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Reactions chelate formation

Figure 7.30 Mechanism of phenol and formaldehyde reaction using base catalyst involving the formation of chelate. Figure 7.30 Mechanism of phenol and formaldehyde reaction using base catalyst involving the formation of chelate.
Co (I I) complex formation is the essential part of copper wet analysis. The latter involves several chemical unit operations. In a concrete example, eight such operations were combined - two-phase formation, mixing, chelating reaction, solvent extraction, phase separation, three-phase formation, decomposition of co-existing metal chelates and removal of these chelates and reagents [28]. Accordingly, Co (I I) complex formation serves as a test reaction to perform multiple unit operations on one chip, i.e. as a chemical investigation to validate the Lab-on-a-Chip concept. [Pg.563]

A new aminocarboxylate chelator of potential therapeutic value, 77(2-hydroxybenzy -Al -benzylethylenediamine-A Al -diacetate, reacts as LH4 and LH3 with Fe(OH)2q by dissociative activation with rate constants of 770 and 13 300 M s-1, respectively. These rate constants are similar to those for reaction of Fe(OH)2q with edta and with nta. These formation reactions are, however, considerably faster than with simple ligands of identical charge thanks to the zwitterionic properties of ami-nocarboxyl ates (334). [Pg.119]

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). [Pg.162]

The linking of a metal to an antibody could, in principle, be accomplished by forming the metal chelate either prior to or after attachment to protein. Success to date has been achieved only by formation of the protein-ligand conjugate before metal chelation. The complexation reaction has several general features. First, reactions between the metallic radionuclides and antibodies are almost always performed with sub-stoichiometric quantities of chelate and metal ion. It is therefore of the utmost importance that no carrier added metals obtained from commercial sources be exceedingly pure or else be purified prior to use. Reactions of "carrier added" metal solutions are not likely to be of use because of the ease with which available chelate sites become saturated. Because the formation of chelate complexes is usually a bimolecular reaction, the complexation will proceed optimally when more chelation sites are available. Similarly, the more isotope in solution, the faster the reaction. Employment of a carrier chelate to insure solubilization of the radiometal is of value to maximize available isotope and the acetate ion has proven useful. [Pg.225]

The pKa of a nitrogen is a convenient measure of its nucleophilicity in proton addition steric effects are unimportant. All other types of electrophilic attack at nitrogen are sensitive in varying degrees to steric effects from a-substituents. (Exception certain ring formation reactions as in metal chelation.)... [Pg.176]

Schiff bases having two nitrogen atoms as donors may be derived either from condensation of dialdehydes and diketones with two molecules of an amine, or from reaction of diamines with aldehydes or ketones. In Section 20.1.2.1, it has been pointed out that coordination through the N atom may occur only under particular circumstances. However, in the case of diimines the formation of chelate rings stabilizes the metal-nitrogen bond. Thus, they can form both mono-41 and bis-chelate42 complexes. [Pg.721]

The design of polydentate ligands containing imines has exercised many minds over many years, and imine formation is probably one of the commonest reactions in the synthetic co-ordination chemist s arsenal. Once again, the chelate effect plays an important role in stabilising the co-ordinated products and the majority of imine ligands contain other donor atoms that are also co-ordinated to the metal centre. The above brief discussion of imine formation will have shown that the formation of the imine from amine and carbonyl may be an intra- or intermolecular process. In many cases, the detailed mechanism of the imine formation reaction is not fully understood. In particular, it is not always clear whether the nucleophile is metal-co-ordinated amine or amide. Some intramolecular imine formation reactions at cobalt(m) are known to proceed through amido intermediates. A particularly useful intermediate (5.24) in metal-directed amino acid chemistry is... [Pg.114]

A full report for the identification of salicylamide through melting point, reactions of the amide group, the phenolic group, the benzene ring, and the formation of chelate compounds has been published (9). [Pg.524]

The attachment to the metal center of the second or third donor atom of the ligand is usually very rapid, so in most cases departure of the first water molecule is rate determining. The rate-determining step is the closure of a chelate ring in complex formation reactions where a steric property of the ligand governs the kinetics of substitution by the second donor atom.241,242 In cases in which Equation 7.21 is applicable, the volume of activation is given by... [Pg.302]

At the same time, some case are known when it is possible to carry out complex-formation reactions with N,S-donor chelating systems having a free SH group, for example (3.51) [61], (3.52) [62], (3.53) [63], and (3.54) [64] ... [Pg.163]

In additions of hydride donors to a-chiral carbonyl compounds, whether Cram or anti-Cram selectivity, or Felkin-Anh or Cram chelate selectivity occurs is the result of kinetic control. The rate-determining step in either of these additions is the formation of a tetrahedral intermediate. It takes place irreversibly. The tetrahedral intermediate that is accessible via the most stable transition state is produced most rapidly. However, in contrast to what is found in many other considerations in this book, this intermediate does not represent a good transition state model for its formation reaction. The reason for this deviation is that it is produced in an... [Pg.412]

Schwarz et al. [326] synthesised a functionalised bis-imidazolium salt with hydroxy end groups on the wingtips [327,328] and used it in the formation of chelating cw-bis-carbene complexes of palladium(ll) applied as catalysts in the Heck reaction. The functional groups were needed to immobilise the catalyst by attachment to a polymeric support [329] (see Figure 3.104). [Pg.135]

Proline hydroxylase has been isolated and characterized and is known to contain a mononuclear nonheme ferrous iron center that is the catalytic active site of the enzyme, coordinated by two histidine and one aspartate side chains. The requirement for Fe(II) is reflected in the in vivo sensitivity of collagen formation to chelators specific for ferrous ion (e g. 2,2 -dipyridyl). In addition to the catalytic metal cofactor, the reaction requires a reducing cosubstrate, 2-oxoglutarate, dioxygen, and the procollagen peptide (equation 1). [Pg.5496]

Studies on ionic hydration and complex formation reactions in solution in the 1960 s quite actively proceeded in Europe, especially in northern Europe. J. Bjerrum, a son of N. Bjerrum in Denmark, established a method for determination of stepwise formation constants of complexes with simple monodentate ligands in solution by the spectrophotomeffic method, which was soon modified to the method by potentiometry. Sillen in Sweden extended the method to polynuclear complex formation reactions and Schwartzenbach in Switzerland, who is a pioneer of chelate chemistry, applied this method to multidentate ligand complexes... [Pg.2]


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See also in sourсe #XX -- [ Pg.219 , Pg.300 ]




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