Competitive complex formation reactions

J. Wang, B.S. Grabaric, Application of adsorptive stripping voltammetry for indirect measurement of nonelectroactive ions using competitive complex formation reactions, Mikrochim. Acta 100 (1990) 31-40. 

Those donor atoms and/or groups are examined as donor centers in the problem of competitive coordination on which, due to the molecular structure of the ligand, the most favorable conditions are created for electrophilic attack by the metal, after taking into account the acceptor properties of the metal and the conditions of complex-formation reactions [19]. Such donor centers are mostly elements of a few main subgroups belonging to Groups V and VI of the Periodic Table, and also the unsaturated, aromatic, and heteroaromatic compounds which form the fundamentals of modern ligands (Chap. 2). 

The reaction control should be emphasized amongst the conditions of reactions of competitive complex formation [19,23], It is necessary to take into account that it is possible to determine, and frequently predict, the direction of the electrophilic attack to the donor center of di- and polyfunctional donors (ligands) only in the case when the thermodynamically stable products are formed under conditions of kinetic control. Thus, the thermodynamic stability of complexes is discussed, when the bond between the metal and di- and polydentate ligands is localized in the place of primary attack on one of any of the donor centers by the electrophilic reagent, without further change of coordination mode in the reaction of complex formation. 

The reactivity of biotin with several reagents can be exploited in some chemical assays like its reaction with diazo derivatives, or the reaction of the ureido ring with p-dimethylaminocinnamaldehyde in an acidic medium, but the sensitivity of these assays is relatively poor. Several methods described for biotin assay involve a competitive complex formation between biotin and avidin bound with a chromophore/ fluorophore probe. In these assays, biotin, which has a higher affinity for avidin, quantitatively displaces the probe from the complex. 

An indirect adsorptive stripping voltammetry (AdSV) method on a mercury drop working electrode has been used for determination of cyanide ions and HCN [51]. The method is based on competitive Cu complex formation reaction between adenine at the electrode surface and CN ions in solution. In this method, cyanide changed the cathodic adsorptive stripping peak height of Cu-adenine. A linear decrease of the peak current of Cu-adenine was observed when the cyanide concentration was increased. 

Although computer treatment is ultimately the most convenient way to interpret the results accurately, a preliminary analytical treatment is useful in defining both the reactions involved and the approximate velocity constant ratios to use in the computer treatment. The analytical treatment also emphasizes the essential simplicity of the method—i.e.9 despite the apparent complexity of the H2 + 02 mechanism, the predominant reactions of the radicals H, O, OH, and H02 are Reactions 4, 3, 1, and 10, respectively. The relative rates of additive consumption and water formation are determined effectively by the competition between these reactions and the reactions of the corresponding radical with the additive, the remaining reactions of the H2 + 02 system merely affecting slightly the relative radical concentrations and the rate of water formation. Thus, with suitable approximations, relatively simple expressions for —d[RH]/d[H20] can be obtained for attack of H, O, and OH on the hydrocarbon, and the expression for H02 attack is more complex only because the competition between Reactions 10 and 24 depends on the H02 concentration. 

Chemical clusters can be obtained also with two monomers, when two reaction mechanisms are in competition, favoring formation of regions of higher and lower crosslink densities. This situation is more complex and more difficult to control. It is certainly the case for dicyanodiamide (Dicy)-cured epoxies with this hardener an accelerator is always used and a competition between step (epoxy-amine addition) and chain (epoxy homopolymerization) occurs (Chapter 2), leading to inhomogeneous networks. 

Meanwhile, the data obtained [87, 116] unambiguously indicate that the catalase activity increase is associated with non-classical peroxidase activity intensification. It is obvious that the last circumstance casts some suspicion on the interpretation of reactions (6.17) and (6.18) as the competing ones, because in this case, intensification of one reaction should cause suppression of the other. Moreover, as follows from Kremer s data [118], the catalase reaction rate is five orders of magnitude higher than the peroxidase reaction rate. Therefore, comparison of these reactions from competition positions is very suspect. An article by Chance and coworkers [119] can be mentioned as evidence that a H202 concentration increase in the system in the presence of ethanol intensifies peroxidase activity (hence, intensification of the catalase activity is implied). Because catalase activity increase causes the Chance complex formation at higher rate, the peroxidase reaction (6.18) rate is also increased owing to chemical induction principle. 

According to the Equation (30) the experimental isotope effect depends not only on the intrinsic isotope effects a,-, but also on the rate constants k2 and k. The intrinsic isotope effects describe the structure of the transition states and the commitment reflects the relative heights of energetic barriers of competitive reactions. If k2i. k -[li(x l), the formation of intermediate B is the rate-limiting step and experimental isotope effect is equal to ai(aexp = i). When intermediate B returns to substrate much faster than forms the product k2, k it (x 1), the experimental isotope effect is aexp = (a1a2)/a 1. For more complex multistep reactions the analysis of isotope effects is analogous, however the commitment factor become a complex collection of kinetic terms.54... 

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