Acid-base equilibrium shift

Scheme VIII has the form of Scheme II, so the relaxation time is given by Eq. (4-15)—appjirently. However, there is a difference between these two schemes in that L in Scheme VIII is also a participant in an acid-base equilibrium. The proton transfer is much more rapid than is the complex formation, so the acid-base system is considered to be at equilibrium throughout the complex formation. The experiment can be carried out by setting the total ligand concentration comparable to the total metal ion concentration, so that the solution is not buffered. As the base form L of the ligand undergoes coordination, the acid-base equilibrium shifts, thus changing the pH. This pH shift is detected by incorporating an acid-base indicator in the solution.

On the other hand, an increase in acidity should shift the acid-base equilibrium of the aniline (Scheme 3-11) further to the side of the anilinium ion, which is much less reactive. The effects of the two equilibria in Schemes 3-8 and 3-11 should therefore be approximately equal and opposite, so that from these arguments alone one would not expect the rate increase observed for region B. 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

HPTS is a highly water-soluble, pH-sensitive dye with a pK of 7.5 in aqueous solution [8], When in alkaline medium, pH > 7.5, acid-base equilibrium is totally displaced toward the anion form (3sPyO ) of the dye. The electronic character of 3sPyO remains unchanged after photo-excitation, and corresponds to a singlet-excited state [9], Fluorescence from this state undergoes a fast 0.4 ps Stokes shift and has a maximum at 515 nm and a lifetime of 5.3 0.1 ns [10],... 

The pL-independent IEs have therefore been interpreted580 as indicating that DOD shifts the acid-base equilibrium from (Fem—OO-) to the protonated intermediate (Fenl—OOH) and increases in that way the rate of synthesis of products formed via oxene chemistry (inverse SDIE) and decreases the rate of products formed through the peroxide chemistry... 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

The plot of the pH-dependence (Fig. 18) indicates qualitatively a participation of an intermediate acid-base equilibrium. Evaluation of rate constants kr and kg is made difficult by the inaccessibility of the dissociation constant of reaction (24 b) which corresponds to protonation of a radical anion. ESR would be a suitable method for the determination of the dissociation constants of at least the more stable radical anions. Another possibility for obtaining at least an approximate value of the equilibrium constant is the measurement of the shifts of the half-wave potentials of the more negative wave at potential 3 with pH. Because the half-wave potential of this wave is known to be sensitive to the... 

In the 1920s and 1930s, Louis Hammett of Columbia University explored the dissociation of substituted benzoic acids (12.1) in water (Scheme 12.1).2 Changing the R-group on the ring shifts the position of the acid-base equilibrium (Equation 12.8). 

They should be the strong acids, i.e., the acid-base equilibrium should be shifted to the right-hand side ... 

In the case of an anionic acid (z = —2 e.g. HSOj), the acid/base equilibrium (4-5) will be shifted to the right with an increase in relative permittivity of the solvent in which HA is dissolved. Here, the ionization will increase much more quickly with an increase in relative permittivity than in the preceding case. 

Changing the pH of a solution shifts the positions of all acid-base equilibria, including those involving polyprotic acids. Acid-base equilibrium expressions and equilibrium constants are used to calculate the amount of the change. For example, the two equilibria that apply to solutions containing H2CO3, HCOJ, and... 

From what we have said above, it follows that the acid-base equilibrium in the solutions containing metal cations and oxide ions in different sections of the titration curve is described either by the dissociation constant (in unsaturated solutions) or by the values of solubility product (in saturated solutions). In Refs. [175, 330] we proposed a method based on the analysis of the scatter in the calculated equilibrium parameters corresponding to the titration process. Indeed, in the unsaturated solution section there is no oxide precipitation and the calculated value of the solubility product increases monotonously (the directed shift) whereas the calculated value of the dissociation constant fluctuates about a certain value, which is the concentration-based dissociation constant of the studied oxide. 

As more and more aniline is reacted, the acid-base equilibrium is shifted to the right. This produces more aniline which will react to give o,p-substitution. 

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