Two Acid-Base Equilibria

When the sigmoidal shape of the rate constant/pH profile associated with (1.207) or the simpler derivatives (1.208) or (1.209) give way to a bell-shape or inverted bell-shape plot, the reactions of at least three acid-base-related species (two equilibria) have to be considered. This may involve acid-base forms of (a) one reactant or (b) two different reactants. 

If the species AH reacts more rapidly or more slowly than either A or AHj, a bell shape or inverted bell shape respectively results for the A /pH profile. 

The ruthenium(III) complex of edta in which the ligand acts only as a five-coordinate species and in which an acetate arm remains free, exists in three pH-related forms  

The second-order rate constants for thiocyanate anation vs pH are shown in Fig. 1.13. The full line represents (1.216) with the values shown in scheme (1.217). This profile had been earlier recognized in the ring closure of the three analogous pH-related forms of Co(III)-edta to give Co(edta) in which the edta is completely coordinated.In the Co(lll) case the reactivities of the three forms are much closer. A plot of A [H+] -1-[H+] is a quadratic curve from which / ah2 ah be obtained. 

If the predominant form of the reactant is AH in the pH region under examination, that is A ah2 [H ] AH then only the middle term of the denominator in (1.216) is important. Thus 

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Scheme 5-14 may be called a two-dimensional system of reactions, in contrast to Scheme 5-1 which consists of a one-dimensional sequence of two acid-base equilibria. In Scheme 5-14 the (Z/E) configurational isomerism is added to the acid-base reactions as a second dimension . The real situation, however, is yet more complex, as the TV-nitrosoamines may be involved as constitutional isomers of the diazohydroxide. In order not to make Scheme 5-14 too complex the nitrosoamines are not included, but are shown instead in Scheme 5-15. The latter also includes the addition reactions of the (Z)- and ( )-diazoates (5.4 and 5.5) to the diazonium ion to form the (Z,Z)-, (Z,E)- and (2 2i)-diazoanhydrides (5.6, 5.7 and 5.8) as well as proto-de-nitrosation reactions (steps 10, 11 and 12). This pathway corresponds to the reverse reaction of diazotization, as amine and nitrosating reagent (nitrosyl ion) are formed in this reaction sequence.

Pyridinium chloride is a salt that generates ions in solution. The major species are the pyridinium cation (C5 H5 NH+), cr, and H2 O. The formula identifies the pyridinium cation as the conjugate acid of the weak base, pyridine (see Table 17-41. There are two acid-base equilibria with major species as... 

Most of the examples in previous sections appear to involve a single acid-base equilibrium. A closer look reveals that nearly any solution that displays acid-base properties has at least two acid-base equilibria. Look again at... 

In the chapters so far we have considered the phase rule somewhat intuitively for example, in solving equilibrium problems we used the obvious principle that an equilibrium problem can be solved if for n unknowns (e.g., activities or concentrations of n species) n equations are available. For example, in a closed dissolved carbonate system we need to define the system (H2CO3, HC03 , CO H, OH ) and two concentration conditions (e.g., Ct and pH, or [Aik] and H2CO ]), in addition to temperature and pressure, because the five species are interconnected by three mass laws (two acid-base equilibria and the ion product of H2O). In the example given P = 1 (aqueous solution), C = 3 le.g., HCO, H", H20(l)], andF = 4 (pressure, temperature, and two concentration conditions). 

For the addition of primary and secondary amines, the rate of which is affected by the two acid-base equilibria considered in scheme (7), it is possible to apply equation (8). On the assumption that P STme < P n> equation (8) can be further simplified as follows ... 

In aqueous solution pristinamycin 1 shows two acid-base equilibria (determined by potentiometric methods [41]) associated with (i) protonation of the dimethylamino function of the iV-methyl 4-dimethylaminophenylalanine residue - pKji =4.1 and (ii) deprotonation of the hydroxy group of the hydroxy-picolinic acid residue - pK,2 = 7.3. The molecule is neutral between pH = 4 and pH = 7 and is water soluble at pH less than 3 as the corresponding acid-salt. 

A zwitterionic amino acid can gain a proton at the carboxyl unit under acidic conditions or lose a proton from the ammonium unit under basic conditions to give two acid-base equilibria, Kj and Kg. The isoelectric point is the pH at which the amino acid is neutral, which corresponds to formation of the zwitterion. When the amino acid contains an acidic side chain such as a carboxyl or a phenol, there is another acid-base equilibrium represented by K3. 

