Dissociation kinetics acid-catalyzed

The formation and dissociation kinetics for the complexation of Cu, Zn, Co and Ni with the quadridentate l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclotetradecane to give five-coordinate species have been reported.1203 The rate order (Cu > Zn > Co > Ni) is the same as that for H2O exchange, but the rates are much slower, probably owing to conformational changes occurring in the ligand. The dissociation is acid-catalyzed the five-coordinate species are found to be much less kinetically inert than four- or six-coordinate complexes. No macrocyclic effect was observed. 

All these facts—the observation of second order kinetics nucleophilic attack at the carbonyl group and the involvement of a tetrahedral intermediate—are accommodated by the reaction mechanism shown m Figure 20 5 Like the acid catalyzed mechanism it has two distinct stages namely formation of the tetrahedral intermediate and its subsequent dissociation All the steps are reversible except the last one The equilibrium constant for proton abstraction from the carboxylic acid by hydroxide is so large that step 4 is for all intents and purposes irreversible and this makes the overall reaction irreversible... 

The important difference between the dissociation mechanism of LnMEDTA complexes and the acid catalyzed dissociation of LnEDTA is the formation of mixed LnMEDTA-Ac" complex. The stability constants of such mixed complexes are small (1 to 10) and this may explain why such mixed complexes do not show up in the kinetics of dissociation or exchange of LnEDTA complexes. Further, in the present case [LnMEDTA]0 association with an acetate anion should be more favourable than [LnEDTA]- with an acetate anion based on electrostatic theory. Reference to Table 7.14 shows that Km values increase with increasing atomic number (decreasing radii) for heavy lanthanides but are independent for La-Nd series. 

In a study of the kinetics of transfer of an acetyl group from 1-acetylimidazolium and 1-acetylimidazole to phenols the rate constants were found to be in close agreement with those obtained for similar acetylation of thiols. As well as this, it was discovered that the acetyl transfer reactions of the non-dissociating model, l-acetyl-3-methylimidazolium chloride, compared very closely with those of 1-acetylimidazolium. This provided evidence that many reactions of acetylimidazolium with reagents of typeHY are merely acid-catalyzed reactions of the anion Y (B-76MI40701). 

Breen et al. (1986) report the results of a stopped-flow investigation of metal ion exchange kinetics between Tb(III) and Ca(EDTA). The progress of the reaction was monitored by Tb(III) luminescence. These authors interpret the data to indicate that metal ion exchange occurs via acid-catalyzed dissociation of the Ca(EDTA) complex with rapid formation of the Tb(III)-EDTA complex, and through a parallel process involving a dinuclear intermediate. The acid-catalyzed dissociation pathway is the preferred mode. 

The kinetics of aquation and base hydrolysis of the cis-[Co(en)2(NH2Et)02CR] ions (R = H or Me) have been studied in detail. Aquation is strongly acid-catalyzed and rate and activation parameters for this process are reported. Similar rates are observed for both complexes in spite of the differences in basicity of coordinated formate and acetate. Aquation rates (k q) are also very similar, but base hydrolysis of the formato complex is some five times faster than that of the acetato complex, consistent with a dissociative SNiCB mechanism and cleavage of the Co—O bond. 

Kinetic parameters have been established for solvolysis of the pentacyanofer-rate(III) derivative [Fe(CN)5(N02)] . For aquation, which is acid-catalyzed at pH <5, A//= = 43kJmol-, =-80 J K" mol", and A =+2 cm" mol". Intrinsic and solvational contributions are presumably closely balanced in the case of A Rate constants for solvolysis of [Fe(CN)5(N02)] in water, methanol, dimethyl sulfoxide, and dimethylformamide correspond with the electron-donating abilities of the respective solvents. Activation volumes for the nonaqueous solvents, between +20 and +27 cm" mor reflect the dissociative nature of these solvoly-ses. " Rate constants for dissociation of the [Fe(CN)5(2,6-Me2pyrazine)]" anion in binary aqueous solvents containing methanol, acetone, or acetonitrile correlate well with acceptor numbers for the respective media, though with a very different... 

Unlike numerous kinetic and mechanistic studies of acid-catalyzed hydrolysis of glycopyranosides [5, 7, 9, 12, 16, 24-30] that led to the conclusion that glycopyranosides are hydrolyzed via an A-1 mechanism (the molecularity of the reaction and the entropy of activation (positive AS ), dissociation of methanol, and the formation of oxocarbenium ion transition state intermediate) (42 in Fig. 3.9), the acid-catalyzed hydrolysis of glycofuranosides has been studied much less [4, 9, 31-34]. 

The kinetic deuterium solvent isotope effects on the acid-catalyzed dissociation of Li(2,l,l) and Ag(2,2,2) have been reported, as have the exchange kinetics " of cryptand (2,2,1) on Tl(2,2,2) and T1(2b,2,2) . The kinetics of complex formation between Li and 18-crown-6 ether in 1,3-dioxalane and 1,2-dimethoxyethane solvents have also been reported. ... 

The new ligand (6) also bears a 3— charge when it is fully ionized, but a recent kinetic study of its formation reactions in fact dealt with 2-h cations, specifically of Ca, Mg, Mn, and Cd. This primarily thermodynamic study reveals some tantalizing hints on kinetic behavior, for example the period of about 5 hours mentioned for the Mn " plus (6) system to come to equilibrium/ Lanthanide(III) cations react with (7) about a hundred times faster, but with (8) about ten times slower, than with the much-studied >NCH2C0 ligand (3)/ Rate constants for dissociation of lanthanide(III) complexes of (7) and (8) are, in the present context, rather rapid—of the order of 10 to 10 s in neutral solution, with the usual acid-catalyzed pathway/ ... 

Kemkes256 assumes that the overall order relative to the esterification of terephthalic acid by 1,2-ethanediol in oligo(l,2-ethanediyl terephthalate) is two no mechanism has however been suggested. Mares257 considers that during the esterification of terephthalic acid with 1,2-ethanediol, two parallel kinetic paths take place, one corresponding to a reaction catalyzed by non-dissociated add and the other to a non-catalyzed process. In fact, Mares257 is reserved about the existence of protonic catalysis. Some other orders were found for the system terephthalic atid/l,2-ethanediol 0 (overall)318 2 (add) andO (alcohol)203 1 (add) and 1 (alcohol)181 1 (add)194 . These contradictory results could be partly due to the low solubility of terephthalic acid in 1,2-ethanediol. 

The kinetics of formation and dissociation of the Ca2+, Sr2+ and Ba2+ complexes of the mono- and di-benzo-substituted forms of 2.2.2, namely (214) and (285), have been studied in water (Bemtgen et al., 1984). The introduction of the benzene rings causes a progressive drop in the formation rates the dissociation rate for the Ca2+ complex remains almost constant while those for the Sr2+ and Ba2+ complexes increase. All complexes undergo first-order, proton-catalyzed dissociation with 0bs — kd + /ch[H+]. The relative degree of acid catalysis increases in the order Ba2+ < Sr2+ < Ca2+ for a given ligand. The ability of the cryptate to achieve a conformation which is accessible to proton attack appears to be inversely proportional to the size of the complexed metal cation in these cases. 

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