Reduction of Cu

The formation of a red precipitate of copper(I) oxide by reduction of Cu(II) is taken as a positive test for an aldehyde Carbohydrates that give positive tests with Benedict s reagent are termed reducing sugars... 

Appendix 3D contains a listing of the standard-state reduction potentials for selected species. The more positive the standard-state reduction potential, the more favorable the reduction reaction will be under standard-state conditions. Thus, under standard-state conditions, the reduction of Cu + to Cu E° = -1-0.3419) is more favorable than the reduction of Zn + to Zn (E° = -0.7618). 

The potential needed for a quantitative reduction of Cu + can be calculated using the Nernst equation... 

If the initial concentration of Cu + is 1.00 X 10 M, for example, then the cathode s potential must be more negative than -1-0.105 V versus the SHE (-0.139 V versus the SCE) to achieve a quantitative reduction of Cu + to Cu. Note that at this potential H3O+ is not reduced to H2, maintaining a 100% current efficiency. Many of the published procedures for the controlled-potential coulometric analysis of Cu + call for potentials that are more negative than that shown for the reduction of H3O+ in Figure 11.21. Such potentials can be used, however, because the slow kinetics for reducing H3O+ results in a significant overpotential that shifts the potential of the H3O+/H2 redox couple to more negative potentials. 

This experiment describes the adaptation of the bicinchoninic acid (BCA) protein assay to a flow injection analysis. The assay is based on the reduction of Cu + to Cu+ by the protein, followed by the reaction of Cu+ with bicinchoninic acid to form a purple complex that absorbs at 562 nm. Directions are provided for the analysis of bovine serum albumin and rabbit immunoglobulin G, and suggestions are provided for additional analyses. 

During the operation of the cell (or during the direct interaction of zinc metal and cupric ions in a beaker) the zinc is oxidised to Zn and corrodes, and the Daniell cell has been widely used to illustrate the electrochemical mechanism of corrosion. This analogy between the Daniell cell and a corrosion cell is perhaps unfortunate, since it tends to create the impression that corrosion occurs only when two dissimilar metals are placed in contact and that the electrodes are always physically separable. Furthermore, although reduction of Cu (aq.) does occur in certain corrosion reactions it is of less importance than reduction of HjO ions or dissolved oxygen. 

The principal cathodic reaction on the upper surface of the membrane is the reduction of Cu " that is formed by the reaction of Cu with dissolved oxygen in the water these Cu ions are provided partly from the diffusion through the pores in the oxide membrane from within the pit and partly from those produced by cathodic reduction (equation 1.154). Lucey s theory thus rejects the conventional large cathode small anode relationship that is invoked to explain localised attack, and this concept of an electronically conducting membrane has also been used by Evans to explain localised attack on steel due to a discontinuous film of magnetite. 

The potentials of film-free a-brass and /3-brass in solutions comparable to those existing inside the alloy at the advancing front of attack were found to be —0-38V and —0-56V (v. S.H.E.), respectively. It was also established, taking into account the activities of copper ions in equilibrium with the sparingly soluble corrosion product CU2CI2, that whereas Cu ions can be reduced to copper at —0 -16 V the reduction of Cu ions is possible only at potentials more negative than —0-41 V. Thus whereas the /3-phase of an a/3-brass can reduce both Cu and Cu ions, the a-brass can reduce only the Cu ion. 

Of course, as time goes on the copper sulfate solution in the cell has to be replaced because it collects undesirable ions such as Fe+2, which have not been reduced because the voltage used is favorable to the reduction of Cu+2 only. 

The enantioselective 1,4-addition addition of organometaUic reagents to a,p-unsaturated carbonyl compounds, the so-called Michael reaction, provides a powerful method for the synthesis of optically active compounds by carbon-carbon bond formation [129]. Therefore, symmetrical and unsymmetrical MiniPHOS phosphines were used for in situ preparation of copper-catalysts, and employed in an optimization study on Cu(I)-catalyzed Michael reactions of di-ethylzinc to a, -unsaturated ketones (Scheme 31) [29,30]. In most cases, complete conversion and good enantioselectivity were obtained and no 1,2-addition product was detected, showing complete regioselectivity. Of interest, the enantioselectivity observed using Cu(I) directly in place of Cu(II) allowed enhanced enantioselectivity, implying that the chiral environment of the Cu(I) complex produced by in situ reduction of Cu(II) may be less selective than the one with preformed Cu(I). 

Maikov et al. [37] prepared a series of C2-symmetric bipyridine-type ligands, the chiral moieties arising from the isoprenoid chiral pool (/3-pinene, 3-carene, 2-carene, or a-pinene, for example). Some representative examples are drawn in Scheme 16 (see 25, 26, 27) and were used as copper ligands of a copper(I) species obtained by an in-situ reduction of Cu(OTf )2 with phenyl-hydrazine. The use of the resulting catalysts in enantioselective cyclopropana-tion proceeded with up to 76% ee (for ligand 27) and high diastereoselectivity (up to 99 1). 

A number of peptide hormones have a carboxyl terminal amide which is derived from a glycine terminal residue. This glycine is hydroxylated on the a-carbon by a copper-containing enzyme, peptidylglycine hydroxylase, which, again, requires ascorbate for reduction of Cu ". ... 

CuCl, especially in a single crystal form, is extensively used as an optical material for its special optical properties. Orel et al. [2] first proposed a new method to obtain CuCl particles by the reduction of Cu with ascorbic acid. Several dispersants were used in the reduction and monodispersed CuCl particles can be obtained by selecting the proper dispersant and reduction conditions. In this work, the above method was used to modify the traditional process of CuCl preparation, namely, by reducing the Cu " with sodium sulfite to obtain the highly active CuCl catalyst to be used in the direct process of methylchlorosilane synthesis. 

