Cupric ion catalysis

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

Bawn and Margerison have observed cupric ion catalysis of the persul-phate-DPPH reaction although the effect is smaller than that with Ag. The kinetics are complex and few details are given. 

The reaction between the monothionate radical and silver ions should occur readily in solution without catalysis, and the silver atoms formed in this way could condense to form new catalyst nuclei. The cupric ion catalysis of the reduction of silver ions by sulfite can be explained in a similar way, the monothionate radical being formed by the reaction... 

An interesting enhancement of the cupric ion catalysis by manganous ion has been reported by Bobtelsky (97). It was found that in neutral solution with 0.05 N manganous sulfate the rate of decomposition increases with increase of copper sulfate but passes through a maximum at about 0.05 N. Here the rate is about ten times that of copper sulfate alone. Zinc and cadmium sulfates are also reported to enhance the Cu++-Mn++ catalysis. 

Addition of cupric ion " greatly increases the relative amount of 4-equivalent oxidation, both by catalysis of (74) and by reducing the back-reaction in step (71). Analysis of the effect of Cu(II) confirms the importance of this reversibility" . 

The following relative second-order rate constants have been obtained for hydroxide ion-catalysed hydrolysis glycine ethyl ester, 1 protonated glycine ethyl ester, 41 and the cupric ion complex of glycine ethyl ester, F3 x 10 (Conley and Martin, 1965). The large effect of the cupric ion cannot be due entirely to electrostatic effects, but rather to catalysis by direct co-ordination with the ester function. 

A reactive intermediate may be responsible for the copper catalysis of the hydroxylamine reaction. The intermediate formed in the silver-catalyzed reaction, if it has any real existence, is not further oxidized but breaks down into nitrogen and water. Oxidation of hydroxylamine by cupric ion, on the other hand, yields predominately nitrous oxide. The intermediate formed by the removal of a single electron from the hydroxylamine in this reaction must be further oxidized to yield the final product. Such an intermediate may react readily with silver ions in solution. 

In addition to the hydrolysis of monoesters of phosphoric acid, the hydrolysis of diesters of phosphoric acid is also susceptible to metal ion catalysis, in particular by multivalent cations such as barium, stannous, and cupric ions. The diesters which undergo metal ion-catalyzed hydrolyses include open-chain diesters and cyclic diesters containing both five- and six-membered rings (54). 

Most reactions in two-phase systems occur in a liquid phase following the transfer of a reactant across an interface these are commonly known as extractive reactions. If the transfer is facilitated by a catalyst, it is known as phase-transfer catalysis [2]. Unusually, reactions may actually occur at an interface (interfacial reactions) examples include solvolysis and nucleophilic substitution reactions of aliphatic acid chlorides [3 ] and the extraction of cupric ion from aqueous solution using oxime ligands insoluble in water [4], see Section 5.2.1.3(ii). 

Catalysis of these reactions has been reported . At pH 3.5, cupric ion is said to catalyse the process (1) and ferric ion, the process (2). The latter appears to be third order in chlorite. Nickel and cobalt salts had less selective action. The first-order reactions reported by Ishi involve rate coefficients comparable to those above and thus the orders may be wrong. The maximum yield of CIO2 near pH 2, which was found by Buser and Hanisch , has been confirmed . A low order (1.69) in chlorate was found with acetate buffers and may be explained by the above mechanism. Further studies in this pH region are required, but it is likely that the process initiated by (11) is predominant, and produces CIO2 more rapidly than either (3) or (10) . 

Later work showed this mechanism to be incorrect. Wiberg showed that 804 when present in the reaction mixture does not give labelled peroxodi-sulphate, as required by the reversible first step of Levitt and Malinowski s mechanism. Furthermore, allyl acetate inhibits the reaction and reduces the rate of consumption of peroxodisulphate to that observed in the absence of 2-propanol. Wiberg proposed a chain mechanism involving sulphate and hydroxyl radicals. In a thorough study of the reaction, Ball et aV showed that all previous studies were complicated by the catalytic effects of trace amounts of metal ions (most likely cupric ions) and inhibition by dissolved oxygen from the atmosphere. In the absence of oxygen there is no catalysis by cupric ions, and the rate equation is... 

The effect of cupric ion on the ferrous catalysis is also shown in the work of Spitalsky and Konovalova (51) but was not recognized as such. 

An interesting observation which must be accounted for in any theory of the ferric ion catalysis was made by Bohnson and Robertson, who found that the catalytic decomposition by ferric salts is considerably enhanced if cupric salts are present (51,63,64). The resulting rate is much higher than that expected from the sum of the individual ferric and cupric ion rates. For a constant ferric ion concentration the enhanced rate at first increases with added cupric ion but ultimately reaches a limit beyond which further cupric ion has little effect. Bohnson and Robertson assumed that in this promoted catalysis the rate was first order in peroxide concentration as had been found for the ferric ion alone, but there is as yet no experimental evidence for this. Analysis of their numerical data (43,62) shows that when the maximum promotion by cupric ion has been reached the rate of peroxide decomposition is proportional to [Fe+++]H/[H+]. 

