Copper pure, activity

Hydrolysis of chlorobenzene and the influence of silica gel catalysts on this reaction have been studied by Freidlin and co-workers (109). Pure silica gel gave up to 45% phenol from chlorobenzene at 600°C. When the silica gel was promoted with 2% cupric chloride, up to 75% phenol was obtained (381). A number of other salts were tested by Freidlin and co-workers as promoters, but they exerted an adverse effect on the activity or selectivity of the catalyst. With 0.2% cupric chloride and 6% metallic copper, the activity of silica-gel was doubled (389). At 500° under the above conditions, the halides were hydrolyzed at rates decreasing in the following order chloride, bromide, iodide, fluoride (110). The specific activation of aryl halides by cupric chloride was demonstrated by conversion of chlorobenzene to benzene and of naphthyl chloride to naphthalene when this catalyst was supported on oxides of titanium or tin (111). The silica promoted with cupric chloride was also found to be suitable for hydrolysis of chlorophenols and dichlorobenzenes however, side reactions were too prominent in these cases (112). 

The experiment outlined above demonstrates that like metallic copper, pure zinc oxide is a very poor methanol catalyst at 75 atm and 250°C. This finding contrasts with the relatively high activity of zinc oxide at pressures exceeding 200 atm and temperatures above 350°C. 

Treatment with zinc/copper couple activated by ultrasound in dimethylacetamide gave the zinc homoenolate 57 stabilised by chelation to either the ester or the carbamate. Acylation with acid chlorides and Pd(0) catalysis gave the enantiomerically pure y-oxo-amino acids 58 in good yield. 

Electrorefining. Electrolytic refining is a purification process in which an impure metal anode is dissolved electrochemicaHy in a solution of a salt of the metal to be refined, and then recovered as a pure cathodic deposit. Electrorefining is a more efficient purification process than other chemical methods because of its selectivity. In particular, for metals such as copper, silver, gold, and lead, which exhibit Htfle irreversibHity, the operating electrode potential is close to the reversible potential, and a sharp separation can be accompHshed, both at the anode where more noble metals do not dissolve and at the cathode where more active metals do not deposit. 

Examples of this procedure for dilute solutions of copper, silicon and aluminium shows the widely different behaviour of these elements. The vapour pressures of the pure metals are 1.14 x 10, 8.63 x 10 and 1.51 x 10 amios at 1873 K, and the activity coefficients in solution in liquid iron are 8.0, 7 X 10 and 3 X 10 respectively. There are therefore two elements of relatively high and similar vapour pressures, Cu and Al, and two elements of approximately equal activity coefficients but widely differing vapour pressures. Si and Al. The right-hand side of the depletion equation has the values 1.89, 1.88 X 10- , and 1.44 X 10 respectively, and we may conclude that there will be depletion of copper only, widr insignificant evaporation of silicon and aluminium. The data for the boundaty layer were taken as 5 x lO cm s for the diffusion coefficient, and 10 cm for the boundary layer thickness in liquid iron. 

The addition of cathodically active elements to pure lead was the main objective of investigations to improve its corrosion resistance to H2SO4 [42,44]. Best known is copper-lead with 0.04 to 0.08% Cu. By adding combinations of alloying elements, it was possible to produce lead alloys that not only had much better corrosion resistance, but also had greater high-temperature strength. Lead alloy with 0.1% Sn, 0.1% Cu and 0.1% Pd is an example [45]. 

Meyers has also reported the use of chiral oxazolines in asymmetric copper-catalyzed Ullmann coupling reactions. For example, treatment of bromooxazoline 50 with activated copper powder in refluxing DMF afforded binaphthyl oxazoline 51 as a 93 7 mixture of atropisomers diastereomerically pure material was obtained in 57% yield after a single recrystallization. Reductive cleavage of the oxazoline groups as described above afforded diol 52 in 88% yield. This methodology has also been applied to the synthesis of biaryl derivatives. 

Especially in the early steps of the synthesis of a complex molecule, there are plenty of examples in which epoxides are allowed to react with organometallic reagents. In particular, treatment of enantiomerically pure terminal epoxides with alkyl-, alkenyl-, or aryl-Grignard reagents in the presence of catalytic amounts of a copper salt, corresponding cuprates, or metal acetylides via alanate chemistry, provides a general route to optically active substituted alcohols useful as valuable building blocks in complex syntheses. 

In the previous sections we have dealt mainly with the catalytic activity of pure substances such as metallic iron, ruthenium, copper, platinum, etc. Real catalyst, however, are often much more complex materials that have been optimized by adding remote amounts of other elements that may have a profound impact on the overall reactivity or selectivity of the catalyst. Here we shall deal with a few prominent examples of such effects. 

If one considers the electrodeposit to be of pure silver and pure copper, then the activities of silver and copper can both be taken as unity. Therefore, the co-deposition condition can be expressed as... 

Electrodes responding to other halides, sulphide, cyanide, silver, lead, copper and cadmium are made using membranes fabricated from pure or doped silver sulphide (Ag2S). The membrane potential is affected by the movement of Ag+ ions between cationic lattice sites which in turn is determined by the activities of the Ag+ ion in the internal and sample solutions. As the activity of the former is fixed, that of the latter alone influences the membrane potential. The electrode will also respond to the presence of S2- ions because of their effect on the Ag+ ion activity via the solubility product expression ... 

The activity of a solid The activity of a pure solid in its standard state is unity, so the activity of pure copper or of zinc metal electrodes is one. We write this as a(cU) or a,zn, = 1. 

Consider the Daniell cell Zn Zn2+ Cu2+ Cu. The cell emf is about 1.1 V when prepared with clean, pure electrodes and both solutions at unit activity. The emf decreases to about 1.05 V after adding lithium chloride to the copper half-cell. Adding more LiCl, but this time to the zinc solution, increases the emf slightly, to about 1.08 V. 

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