Acid-dissociation constants derivation

Since the position of an acid-base equilibrium depends on the pH, the distribution ratio must also be pH-dependent. To derive an equation for D showing this dependency, we begin with the acid dissociation constant for HA. 

If a methyl group replaces a hydrogen atom on the carbon of the C==N bond across which addition of water occurs, a considerable reduction in the extent of water addition is observed. Conversely, the existence of such a blocking effect can be used as a provisional indication of the site at which addition of water occurs, while the spectrum and acid dissociation constant of the methyl derivative provide a useful indication of the corresponding properties of the anhydrous parent substance. Examples of the effect of such a methyl group on equilibria are given in Table IV. 

Tam, K. Y. Takacs-Novak, K., Multiwavelength spectrophotometric determination of acid dissociation constants. Part II. First derivative vs. target factor analysis, Pharm. Rese. 16, 374-381 (1999). 

In a similar investigation of the tautomeric tridentate ligand 2 -hydroxyphenylazo-2-naphthol (5.65 in Scheme 5.17), the first and second acidic dissociation constants (pKa) related to the two hydroxy groups in the parent structure (X = H) were found to be 11.0 and 13.75 respectively. On introduction of an electron-withdrawing substituent (X) the first dissociation constant decreased from 11.0 to 10.55 (X = Cl) or 7.67 (X = N02). The stability constants (log K1 1) of the derived 1 1 complexes were dependent on the metal ion introduced [46], being particularly high for nickel(n) at 19.6 and copper(II) at 23.3. 

The pronounced electron-withdrawing nature of the 1,2,5-thiadiazole system is also evidenced by strong carbonyl electrophilic activation and by enhancement of carboxy acidity. The acid dissociation constants of thiadiazole acids, discussed in Section 4.09.4.1, fall in the range 1.5-2.5. The 1,2,3-thiadiazole carboxylic acids are easily decarboxylated at 160-200 °C. This reaction has been used for the synthesis of monosubstituted derivatives as well as the parent ring and deuterated derivatives <68AHC(9)107>. An efficient bromo-decarboxylation of 3-amino-1,2,5-thiadiazole-carboxylic acid has also been reported <70BRP1190359>. 

The bipyridines are dibasic, and the two acid dissociation constants Ki and K2, for all the bipyridines have been determined. Typical values are recorded in Table I. There has been considerable interest in the first dissociation constants Ki of 2,2 -bipyridine and substituted 2,2 -bipyridines because of their use as metal complexing agents. In general, the order of relative basic strengths of derivatives of 2,2 -bipyridine is as expected. Electron-attracting substituents reduce the basicity, whereas electron-donating substituents increase the basicity of the molecule. " The dissociation constants of several substituted bipyridines correlate well with the Hammet equation. 2,2 -Bipyridines with an electron-donating substituent at position 4 are monoprotonated at N-1 and not at... 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

The standard notation for successive acid dissociation constants of a polyprotic acid is Kt, K2, K2, and so on, with the subscript a usually omitted. We retain or omit the subscript as dictated by clarity. For successive base hydrolysis constants, we retain the subscript b. The preceding examples illustrate that Kal (or K ) refers to the acidic species with the most protons, and Kbl refers to the basic species with the least number of protons. Carbonic acid, a very important diprotic carboxylic acid derived from COz, is described in Box 6-4. 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

Figure 2.3 shows how the chemistry of dissolved arsenious acid varies with pH. An analogous graph for arsenic acid is in Figure 2.4. As expected, protonated species of the acids are more common under low pH conditions were H+ is abundant. For both weak acids, dissociation constants (Ka values) may be derived to describe their gain or loss of H+ with changing pH conditions (Table 2.10 (Faure, 1998), 119-120). For example, the following reaction involving the dissociation of H3ASO3 in water at 25 °C and 1 bar pressure has a dissociation constant (K ) of HP9 2 (Wolthers et al., 2005), 3490 ...

Various methods are available for predicting the acid dissociation constant (pKa) within homologous series. Most are based on the Hammett equation (benzene derivatives) and the Taft correlation (aliphatics and alicyclics). A comprehensive review of methods was published by Perrin et al. (1981). 

A linear relationship was obtained between the logarithmic interfacial formation constant (log / ,) for PdL+-diazine derivative complexes and the logarithmic ratio of the distribution constant (A D) to the acid-dissociation constant (Ka) for the two groups. PdL+-pyridazine derivative complexes showed much higher stability at the interface than pyrimidine and pyrazine derivative complexes. This result suggests that pyridazine derivative complexes become more... 

The molecular recognition ability of Pd(II)-5-Br-PADAP for the isomers of diazine derivatives has been evaluated. Figure 10.14 summarizes the molecular structures of the diazines (Dzs or N) studied, the acid-dissociation constant Kf) and the distribution constant values between toluene and water Kd). PdLCl reacted with a neutral N with one or two nitrogen atoms, forming a PdLN" complex at the toluene/water interface. The interfacial formation constant (/6j) of the PdLN+ complex was determined as follows [52] ... 

Example 2.5 (a) Derive the two equations to calculate /m and /ml as functions of only [L] for a system where only one product (ML) of M and L is formed, (b) Select a monoprotic acid as an example and find its corresponding acid dissociation constant (Analytical Chemistry textbooks are normally good initial sources), (c) Plot the results for pL = 0 to pL = 14. Note that in this example, pL = pH. 

In the care of reaction (51) it is the o-complex which is undergoing deprotonation. The pKj, values of these ketone-like o-coraplexes can be derived from the constant Kt for the tautomeric keto-enol equilibrium of the phenol and the acid dissociation constant K, for the phenol. One gets pK pteto) 1-... 

When HX is a carbon acid the value of the rate coefficient, ) for a thermodynamically favourable proton transfer rarely approaches the diffusion limit. Table 1 shows the results obtained for a few selected carbon acids which are fairly representative of the different classes of carbon acids which will be discussed in detail in Sect. 4. For compounds 1—10, the value of k i is calculated from the measured value of k, and the measured acid dissociation constant and, for 13, k, is the measured rate coefficient and k1 is calculated from the dissociation constant. For 11 and 12, both rate coefficients contribute to the observed rate of reaction since an approach to equilibrium is observed. Individual values are obtained using the measured equilibrium constant. In Table 1, for compounds 1—10 the reverse reaction is between hydronium ion and a carbanion whereas for 11, 12 and 13 protonation of unsaturated carbon to give a carbonium ion is involved. For compounds 1—12 the reverse reaction is thermodynamically favourable and for 13 the forward reaction is the favourable direction. The rate coefficients for these thermodynamically favourable proton transfers vary over a wide range for the different acids. In the ionization of ketones and esters, for which a large number of measurements have been made [38], the observed values of fe, fall mostly within the range 10s—101 0 1 mole-1 sec-1. The rate coefficients observed for recombination of the anions derived from nitroparaffins with hydronium ion are several orders of magnitude below the diffusion limit [38], as are the rates of protonation and deprotonation of substituted azulenes [14]. For disulphones [65], however, the recombination rates of the carbanions with hydronium ion are close to 1010 1 mole-1 sec-1. Thermodynamically favourable deprotonation by water of substituted benzenonium ions with pK values in the range —5 to —9 are slow reactions [27(c)], with rate coefficients between 15 and 150 1 mole-1 sec-1 (see Sect. 4.7). 

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