The accelerating effect of cupric ions on the ferric ion catalysis which was observed by Bohnson and Robertson is considered by Barb el al. to be due to reaction (5 ), as was the analogous effect of cupric salts on the ferrous ion catalysis. For conditions in which Scheme A applies reaction (4 ) is in effect catalyzed by (5 ). At high cupric ion concentrations (5 ) will eliminate (3), since effectively all the radical HO2 will react in (5 ). In these conditions the enhancement reaches a limit as was observed by Bohnson and Robertson. However (1) now becomes the operative chain-terminating step, and hence the kinetics of Scheme B should apply (Eq. g). Unfortunately no data on the peroxide dependence is available, but analysis of the data (43,66) shows the rate to be proportional to [Fe+++]w as required by (g). There is the same discrepancy in hydrogen ion dependence as was found in the simple ferric reaction. 

Kiss and Lederer (93) have made the only significant quantitative study of the decomposition of hydrogen peroxide by cupric ions alone and this is not very extensive. The reaction is much slower than the ferric ion catalysis at the same concentrations and acidities. For three... 

Hydroxyperoxy radicals can induce both oxidation and reduction. If the inhibitor is present in two states, oxidized and reduced, and each state reacts with hydroxyperoxy radicals only, terminating the chains, then negative catalysis will take place, each inhibitor molecule terminating chains an infinite number of times. This is the case on addition of CuS04 to cyclohexanol [83]. Cupric ions in a concentration of 10-smolel I virtually stop the initiated oxidation of cyclohexanol. The mechanism of the retarding action of cupric ions is... 

Breslow et al. 19) have investigated the mechanism of the divalent metal ion catalyzed hydration of 2-cyano-l,10-phenanthroline to the corresponding amide [see Eq. (7) below]. The cupric ion catalyzed reaction is extremely rapid lyz < 10 sec at 25°, pH 6—7). The Ni(II) and Zn(II) reactions, although slower, are greatly accelerated [the Ni(II) reaction by a factor of 10 ] in comparison to the rate of hydration observed in the absence of divalent metal ion catalysis. The reaction obeys a rate law which is second-order over all first-order with respect to hydroxide ion concentration, and first-order with respect to the 1 1 metal ion-substrate complex concentration. These authors suggest that the metal ion acts as a Lewis (general) add in activating the nitrile for external nucleophilic attack by hydroxide ion, as illustrated in Eq. (7) ... 

Schiff base formation between pyridoxal phosphate and amino acids are the basis for most enzymatic transformations of amino acids including transamination, decarboxylation, and racemization. Schiff bases formed between amino acids and pyridoxal phosphate or other heteroaromatic or aromatic aldehydes are, however, not only transformed enzymatically, but can, without enzymatic catalysis, undergo a large number of reactions, although at lower rate and/or higher temperatures than those for the corresponding enzymatic reactions. The enzymatic reactions require metal ions as cofactors and in analogy the nonenzymatic reaction are also catalyzed by metal ions, most effectively by cupric ions. 

AAButadiene Dimerization. The production of vinylcyclohexene is of a significant industrial importance as it is a route to styrene manufacture via the dehydrogenation of VCH. Cu zeolites, particularly Cu(I) Y zeolites proved to be active, selective and stable. They prevented the formation of cycloocta 1,5 diene and divinyl cyclobutane and also the formation of trimers. MAXWELL et al. (47) have studied the deactivation of this cuprous catalyst and found that the lower the acidity, the longer the lifetime. Consequently they devised a preparation procedure that involved reduction of cupric ions with NH3 at moderate temperature so that the Bronsted acidity generated by the reduction of cupric ions is neutralized by ammonia. As the temperature is increased little NH3 reamined as NH4 ions. These observations confirm that indeed this is not a Bronsted acid catalysis and confirm the involvement of the cuprous ions. 

HrabdrovdE., ValachovdK., Juranek ., Soltes L. Free-radical degradation of high-molar-mass hyaluronan induced by ascorbate plus cupric ions. Anti-oxidative properties of the PieSt any-spa curative waters from healing peloid and maturation pool. In Kinetics, Catalysis and Mechanism of Chemical Reactions G. E. Zaikov (eds). Nova Science Publishers,New York, 2011 pp. 29-36. 

There are several observations which can be derived from these data, and which are relevant to the general problem of the relationship between structure and catalysis in ma-cromolecular systems. For example, the fact that PLL-Cu(II) is a more active catalyst than Cu(rt-butylamine)4 at high pH, was ascribed to the fact that PLL-Cu(II) can maintain cupric ions in solution at a high pH, whereas the catalytic activity of the low molecular... 

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