Constant basicity

Basicity constants are not necessary in the Brpnsted-Lowry approach Basicity is measured according to the pA of the conjugate acid The weaker the conjugate acid the stronger the base... 

As we discussed in Section 1 14 it is more useful to describe the basicity of amines m terms of the pA a s of their conjugate acids than as basicity constants Always bear m mind that... 

Basicity constant Ki, (Section 1 14) A measure of base strength especially of amines... 

Ill spite of their importance, basicity constants rarely figure in descriptions of alkaloids. Figures for a series of alkaloids and related substances were published by Kolthoff in 1925 and have been extensively used. Recently a few more have been added by Schoorl, and Adams and Mahan have provided figures for the whole group of necines, the amino-alcohols resulting from the hydrolysis of the pyrrolizidine group of alkaloids. ... 

Gas Phase (DGB) Basicities (333 K) [94JCS(P2)2341], Basicity Constants (pA bh+) Measured in Acetonitrile (25°C) (91KGS836), and MNDO PM3 Calculation [94JCS(P2)2341] PROTON AEEiNmES (PA) OF Compounds... 

Various amines find application for pH control. The most commonly used are ammonia, morpholine, cyclohexylamine, and, more recently AMP (2-amino-2-methyl-l-propanol). The amount of each needed to produce a given pH depends upon the basicity constant, and values of this are given in Table 17.4. The volatility also influences their utility and their selection for any particular application. Like other substances, amines tend towards equilibrium concentrations in each phase of the steam/water mixture, the equilibrium being temperature dependent. Values of the distribution coefficient, Kp, are also given in Table 17.4. These factors need to be taken into account when estimating the pH attainable at any given point in a circuit so as to provide appropriate protection for each location. 

Basicity constant, Kh (Section 24.3) A measure of base strength. For any base B, the basicity constant is given by the expression... 

The base properties of the amine are represented by its basicity constant (basicity dissociation constant, Kh), which identifies the amount of the amine (in moles) that is ionized (i.e., available to raise pH) in liquid water-condensate at any given temperature and pressure. The dissociation reaction for a primary amine is shown in equation 1, and the value of the dissociation constant is shown in equation 2. 

Basicity also may be regarded as a measure of amine hydrolysis, and it governs the increase in pH level, per additional unit measure of amine. The law of diminishing returns applies of course, and beyond a certain point there is little further increase in system pH. Also, the basicity constant for individual amines varies considerably with temperature, so it is important to consider Kb at the relevant operating temperature, rather than simply at ambient temperature. 

The final pH, after neutralization, is determined by the remaining amine hydrolysis (basicity constant). 

In dilute solutions, the water is almost pure and its activity can be set equal to 1. With this approximation, we obtain the basicity constant, Kb. If we make the further approximation of replacing the activities of the solute species by the numerical values of their molar concentrations, we can write the basicity constant expression for ammonia as... 

The experimental value of Kb for ammonia in water at 25°C is 1.8 X I(T5. This small value tells us that normally only a small proportion of the NH molecules are present as NH4+. Equilibrium calculations show that only about 1 in 100 molecules are protonated in a typical solution (Fig. 10.16). In general, the basicity constant for a base B in water is... 

The value of Kb tells us how far the reaction proceeds to the right. The smaller the value of Kh the weaker is the ability of the base to accept a proton. Table 10.2 lists the basicity constants of some weak bases in aqueous solution. 

Acidity and basicity constants are commonly reported as their negative logarithms, by defining... 

The proton-donating strength of an acid is measured by its acidity constant the proton-accepting strength of a base is measured by its basicity constant. The smaller the constants, the weaker the respective strengths. The larger the value of pK, the weaker the acid or base. 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

In this expression, Ka is the acidity constant of a weak acid and Kh is the basicity constant of the conjugate base of that acid. The acid and base must form a conjugate acid-base pair (such as CH C00H/CH3C02 or NH4+/NH3). We can express Eq. 1 la in another way by taking logarithms of both sides of the equation ... 

Proton transfer equilibrium is established as soon as a weak base is dissolved in water, and so we can calculate the hydroxide ion concentration from the initial concentration of the base and the value of its basicity constant. Because the hydroxide ions are in equilibrium with the hydronium ions, we can use the pOH and pKw to calculate the pH. 

For each of the following weak acids, write the proton transfer equilibrium equation and the expression for the equilibrium constant Kv Identify the conjugate base, write the appropriate proton transfer equation, and write the expression for the basicity constant Kb. (a) HC102 (b) HCN ... 

Relation between acidity and basicity constants of a conjugate acid—base pair ... 

The extended Hiickel method has been used in a discussion of properties and reactivity of radicals and biradicals (75). We have found it possible to correlate the basicity constants, pKbh. of radical anions with extended Hiickel data (76). 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

Oxadiazoles are weak Hammett bases. The basicity constants of 2,5-diphenyl-1,3,4-oxadiazole (pK -2.49) and of 2-(4-methylphenyl)-5-phenyl-l,3,4-oxadiazole (pAa -1.15) were measured by the method of Yates and MacClelland in an aqueous solution of sulfuric acid in the range from pH 7 to H0 -10. Both compounds exhibited luminescence properties depending on the acid concentration <1996SAA1875>. 

It is convenient to define the basicity constant, K]of the indicator, BH, in the micellar pseudophase as a dimensionless quantity in terms of the mole ratio of micellar bound OH- (p. 225) using (21). The quantity [OHm] (or m oh) can be calculated from the ion-exchange relation and the experimental data fitted, usually by computer simulation, following the approach discussed for treatment of rate constants (p. 229). This treatment fits micellar effects upon the deprotonation of benzimidazole for a variety of CTAX surfactants (X = Cl, Br, N03) over a range of concentrations of NaOH and of added salts (Bunton et al., 1982a). A similar, but less general approach, was also applied to deprotonation of phenols and oximes (Bunton et al., 1980c). 

The basicity constants, K] cannot be compared directly with the classical basicity constants in water, Kb, but comparison can be made by converting Wqh into moles per litre of micellar Stern layer using (22), based on the micellar volume applied to reaction rates (Section 5). 

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