Acid-dissociation constant defined

The numerical values of the terms a and p are defined by specifying the ionization of benzoic acids as the standard reaction to which the reaction constant p = 1 is assigned. The substituent constant, a, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The a values so defined are used in the correlation of other reaction series, and the p values of the reactions are thus determined. The relationship between Eqs. (4.12) and (4.14) is evident when the Hammett equation is expressed in terms of fiee energy. For the standard reaction, o%K/Kq = ap. Thus,... 

Another approach to evaluating cri was taken by Roberts and Moreland,who defined inductive substituent constants in terms of the acid dissociation constants of 4-substituted bicyclof2.2.2]octane-l-carboxylic acids, 3. 

Consider a neutral base B of such strength that it can be protonated in dilute aqueous solution in the acidic range, say pH 1-2. In the conventional manner the acid dissociation constant /ibh + is defined. 

The first acid dissociation constant of sulfurous acid in aqueous solution is therefore defined as ... 

Numbering the particles in (87) from left to right, the conventional acid dissociation constant is defined at infinite dilution by... 

The acid dissociation constant Ka is defined as the equilibrium constant for this reaction ... 

The dioxo complexes of W(IV) and Mo(IV), having high pKa values (Table II), are formed via hydrolysis as the rate-determining step (Scheme 4) and the observed rate constants for the inversion along the O-M-O axis for the W(IV) and the Mo(IV) complexes are therefore defined by Eq. (18). These were calculated as a function of pH, using the proton exchange rate constants (Table IV) and the acid dissociation constants (Table II)... 

It is possible to compare the strengths of weak acids by the values of their acid dissociation constants Ka. Figure 3.1 shows the titration curves for acids (HA or BH+) of different Ka values. The ordinate shows poH, which is defined by paH = -loga(SI I)). paH corresponds to the pH in aqueous solutions (see Section 3.2). The poH of non-aqueous solutions can be measured with a glass pH electrode or some other pH sensors (see Sections 3.2.1 and 6.2). For the mixture of a weak acid A and its conjugate base B, poH can be expressed by the Henderson-Hassel-balch equation ... 

If a solute is an acid or base, its charge changes as the pH is changed. Usually, a neutral species is more soluble in an organic solvent and a charged species is more soluble in aqueous solution. Consider a basic amine whose neutral form, B, has partition coefficient A" between aqueous phase 1 and organic phase 2. Suppose that the conjugate acid, BH+, is soluble only in aqueous phase I. Let s denote its acid dissociation constant as Kir The distribution coefficient, D, is defined as... 

This equation shows that not only a high metal-ion concentration, but also a high pH, often favors the formation of higher polynuclear species, since y generally increases more rapidly than x. For many aqua metal ions, however, the precipitation of insoluble hydroxides sets an upper pH limit, so that in practice it is possible to study the oligomerization reactions only within a narrow pH region defined by the magnitude of the first acid dissociation constant of the monomeric aqua ion and the pH at which insoluble hydroxide formation occurs. 

Condensation to monohydroxo-bridged complexes is often described by Eq. (28), for which the equilibrium constants Kd are related to those defined by Eq. (27) by Kd = Q2i/Qh, where is the first acid dissociation constant of the mononuclear aqua ion. 

In the Bronsted relationship, the reference equilibria are dissociations of protic acids, used directly as ApK i, and the constant of proportionality r = fi (or a, see Chapter 11). In the Hammett relationship, acid dissociation constants are also used for the reference equilibria, but indirectly. The dissociation constants of substituted benzoic acids in water at 25°C are used to define a set of substituent parameters, cr, which are then used in the equation of correlation, and r = p in Equation 9.18. 

The acid dissociation constant (pKa) of an organic compound is useful in assessing its environmental fate. The pKa value can be used to define the degree of ionization of a compound at a given pH and the potential for sorption to surfaces by cation exchange. The extent to which a compound is sorbed can have a significant effect on its bioavailability, transport, photolysis, and biodegradability. 

As discussed in Section 3.10.3, in the gas phase the basicity of simple amines follows the order NMe3 > NHMe2 > NH2Me > NH3 because of the electron donating effect of the methyl (Me) groups. In solution, however, we can define a basicity constant as the equilibrium constant for the reaction shown in Equation 3.4. Note it is important to specify temperature, solvent (usually water) and solution ionic strength, 1 Basicity constants are related to the acid dissociation constants (/Q of the base s conjugate acid via the dissociation constant of water, K = 10 14 at 25 °C. Thus Kbx K = Kw. 

Before discussing the effect of pH on protein-ligand equilibria, it is necessary to discuss an aspect of acid dissociations that was too advanced for Chapter 1. Consider a protein A that has two acid groups. The acid dissociation constants are defined by... 

Because they span such a wide range, acid-dissociation constants are often expressed on a logarithmic scale. The pKa of an acid is defined just like the pH of a solution as the negative logarithm (base 10) of Ka. 

The acidities of alcohols vary widely, from alcohols that are about as acidic as water to some that are much less acidic. The acid-dissociation constant, Ka, of an alcohol is defined by the equilibrium... 

By use of the a value, an apparent acid dissociation constant, pK,pp, is defined as... 

A search [10] of the World Drug Index revealed that 62.5% of marketed drugs are ionizable, which implies that these substances can exist in various charged states depending on the pH of the media. For ionizable drugs, solubility is pH dependent, and it is therefore important to understand the solubility in the context of pH. Ionization of a compound can be defined by the acid dissociation constant, p/C,. For the case of monoprotic compounds, the solubility at a given pH can be described by the following equations ... 

Following the approach of Tanford (1961), it is useful to examine the ionization of a hypothetical diprotic acid (HA-BH). Thermodynamically, the acid-base properties of this acid are completely described by two acid dissociation constants ( fi and K2). However, at the molecular level, it is clearly more appropriate to define four dissociation constants as depicted in Equation (2) ... 

The radical difference between the classical and modern views is readily seen from equations (17) and (18). The basic character, as defined by the older theory, actually is determined by the true acidic dissociation constant based on the hybrid ion conception. Conversely, the acidic nature is governed by the true basic dissociation constant. 

A dissociation constant is a constant whose numerical value depends on the equilibrium between the dissociated and undissociated forms of a molecule. Higher the dissociation constant, greater the dissociation. Examples of dissociation constants are substrate-enzyme dissociation constant and the acid dissociation constant pfC. This latter is defined as... 

The equilibria can be quantihed by defining the apparent acid dissociation constant, p/ app, calculable by the use of the measured pH and the degree of dissociation of the functionalities, a, at equilibrium ... 

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