Protonation and deprotonation reactions

Protonation and deprotonation reactions of corroles have already been mentioned (see Introduction). Attempts to achieve electrophilic substitution reactions, at the corrole, e.g. Friedel-Crafts acylation, have been unsuccessful.1 Heating corroles with acetic anhydride yields the corresponding 21-acetyl derivatives l.8a,b... 

The rates of protonation and deprotonation reactions are sufficiently large in comparison with corresponding diffusion rates, so that it can be assumed that these reactions are at equilibrium even when current is flowing, that is, the following equations are valid everywhere at t > 0, t being the time ... 

Fig. 2.1-44. I somerization induced by protonation and deprotonation reactions of B20H184. ...

After a while longer, compounds 5-9 are converted into compounds 10-12. Note that since all of 5-9 are easily interconverted by protonation and deprotonation reactions, any of them could be the precursors to any of 10-12. 

The final response of the pH sensor depends on the balance of all the equilibria involving H+ protonation and deprotonation reactions of the enzyme reaction products buffering capacity etc. A recent theoretical kinetic model, considering all these associated transport phenomena predicts the steady state response of an enzyme based pH sensor (9). 

Protonation and deprotonation reactions have been reported for Group IB metal heteronuclear clusters (e.g.,66,103), but cleavage of the coinage metal moiety from the cluster often occurs instead (e.g., 20). 

The results of potentiometric titration, the number of edge sites, and intrinsic stability constants of the protonation and deprotonation reactions of calcium-, copper-, zinc-, manganese(II)-montmorillonites, and KSF montmorillonite are shown in Table 2.4. As a comparison, some similar data for other montmoril-lonites are also listed. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

Because of the difficulties discussed above, our classification of the reactions of HNCC is based on the products obtained from those reactions. They are discussed in six separate sections A. Oxidation Reactions, B. Protonation and Deprotonation Reactions, C. Reduction Reactions, D. Electron-Transfer Reactions and Electrochemical Studies, E. Reactions with Soft Nucleophiles, and F. Oxidative Addition of the Small Molecules H2,12, and HX. 

Metal cations in the solid sorbent surface are symbolized by S in MINTEQA2. At mineral surfaces exhibiting amphoteric behavior, that behavior is often attributed to the successive dominance of surface SOH, SOH, and SO species with increasing pH (Fig. 10.4). In MINTEQA2, surface protonation and deprotonation reactions that are basic to the three SC models are written in the form... 

The intrinsic constant expressions written with Boltzmann function terms in Eqs. (10.35) and (10.36), correspond to hypothetical protonation and deprotonation reactions written... 

Index h applies to 1-pK model, while indices p and d denote protonation and deprotonation reactions when the amphoteric (2-pK) concept is applied. The assumed adsorption mechanism is characterised with, indices bp and bh. 

When the "amphoteric concept (2-pK model) is applied one deals with two surface reactions [22], i.e. with protonation (3) and deprotonation (4). The extent of these reactions is same in magnitude but opposite in sign if the Nernstian equation for the surface potential holds, which is not true for real systems [69]. However, in the case of the symmetric experiment the protonation and deprotonation reactions are opposite in direction with the same magnitudes of their extents, regardless to the obedience of the Nernst equation. The extents of surface reactions p and d are ... 

A problem in interpretation on the basis of the 2-pK model is calculation of the extent of the protonation and deprotonation reactions. The following equation describes this situation ... 

The advantage of the LMs is integrating extraction and back-extraction of the desired analyte(s) into one step. Using protonation and deprotonation reactions, selected hydrophobic carriers with carboxyl groups have been shown effective in the separation of amino acids, if the carboxyl functionality was ionized [123]. Optimum values of the stability constants of the complexes between particular amino acids and carrier(s) can be found to increase extraction efficiencies. However, the kinetics of mass transfer often has a more pronounced impact on the efficiency of extraction [118]. 

Whereas the mechanism of the C2-H deprotonation of ThDP has been shown to be identical in all ThDP-dependent enzymes investigated, the following steps in catalysis of the different enzymes require different protonation and deprotonation reactions of the intermediates formed along the process. In order to identify side chains involved in proton transfer steps, the distribution of reaction intermediates during catalysis of any wild type enzyme can be compared with that of active site mutant enzymes. Rate constants for single steps in catalysis can be calculated from... 

Similar results were obtained in our studies on the thiamin-dependent indole-pyruvate decarboxylase, where protonation and deprotonation reactions are also catalyzed by a Glu-cofactor proton shuttle and a His-Asp-Glu relay (Schiitz et al., 2005). 

Obviously, protons near the surface are affected by its electrostatic field, and their activities must include the Boltzmann factor. Because of this at equilibrium protonation and deprotonation reactions must be subjects to equalities... 

When a semiconductor is immersed in an aqueous solution, and OH ions in the solution will continuously adsorb and desorb from the surface. A dynamic equilibrium will be established, which can be described by the following protonation and deprotonation reactions ... 

FIGURE 12.12 Scheme of surface protonation and deprotonation reactions according to the quad layer model. (From Bowden, J. W. et al., Soil Res., 15, 2, 121-136, 1977.)... 

Two main mechanisms involving protonation and deprotonation reactions have been proposed. In die first the initiation step is associated with the protonation of the aromatic nucleus and yields a tr-complex (4) which in turn reacts with the monomer and gives a first unstable cationic dimer (5). It is eonsidered that 5 is then chemically oxidized to a new cationic aromatic dimer (6) and that chain propagation occurs, leading to a cationic polymer (7) which, after loss of a proton, gives the PPP chain (8) (Scheme 6.2) [8]. 

A simple combination of equations for surface reactions, i.e., subtracting Eq. (3) from Eq. (2), gives the usual form of ion exchange in Eq. (1). The protonation and deprotonation reactions are defined only on the amphoteric -OH sites of the edges,... 

Protonation and deprotonation reactions of carbonyl cluster species are in general rather slow. A screening effect of the carbonyl groups around the metal nucleus which makes the direct interaction of the reactants difficult is considered to be the cause of large activation barriers. 

More recently, in addition to the adsorption equilibria studies, calorimetry was employed in examining of the metal oxide aqueous interface. The results deal either with relatively simple situations, such as protonation and deprotonation reactions at the surface or with the heats of the adsorption of organic molecules onto surfaces. In both cases, one encounters two problems. The first one is due to the fact that, upon addition of a reactant, several reactions take place in the calorimeter. In order to evaluate enthalpy of a specific surface reaction, one has to be able to distinguish between different contributions to the heat and also to determine the extents of all reactions taking place in the system. The next problem is related to the electrostatic contribution to the enthalpy. Recently, an experimental design was developed which enabled the proper interpretation of the calorimetric data so that one... 

Most oxide and hydroxides, as well as the broken edge sites and basal-plane hydroxyl groups of clay minerals exhibit amphoteric behavior. The formation of eleciric charge can be explained by the acid-base behavior of surface hydroxyl groups. A detailed description can be found in several excellent book chapters and articles [1,2,6,8,13,14,35,45], Charge development on amphoteric surface sites (S—OH) could occur by direct proton transfer. The surface hydroxyl groups can be capable of ionization [8,46], Surface ionization (protonation and deprotonation) reactions can take place on these sites, depending on the pH of the solution ... 

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