Cr207 + H2O 2HCr04, K = 10 on which the following two acid-base equilibria are superimposed ... 

A. A. Tamburello-Luca, P. Hebert, R. Antoine, P. F. Brevet, and H. H. Girault, Langmuir, 13, 4428 (1997). Optical Surface Second Harmonic Generation Study of the Two Acid/Base Equilibria of Eosin B at the Air/Water Interface. 

Two important points come from using relative pK s to analyze acid-base equilibria... 

Tucker, S. A. Amszi, V. L. Acree, Jr., W. E. Studying Acid-Base Equilibria in Two-Phase Solvent Media, /. Chem. Educ. 1993, 70, 80-82. 

Aromatic pyrazoles and indazoles, in the broad sense defined in Sections 4.04.1.1.1 and 4.04.1.1.2, will be discussed here. Tautomerism has already been discussed (Section 4.04.1.5) and acid-base equilibria will be considered in Section 4.04.2.1.3. These two topics are closely related (Scheme 10) as a common anion (156a) or a common cation (156b) is generally involved in the mechanism of proton transfer (e.g. 78T2259). For aromatic pyrazoles with exocyclic conjugation there is also a common anion (157) for the three tautomeric forms... 

Analogous, but slightly different, is the treatment of the acid-base equilibria and the basic hydrolysis rates of the phthalimids (19). In both of these cases, the two paths to the reaction site are equivalent, hence pi and po of Eq. (3) are equal, and the equation reduces to ... 

To treat acid-base equilibria involving zwitterions, it is convenient to consider the cation stable at low pH to be a diprotic add (analogous to H2C03), which ionizes in two steps. Using the symbols C+, Z, and A- to stand for the cation, zwitterion, and anion, respectively, we have... 

Our goal in this chapter is to help you continue learning about acid-base equilibrium systems and, in particular, buffers and titrations. If you are a little unsure about equilibria and especially weak acid-base equilibria, review Chapters 14 and 15. You will also learn to apply the basic concepts of equilibria to solubility and complex ions. Two things to remember (1) The basic concepts of equilibria apply to all the various types of equilibria, and (2) Practice, Practice, Practice. 

The structure of [Cr(edta)] 1- in aqueous solution is still under active investigation. Two series of salts can be isolated Rb+ (92) or K[Cr(edta)] 2H20 with N204 hexadentate coordination in the solid state (93), and [Cr(Hedta)(OH2)] (94, 95) with one G-ring dechelated and protonated. The system is very labile (rechelation rates exceed stopped-flow measurement), and several acid-base equilibria are involved (Fig. 14). 

A.2.1 Dissociation Reactions. A glance at the list of fragments which can be produced from the dissociation of a molecule shows that these apparently simple reactions can in fact follow several mechanisms. Of these, two are treated separately the loss of an electron, which is the process of photoionization and the loss of a proton, which is one side of the acid-base equilibria considered in section 4.4.3. 

Exercise 18-48 The Michael-addition product that results from ethyl 3-phenyl-propenoate and diethyl propanedioate, in principle, also can be formed by sodium ethoxide-catalyzed addition of ethyl ethanoate to ethyl (2-carbethoxy)-3-phenyl-propenoate. Work out the course of this reaction along the lines of Equations 18-25 and 18-26 and explain why it is less likely to be successful than the addition of diethyl propanedioate to ethyl 3-phenylpropenoate. It will be helpful to compare the various possible acid-base equilibria involved in the two possible routes to the same Michael-addition product. 

To affect the limiting current the acid-base equilibria must either precede the electrode process or occur between two electrode processes. In the former case the simplest system involves a monobasic acid (HA), the anion of which (A-) is protonated2 only by hydroxonium ions (18) ... 

The proposal of a mechanism obviously requires identification of the species able to react in this case, at first sight, there are up to six species four from chlorine (Cl2, Cl3-, HOC1 and CIO-) and two from the amine (RiR2NH2+ and RiR2NH). The eight elementary bimolec-ular steps shown in Scheme 4.6, therefore, are possible in principle. For the chlorination of amino acids, of course, there are additional acid-base equilibria to be taken into account, which expands the number of mechanistic possibilities. 

Additional acid-base equilibria involve the phosphate groups in mono- and oligonucleotides and the sugars in nucleosides. In mononucleotides, two ioniz-able protons with pK values of 0.9 and 6.2 are present (21) ... 

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