Ionization constants of bases

To accurately measure these n macroscopic values by standard methods, it is necessary to obtain information abont the ratio of eqnilibrinm concentrations of the n ionized and one neutral form of the acid by the most noninvasive method possible, while monitoring pH, temperature and ionic strength of the solntion. Ionization constants of bases and mixed systems are determined nsing the same principle. The problem of macroscopic determination thus becomes a problem of accurate measurement of relative concentrations in water solntion. Widely applied methods for this purpose are ... 

In the older literature the ionization constants of bases referred to the equilibrium described by equation (9-4). These constants were known as K5. Thus for ammonia... 

Perrin, D. D., 1982. Ionization Constants of Inorganic Acids and Bases in Aqueous Solution. New York Pergamon Press. 

It follows that Kt can be theoretically calculated from Kt and the ionization constant of either of the alkylated derivatives. In practice, however, this can only be done if there is an appreciable difference between the constant for the alkylated derivative and the value of Kt. Mason has argued that it is better to use the ionization constants of both alkylated forms in the equation Kt = Xmethyl groups can be base-weakening as well as base-strengthening the data shown in Table I indicate that the base-weakening inductive... 

That the disruption of the benzenoid structure is an important factw in the destabilization of the bases is shown by the work of Adler and Albert [J. Chem. Soc. 1794 (I960)] on the ionization constant of the diazaindenes where this factor is not present and the order of base strengths is 1,5- > 1,6- > 1,4- > 1,7-diazaindene. 

A. Albert ajid E. P. Serjeant, Ionization Constants of Acids ajid Bases, pp. 140 f. Methuen, London, 1962. 

It is a simple matter to determine an ionization constant and also to predict its magnitude. When these values do not agree, and if ringopening has been carefully excluded, the likelihood of covalent hydration must be considered. Equilibria encountered during the determination of the ionization constant of a hydrating heteroaromatic base are shown in the following diagram. Similar equilibria exist for... 

Most of the chemistry of PA is determined by its acidic nature. It is a strong acid whose ionization constant of 1.6 x 10"1 (Ref 31) makes it comparable in acid strength to pyrophosphoric acid and trichloroacetic acid. PA readily forms salts with bases and esters with alcohols. The salts are known as Picrates. Many of them are expl and will be described in a separate article in this Vol. The esters are phenol ethers, eg, Trinitro-anisolc (see Vol t, A450-L)... 

LFER. Consider the rate of base hydrolysis of a series of ethyl benzoates given by d[C2HsOH]/dt = LfdALQ COOQHsHOH"]. Show that a plot of log Jfcx versus log Ka, where Ka is the ionization constant of the parent benzoic acid, should be linear, and relate its slope to that given by a conventional Hammett plot of log k versus a. 

Hydroxyl radicals. The acid ionization constant of the short-lived HO transient is difficult to determine by conventional methods but an estimate can be made because HO, but not its conjugate base, O -, oxidizes ferrocyanide ions HO + Fe(CN) — OH- + Fe(CN)g . Use the following kinetic data26 for the apparent second-order rate constant as a function of pH to estimate Ka for the acid dissociation equilibrium HO + H20 =... 

None of the other reactions so far discussed involve interaction between a pair of charged species. This is but another instance of the electrostatic effect shown by Kirkwood and Westheimer to be responsible for the disparity between the first and second ionization constants of dibasic acids, for the effect of the carboxylate ion on the basicity of an a-amino acid, and for the difference in reactivity of ionic compounds compared with analogous nonionic species in acid- or base-catalyzed reactions. ... 

Figure 3.3 The six apparent ionization constants of vancomycin plotted as a function of weight % methanol. Unfilled circles denote acid groups, and filled circles denote basic groups. Acids usually are indicated by positive slopes and bases, by negative slopes. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

The cosolvent will lower the dielectric constant of the mixed solvent, independent of the properties of the solute molecule. The ionization constant of acids will increase and that of bases will decrease (see Sections 3.3.3 and 3.3.4), the result of which is to increase the fraction of uncharged substance in... 

