Copper!II reduction

Copper(II) reduction Methods based on the ability of certain sugars, including all monosaccharides, to reduce copper(II) salts to copper(I) are recommended for measuring sugars in vegetable products. In the USA the Munson-Walker method as detailed by the AO AC is most commonly used, whereas in Europe the modified Lane-Eynon titration method is standard. 

Sugars A wide range of methods is available for the determination of sugars in fruit and fruit products. These include relatively simple refractometry and hydrometry techniques, polarimetry, copper(II) reduction, enzymatic/color detection, and various chromatographic techniques. These vary greatly in their sensitivity, specificity, and in the level of equipment and expertise required, and these considerations generally dictate the method chosen. 

Copperil) oxide, CujO, occurs naturally as the red cuprite. It is obtained as an orange-yellow precipitate by the reduction of a copper(II) salt in alkaline solution by a mild reducing agent, for example glucose, hydroxylamine or sodium sulphite ... 

By the reduction of copper(II) chloride or a mixed solution of copper(II) sulphate and common salt by sulphur dioxide. 

The desired pyridylamine was obtained in 69 % overall yield by monomethylation of 2-(aminomethyl)pyridine following a literature procedure (Scheme 4.14). First amine 4.48 was converted into formamide 4.49, through reaction with the in situ prepared mixed anhydride of acetic acid and formic acid. Reduction of 4.49 with borane dimethyl sulfide complex produced diamine 4.50. This compound could be used successfully in the Mannich reaction with 4.39, affording crude 4.51 in 92 % yield (Scheme 4.15). Analogous to 4.44, 4.51 also coordinates to copper(II) in water, as indicated by a shift of the UV-absorption maximum from 296 nm to 308 nm. 

Also the arene-arene interactions, as encountered in Chapter 3, are partly due to hydrophobic effects, which can be ranked among enforced hydrophobic interactions. Simultaneous coordination of an aromatic oc amino acid ligand and the dienophile to the central copper(II) ion offers the possibility of a reduction of the number of water molecules involved in hydrophobic hydration, leading to a strengthening of the arene-arene interaction. Hence, hydrophobic effects can have a beneficial influence on the enantioselectivity of organic reactions. This effect is anticipated to extend well beyond the Diels-Alder reaction. 

The first equation is an example of hydrolysis and is commonly referred to as chemical precipitation. The separation is effective because of the differences in solubiUty products of the copper(II) and iron(III) hydroxides. The second equation is known as reductive precipitation and is an example of an electrochemical reaction. The use of more electropositive metals to effect reductive precipitation is known as cementation. Precipitation is used to separate impurities from a metal in solution such as iron from copper (eq. 1), or it can be used to remove the primary metal, copper, from solution (eq. 2). Precipitation is commonly practiced for the separation of small quantities of metals from large volumes of water, such as from industrial waste processes. 

Electroplating. Aluminum can be electroplated by the electrolytic reduction of cryoHte, which is trisodium aluminum hexafluoride [13775-53-6] Na AlE, containing alumina. Brass (see COPPERALLOYS) can be electroplated from aqueous cyanide solutions which contain cyano complexes of zinc(II) and copper(I). The soft CN stabilizes the copper as copper(I) and the two cyano complexes have comparable potentials. Without CN the potentials of aqueous zinc(II) and copper(I), as weU as those of zinc(II) and copper(II), are over one volt apart thus only the copper plates out. Careful control of concentration and pH also enables brass to be deposited from solutions of citrate and tartrate. The noble metals are often plated from solutions in which coordination compounds help provide fine, even deposits (see Electroplating). 

Copper compounds, which represent only a small percentage of ah copper production, play key roles ia both iadustry and the biosphere. Copper [7440-50.8] mol wt = 63.546, [Ar]3/°4.t is a member of the first transition series and much of its chemistry is associated with the copper(II) ion [15158-11-9] [Ar]3i5. Copper forms compounds of commercial iaterest ia the +1 and +2 oxidation states. The standard reduction potentials, for the reasonably attainable valence states of copper are... 

