Protonation-deprotonation reactions sites

Surface protonation/deprotonation reactions at the edge of the silanol and aluminol sites (>SOH) of montmorillonite, which can be exemplified by the following reactions ... 

Complexation/decomplexation of metal ions or of neutral organic molecules, protonation/deprotonation reactions, and oxidation/reduction processes can all be exploited to alter reversibly the stereoelectronic properties of one of the two recognition sites, thus affecting its ability to sustain noncovalent bonds [30-34, 41]. These kinds of switchable [2 catenanes can be prepared following the template-directed synthetic strategy illustrated in Figure 5, wherein one of the two macrocyclic components is preformed and then the other one is clipped around it with the help of noncovalent bonding interactions. 

Often there is more than one basic or acidic site in a molecule. Remember that protonation-deprotonation reactions are very rapid and reversible reactions, so just because a substrate has an acidic or basic site, it doesn t mean that protonation or deprotonation of that site is the first step in your mechanism. It s up to you to figure out which site must be deprotonated for the reaction in question to proceed. 

We usually think of protonation-deprotonation reactions occurring in solution where protons can move with solvent molecules. In an enzyme active-site, there is no "solvent", so there must be another mechanism for movement of protons. Often, conformational changes in the protein will move atoms closer or farther. Histidine serves the function of moving a proton toward or away from a particular site by using its different nitrogens in concert as a proton acceptor and a proton donor. 

Fig. 3 Experimental points of net proton surface excess amounts from the reversible backward titration cycles of sodium montmoril-lonite at different NaCl concentrations. The different lines represent the results of numerical fitting (FITEQL [28]) using the diffuse-double-layer option of the surface complexation model assuming reactions of and Na" ions with permanently charged ion-exchange sites in parallel with protonation/deprotonation reactions on amphoteric edge sites...

The solvent dielectric constant, ionic strength, and temperature are chosen to fit the conditions of the experimental studies. We have found a dielectric constant of 20 to work well for the single-site pK method. It seems that the fewer details the model includes, the larger the dielectric constant of the protein should be. When more details are included (e.g., rearrangement of the charge distribution upon the protonation/deprotonation reaction, local flexibility following the protonation/deprotonation reaction), the protein dielectric constant can be set lower (e.g., 4). In an ideal case where only the electronic polarizability needs to be included, one could use a dielectric constant of 2. In the work of others, a value for the dielectric constant is typically 2-4. 

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. 

With minor adjustments, the treatment presented above for the reactions of Feaq3+/FeaqOH2+ is applicable to any reaction that involves two hydrolytic forms of one or more reactants and is easily extended to cases with more than one protonation/deprotonation site at a single center. The experimental rate law will depend on the pH range used and relative reactivities of acidic and basic form(s). The two general cases in Figure 8.3 schematically cover various possibilities for a reaction with a single acid-base site. The two curves differ only in relative reactivities of the acidic and basic forms. 

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. 

However, the zeolite framework effect on the reaction is not limited only to a stabilization of charged species. We saw already that a transition state from the cluster approach turns to be an inflection point when the zeolite framework contribution is considered. An effect exists also on transition state. In the case of the shift isomerization transition state, it is found an alternative geometry. Before protonated toluene changes its orientation with respect to the deprotonated Brytasted site, the methyl shift reaction step can be achieved (see Figure 12). 

The conjugate base of a cluster generated by deprotonation (see Section 5.1) is a Lewis base. Any metal-metal, metal boron, or boron boron bond is a potential Lewis base. The former have considerable nucleophilic character if anionic and effectively interact with electrophiles more complex than the proton. As the site of basicity is associated with framework bonding, this reaction type results in cluster building, for... 

These reactions involve proton and electron transfer and kinetic data suggest that the mechanism involves a protonated intermediate followed by base-catalyzed deprotonation of the ring [32]. In black-and-white developers the amount of sulfite present is usually very high at about 60 g L of sodium sulfite (0.5 m), and almost all the oxidized developer formed is immediately removed from the reaction site. (At low sulfite levels the effects of accumulated oxidation products can have a significant effect on the development rate, particularly for hydroquinone developers see the section on Lith development ). In the case of hydroquinone the reaction with sulfite is similar to that of p-phenylenediamine (Eq. (22)). 

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