The rate of reduction of Cu(II) by Cr(II) has been measured in aqueous perchloric acid - . With Cu(II) in excess and in the absence of oxygen, the reaction corresponds to... 

The direct reduction of Cu(Il) acetate to Cu(I) by CO at high pressures (up to 1360 atm) in aqueous solution at 120 °C shows several kinetic paths, the rate... 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

Special attention was paid to the detection of residual Cu-fl quantities accompanying the metallic Cu. The relative amounts of Cu+1 and Cu were determined by curve-fitting the Cu (LMM) spectra using the Physical Electronics Version 6 curve-fitting program. The catalyst showed reduction of Cu+2 Into a mixture of Cu+1 and Cu after reduction In H2 at 250 C for one hour (Figure 6) as evidenced by the two resolved peaks In the Cu (LMM) spectrum at 568.0 and 570.3 eV which are characteristic of Cu and Cu+1, respectively, and by the disappearance of the Cu+2 2p satellite structure. It could be shown that less than 2%, If any, of the total Cu could be present In the +1 oxidation state during methanol formation. However, when the catalyst was briefly exposed to air (1 minute), a few percent of Cu+1 readily formed (7). Thus, any kind of oxidation environment has to be avoided between methanol synthesis and catalyst analysis. Otherwise, appreciable amounts of Cu+1 will be detected. 

Maximum temperatures and hydrogen consumption during the temperature programmed reduction of Cu/oxides by hydrogen, and NO taken up by Cu atom. 

In the case of zeolites (Table 4), a very broad reduction peak is observed on the sample partially exchanged, with a poorly defined maximum. On the other hand, two reduction peaks appear for Cu exchanges higher than 100%. In both cases, however, the consumption of hydrogen corresponds to the reduction of Cu + to metallic Cu. A similar situation was previously reported for Cu-MOR [6]. By... 

Reasonable NO conversion can be achieved using n-decane as reductant. In the absence of sulfur dioxide, the catalytic activity is roughly related to the r ucibility of the Cu phase of Cu ions in zeolites the reaction temperature needed to reach 20% NO conversion parallels that of the TPR peak (Table 7). This relation also practically holds for Cu on simple oxides, therefore a redox mechanism in which reduction of Cu + cations is the slow step could account for the results. 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

Initial one-electron oxidation of the diazo compound by Ag(I), Hg(II) or Cu(II) acetates may also be responsible for the formation of Ph2C(OAc)—C(OAc)Ph2 from diazodiphenylmethane and of EtOOCCH(OAc)—CH(OAc)COOEt from ethyl diazoacetate in DMF/H20 417). Direct evidence for reduction of Cu(II) triflate to the Cu(I) salt by alkyl diazoacetates has been furnished by the disappearance of the... 

Cu(II/I) redox couples often present unusual complexities arising from the dual features of high lability in both oxidation states and a preference for highly different structures in the two oxidation states. The structural differences arising from reduction of Cu(II) to Cu(I) can appear either as a loss of coordination number (e.g. from 5 to 4) or as a... 

Reports by Li and Zuberbuhler were in support of the formation of Cu(I) as an intermediate (16). It was confirmed that Cu(I) and Cu(II) show the same catalytic activity and the reaction is first-order in [Cu(I) or (II)] and [02] in the presence of 0.6-1.5M acetonitrile and above pH 2.2. The oxygen consumption deviated from the strictly first-order pattern at lower pH and the corresponding kinetic traces were excluded from the evaluation of the data. The rate law was found to be identical with the one obtained for the autoxidation of Cu(I) in the absence of Cu(II) under similar conditions (17). Thus, the proposed kinetic model is centered around the reduction of Cu(II) by ascorbic acid and reoxidation of Cu(I) to Cu(II) by dioxygen ... 

The autoxidation of 3,5-di-terf-butylcatechol (H2DTBC) was frequently used to test the catalytic activity of various metal complexes. Speier studied the reaction with [Cu(PY)Cl] (PY = pyridine) in CH2C12 and CHCI3, and reported second-, first- and zeroth-order dependence with respect to Cu(I), 02 and substrate concentrations, respectively (41). The results are consistent with a kinetic model in which the rate determining oxidation of Cu(I) is followed by fast reduction of Cu(II) by H2DTBC. 

Alternative kinetic models were considered for this reaction. Both of them predict rapid reduction of Cu(II) Cu(II) to Cu(I) Cu(I) by H2DTBC and subsequent formation of the Cu(II)(0 -)Cu(II) intermediate in the reaction of the reduced form with 02. The first model assumes that the rate determining formation of the intermediate is followed by a fast, acid assisted dissociation into the oxidized form of the catalyst and H202. In the other model, the rate determining step is the oxidation of H2DTBC by the intermediate. The two models predict... 

In the conjugate addition of diethylzinc to enones catalyzed by copper reagent CuOTf or Cu(OTf)2 in the presence of 90, an ee of over 60% has been obtained. Study also shows that the actual catalyst in the reaction may be a Cu(I) species formed via in situ reduction of Cu(II) complexes. 

The nitric oxide reduction of Cu(dmp)2(H20)2+ in aqueous media gives a Cu(II)-NO complex via an inner-sphere mechanism [216] (dmp = 2,9-dimethyl-l,10-phen-... 

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