Yang et al. [12] determined the ionization constants of primaquine by a titrimetric method and studied its coordination ratio with vitamin C. The ionization constants of primaquine in 50% (v/v) ethanol in water determined at 25 °C in the ionic strength range of 5 x 10 to 5 x 10-2 mol/L are given. The coordination ratio of primaquine to vitamin C is determined by continuous variation and mole ratio methods based on pH and conductance measurements to be 1 1, indicating that the coordination compound formed in the solution is mainly a 1 1 compound. 

Dissociation constant of silicic acid calculated according to the a + = [(Kacx Kw)/c]1/2fbrmula for dissociation of salts formed from weak acid and strong base a+ is the activity of protons (from pH), K w is the ionization constant of water, and c is the concentration of silicate solution. 

Sarmini, K., and Kenndler, E. (1999). Ionization constants of weak acids and bases in organic solvents. /. Biochem. Biophys. Methods 38, 123—137. 

Figure 20.1. Extrapolation of K values in the determination of the ionization constant of acetic acid at 25°C. Based on data from H. S. Hamed and R. W. Ehlers, J. Am. Chem. Soc. 54, 1350 (1932).

Strong et al. [8] have determined the ionization constant of benzoic acid in H2O as a function of ternperamre by conductance methods. Their data are listed in Table 20.3, with the pK based on a hypothetical standard state of 1 mol dm. Temperature was controlled to +0.002°C. 

The first equation defines the ionization constant of water at 25 °C (we omit the sign of the charges to simplify notation). The second is the same as Eq. (2.6.3), while the third is the conservation of the total initial concentration of the (weak) acid Nj- (we assume that there is no change in volume during the titration, hence this is the same as the conservation of the total number of acid molecules). The fourth equation is the electroneutrality condition, where [iV ] is the concentration of the added (strong) base. 

More important and more widely applicable criteria of protonation sites are available in the effects of more distant substituents, in particular meta and para in benzene derivatives, which are as a rule purely electronic in origin. Exceptionally, they may be complicated by steric effects (e.g., in polysubstituted derivatives). The most widely useful approach is that of Hammett (1940), who suggested that the effects of substituents on the ionization constants of benzoic acid may be taken as a measure of their effectiveness in other systems involving other reaction centres and in reactions other than acid-base equilibria. He thus defined substituent constants, a, by the equation... 

The basic ionization constants of a large number of 2,4-diaminopyrimidines and condensed pyrimidine derivatives have recently been measured and earlier values verified (Roth and Strelitz, 1969). The first protonation occurs at the N-1 ring atom (pAT = 7 40) and the effect of 5-substituents is correlated quite well with -constants for the -t-M substituents (p = 4 85). The —M substituents (—NO2, —CN) show an enhanced base-weakening effect, which may be accounted for in terms of the direct conjugation shown in [65]. This provides additional evidence for protonation at N-1. 

The rate coefficient (k2 enc) for the collision of two species is given by 8RT/3Z (where Z is the viscosity of the medium at the reaction temperature), the Smoluchowski equation. This is the maximum possible rate of reaction, which is controlled by the rate at which the two reacting species diffuse together. For nitration in >90% H2S04, where nitric acid is completely ionized, if exclusively the free base nitrates the rate coefficient (k2 fb) would equal k2 obs KJhx (where Ka is the ionization constant of the base, and hx the acidity function that it follows). Thus, if k2 fb> k2 enc free base nitration is precluded, but if... 

The ionization constants of a number of 1,2,4-thiadiazoles have been determined potentiometrically87,88 or by Hammett s method205 based on the measurement of ultraviolet absorption spectra in media of different hydrogen ion concentration.126 The results are given in Table VTI. 2- and 4-Aminopyrimidine differ in their basicities (pA ... 

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