The tri- or tetraamine complex of copper(I), prepared by reduction of the copper(II) tetraamine complex with copper metal, is quite stable ia the absence of air. If the solution is acidified with a noncomplexiag acid, the formation of copper metal, and copper(II) ion, is immediate. If hydrochloric acid is used for the neutralization of the ammonia, the iasoluble cuprous chloride [7758-89-6], CuCl, is precipitated initially, followed by formation of the soluble ions [CuClj, [CuCl, and [CuCl as acid is iacreased ia the system. 

Irradiation of 3,5-disubstituted isoxazoles in alcoholic solvents gave reaction products such as acetals incorporating the reaction solvent. The use of triethylamine in acetonitrile media produced ketene-aminals by reductive ring cleavage. The reductive ring cleavage product was also obtained by irradiation of the isoxazole in alcohol in the presence of copper(II) salts (Scheme 3) (76JCS(P1)783). 

The copper(II) transport rate increases, as a rule, as Cu + initial concentration in the feed solution increases. The increase of the caiiier s concentration from 10 to 30 vol.% results in a decrease of both metal fluxes and in an increase of Cu transport selectivity. The increase of TOA concentration in the liquid membrane up to 0.1 M leads to a reduction of the copper(II) flux, and the platinum(IV) flux increases at > 0.2 M. Composition of the strip solution (HCl, H,SO, HNO, HCIO, H,0)does not exert significant influence on the transport of extracted components through the liquid membranes at electrodialysis. 

This synthesis is only one example of a wide range of reactions which involve aryl (or alkyl) radical addition to electron-deficient double bonds resulting in reduction.The corresponding oxidative reaction using aryl radicals is the well known Meerwein reaction, which uses copper(II) salts. 

Sometimes the metal may be transformed into a different oxidation state thus copper(II) may be reduced in acid solution by hydroxylamine or ascorbic acid. After rendering ammoniacal, nickel or cobalt can be titrated using, for example, murexide as indicator without interference from the copper, which is now present as Cu(I). Iron(III) can often be similarly masked by reduction with ascorbic acid. 

Discussion. Copper(II) ions are quantitatively reduced in 2M hydrochloric acid solution by means of the silver reductor (Section 10.140) to the copper(I) state. The solution, after reduction, is collected in a solution of ammonium iron(III) sulphate, and the Fe2+ ion formed is titrated with standard cerium(IV) sulphate solution using ferroin or AT-phenylanthranilic acid as indicator. 

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

Reductions of 5//-dibenz[/j,/]azepines to their 10,11-dihydro derivatives have been accomplished in high yield with sodium in ethanol,29 133 with copper(II) chromite (2CuO Cr203) and barium carbonate,224 with 5 % palladium on charcoal29 or platinum(IV) oxide30 in ethanol, and with magnesium in methanol.225 4//-Thieno[3,2-/)][1]benzazepine is reduced similarly with hydrogen and palladium on charcoal in ethanol.137... 

The oxidation number of zinc changes from 0 to +2 (an oxidation), whereas that of copper decreases from +2 to 0 (a reduction). Therefore, because zinc is oxidized, zinc metal is the reducing agent in this reaction. Conversely, because copper is reduced, copper(II) ions are the oxidizing agent. 

The effect of copper(II) is essentially similar to that of iron(Ill). In reaction (48) copper(I) is formed which results either indirectly by reaction (49) or directly by reaction (50) in the reduction of a peroxydisulphate ion... 

In the absence of oxygen the addition of arsenic(Iir) has only a slight effect on the rate of reduction of peroxydisulphate. In the presence of air the rate of reduction of persulphate increases nearly fortyfold (Table 12). The oxidation of arsenic(III) by SO4 from reaction (62) is not a chain process, thus it need not be considered in the iron(III)- and copper(II)-catalyzed reaction between peroxydisulphate and arsenic(III). 

As the concentration of copper(II) is increased the steady-state concentration of copper(I) is also increased, and the SO4 radical reacts with copper(I) rather than with arsenic(IV). In such a case the rate of reduction of peroxydisulphate, taking /c5o[Cu(I)]//c 6i > 1, can be given by... 

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... 

It should be mentioned that there is no decrease in the peroxydisulphate current in the presence of both iron(ril) and arsenic(III). Presumably, the iron(II)-peroxydisulphate reaction is too slow to compete with the reduction of peroxydisulphate at the DME at the given concentration. However, iron(III) reduces the kinetic current in the presence of copper(II) and arsenic(ril). This can be accounted for by the termination reactions... 

---