity in reactions with chiral secondary organozinc-copper reagents such as 108 (obtained by a hydroboration/ boron-zinc exchange sequence), producing the alhyne 109 in 42% overall yield (Scheme 2.40) [73]. 

Amino acid synthons can be prepared from iodoalanine with no loss of optical integrity (Scheme 48). The amino acid was transformed into a novel zinc reagent through reductive metallation with a zinc-copper couple in benzene/dimethyl acetamide. This organometallic was acylated under palladium catalysis in good overall yield. ... 

Left) A strip of zinc was placed in a blue solution of copper(II) sulfate, CUSO4. The copper has been displaced from solution and has fallen to the bottom of the beaker. The resulting zinc sulfate solution is colorless. This is the same overall reaction as the one that occurs when the two half-reactions are separated in the zinc-copper cell (see Figure 21 -6). (Right) No reaction occurs when copper wire is placed in a colorless zinc sulfate solution. The reaction... 

In order to carry out most biochemical reactions, metalloenzymes generally utilize the rarer transition metal ions. Elements such as zinc, copper, iron, nickel, and cobalt are found in low concentrations in plasma and seawater and yet the enzyme has to select the appropriate metal ion from them. There is evidence for the existence of proteins that can chaperone specific metal ions to their appropriate sites in apoenzymes, protecting the metal ions from adverse reactions as they are guided to their required location [5]. How does the enzyme attempt to select out the one metal ion it requires The answer is that the chemistry of the metal ion is used as a basis for selection. Each metal ion has some property that is different from that of most others, but, in fact, there is often considerable overlap in these properties so that a given enzyme may bind one of several different cations in one specific site. Some relevant data are provided in Tables 1 and 2. The metalloenzyme contains within its overall design an arrangement of preferred side-chain functional groups with the correct size hole to bind the required metal ions in an appropriate hydrophobic or hydrophilic environment. Thus the metalloenzyme binds metal ions... 

Calculating gn from fhalt-cell The overall redox reaction for the zinc-copper cell is the sum of its half-reactions ... 

Naphthenates of cobalt, manganese, calcium, copper, iron, zinc, and zirconium are used as driers in printing inks. Their use in coatings is declining as a result of the use of higher metal content synthetic driers and the overall trend to latex paint in architectural coatings. 

Dezincification Dezincification is corrosion of a brass alloy containing zinc in which the principal product of corrosion is metallic copper. This may occur as plugs rilling pits (plug type) or as continuous layers surrounding an unattacked core of brass (general type). The mechanism may involve overall corrosion of the alloy followed by redeposition of the copper from the corrosion products or selective corrosion of zinc or a high-zinc phase to leave copper residue. This form of corrosion is commonly encountered in brasses that contain more than 15 percent zinc and can be either eliminated or reduced by the addition ox small amounts of arsenic, antimony, or ph osphorus to the alloy. 

A salt bridge serves as an ionconducting connection between the two half-cells. When the external circuit is closed, the oxidation reaction starts with the dissolution of the zinc electrode and the formation of zinc ions in half-cell I. In half-cell II copper ions are reduced and metallic copper is deposited. The sulfate ions remain unchanged in the aqueous solution. The overall cell reaction consists of an electron transfer between zinc and copper ions ... 

For the most part, iron, copper, nickel, and zinc deposits found in the boiler section are the result of corrosion processes taking place somewhere else within the overall system. Similarly, mill scale from new boilers may be released and redeposited elsewhere, plugging BD lines and boiler tubes. 

Metallothioneins are a group of small proteins (about 6.5 kDa), found in the cytosol of cells, particularly of liver, kidney, and intestine. They have a high content of cysteine and can bind copper, zinc, cadmium, and mercury. The SH groups of cysteine are involved in binding the metals. Acute intake (eg, by injection) of copper and of certain other metals increases the amount (induction) of these proteins in tissues, as does administration of certain hormones or cytokines. These proteins may function to store the above metals in a nontoxic form and are involved in their overall metaboHsm in the body. Sequestration of copper also diminishes the amount of this metal available to generate free radicals. 

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