Weak bases defined

When placed in water, some acids and bases completely ionize, or dissociate into their ions—but others do not. Acids and bases that completely ionize are called strong acids and strong bases. Strong acids are defined as acids that have a pH of 0-4. Strong bases have pH values of 10-14. On the other hand, weak acids and weak bases do not disassociate completely in water. This leads to a pH value that is closer to neutral, because some of the hydrogen ions are still attached to other atoms, decreasing the hydrogen ion concentration. 

It is convenient to summarize the various reactions in a box diagram, such as Fig. 4.1 [17,275,280], illustrated with the equilibria of the weak base, propranolol. In Fig. 4.1 is an equation labeled pA °et. This constant refers to the octanol pKa, a term first used by Scherrer [280]. When the concentrations of the uncharged and the charged species in octanol are equal, the aqueous pH at that point defines p which is indicated for a weak acid as... 

Figure 5.1 shows a tetrad of equilibrium reactions related to the partitioning of a drug between an aqueous environment and that of the bilayer formed from phospholipids. (Only half of the bilayer is shown in Fig. 5.1.) By now, these reaction types might be quite familiar to the reader. The subscript mem designates the partitioning medium to be that of a vesicle formed from a phospholipid bilayer. Equations (4.1)-(4.4) apply. The pAi m in Fig. 5.1 refers to the membrane pKa. Its meaning is similar to that of pAi when the concentrations of the uncharged and the charged species in the membrane phase are equal, the aqueous pH at that point defines pAi em, which is described for a weak base as...

H0 is defined so as to be similar to pH, and to reduce to it in dilute solution, i.e. to pH = pKa — log I. The idea is that versions of equation (8) can be written for weak base indicators that protonate to different extents in the same add solutions (overlapping indicators indicators because they indicate the solution acidity) subtracting two of these (say for indicators A and B) leads to equation (9), and if the activity coefficients for A and B, and for AH+ and BH+, approximately cancel, the value of pA bh+ can be calculated from the measured ionization ratios for A and B if pAah+ is known ... 

It was soon realized that there are problems with this approach.24,25 Log ionization ratios for weak bases that are not primary aromatic amines, while linear in H0, do not give the unit slope required by equation (8). This soon led to many other acidity functions, defined for other types of weak base, HA for amides,24 Hq for tertiary aromatic amines,25 C0 or HR for carbocations,26,27 and so on. In a recent review of addity functions,28 28 different ones were listed... 

A buffer comprises (1) a weak acid and a salt of that acid, (2) a weak base and a salt of that base, or (3) it may contain an acid salt. We define an acid-base buffer as a solution whose pH does not change after adding (small amounts of) a strong acid or base . Sodium ascorbate is a favourite buffer in the food industry. 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

In these equations, pIQ is defined as the negative logarithm of IQ. The equivalent expression, derived from Equation (5.25), for a weak base would be... 

A drug that is a weak base can be defined as a neutral molecule that can form a cation (a positively charged molecule) by combining with a proton. For example, pyrimethamine, an antimalarial drug, undergoes the following association-dissociation process ... 

Being the opposite of an acid, a base will be defined as a compound that has a tendency to combine with protons. In this definition the base9 in an alkaline solution is the OH ion. This ion is one of the strongest bases known to exist. When combining with a proton it forms water, that itself is a weak base, since it is able to add one more proton to form an OHJ, hydronium ion, and a weak acid at the same time, since water can dissociate into OH and H+ ions. Water being a base, too, the actual dissociation reaction will be... 

The acidity of a solvent that protonates the weak base, B, is defined as the Hammett acidity function ... 

The acidity constants of protonated ketones, pA %, are needed to determine the free energy of reaction associated with the rate constants ArG° = 2.3RT(pKe + pK ). Most ketones are very weak bases, pAT < 0, so that the acidity constant K b cannot be determined from the pi I rate profile in the range 1 < PH <13 (see Equation (11) and Fig. 3). The acidity constants of a few simple ketones were determined in highly concentrated acid solutions.19 Also, carbon protonation of the enols of carboxylates listed in Table 1 (entries cyclopentadienyl 1-carboxylate to phenylcyanoacetate) give the neutral carboxylic acids, the carbon acidities of which are known and are listed in the column headed pA . As can be seen from Fig. 10, the observed rate constants k, k for carbon protonation of these enols (8 data points marked by the symbol in Fig. 10) accurately follow the overall relationship that is defined mostly by the data points for k, and k f. We can thus reverse the process by assuming that the Marcus relationship determined above holds for the protonation of enols and use the experimental rate constants to estimate the acidity constants A e of ketones via the fitted Marcus relation, Equation (19). This procedure indicates, for example, that protonated 2,4-cyclohexadienone is less acidic than simple oxygen-protonated ketones, pA = —1.3. 

To answer this question one must think in terms of fundamental thermodynamic principles. Thus, it can be expected that the reaction between a strong acid and a strong base should be highly exothermic (releasing a lot of heat), while that between a weak acid and a weak base is little exothermic. If a constant a is defined as a measure of the tendency of a binary oxide to accept an O2- ion (i.e., its acidity in terms of Lux and Flood), it would be reasonable to expect that for a reaction between an acidic oxide, A, and a basic oxide, B, the A B of the reaction between A and B to produce an oxysalt, C (i.e., A + B = C), is proportional to their difference in acidity. 

The Bronsted theory of acids and bases defines an acid as a proton donor and a base as a proton acceptor, i.e. a pro tic acid such as hydrochloric acid is a source of protons. Although the idea of an acidic hydrogen in organic compounds may initially be understood in terms of a carboxyl hydroxyl group, a hydrogen atom may become weakly acidic in a number of other circumstances, e.g. when it is attached to a carbon atom that is adjacent to a carbonyl group. On the other hand, a base such as an amine, or a carboxylate anion, is capable of accepting a proton. 

This is in accordance with the general properties and isomorphic relations of the acid and its salts, but such a formula indicates a deea-basic acid, whereas, with the exception of an ill-defined mercury salt (see p. 239), only hexabasicsalts, M 3H4[H2(W207)g], have been obtained. Even with such a weak base as dimethylpyrone, the salt produced is... 

A strong base is a substance that reacts completely with water, so that the base ionization constant defined in Eq. (14) or (15) is effectively infinite. This situation can only be realized if the conjugate acid of the strong base is very weak. A weak base will be characterized by a base ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (12) favors the existence of unreacted free base. 

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