Conjugate bases basicity

Because of the conjugate relationship between acidity and basicity the strongest acid (HI) has the weakest conjugate base (I ) and the weakest acid (HF) has the strongest conjugate base (F )... 

Procedures to compute acidities are essentially similar to those for the basicities discussed in the previous section. The acidities in the gas phase and in solution can be calculated as the free energy changes AG and AG" upon proton release of the isolated and solvated molecules, respectively. To discuss the relative strengths of acidity in the gas and aqueous solution phases, we only need the magnitude of —AG and — AG" for haloacetic acids relative to those for acetic acids. Thus the free energy calculations for acetic acid, haloacetic acids, and each conjugate base are carried out in the gas phase and in aqueous solution. 

When a Br nsted plot includes acids or bases with different numbers of acidic or basic sites, statistical corrections are sometimes applied in effect, the rate and equilibrium constants are corrected to a per functional group basis. If an acid has p equivalent dissociable protons and its conjugate base has q equivalent sites for proton addition, the statistically corrected forms of the Br insted relationships are... 

A biologically important point is revealed by the basic shape of the titration curves of weak electrolytes in the region of the pK, pH remains relatively unaffected as increments of OH (or H ) are added. The weak acid and its conjugate base are acting as a buffer. 

The methods outlined, of course, are readily applicable to a wide variety of substituted heterocycles like the carboxyl, hydroxy and mercapto derivatives of pyridines, pyridine 1-oxides, pyrroles, etc. The application to amines and to diaza compounds such as pyrimidine, where the two centers are basic, is obvious except that now 23 takes the role of the neutral compound, 21 and 22 the roles of the tautomeric first conjugate bases, and 20 the role of the second conjugate base. Extensions to molecules with more than two acidic or basic centers, such as aminonicotinic acid, pyrimidinecarboxylic acids, etc., are obvious although they tend to become algebraically cumbersome, involving (for three centers) three measurable Kg s, four Ay s, and fifteen ideal dissociation constants (A ), a total of twenty-two constants of which seven are independent. 

As pointed out in Section 13.5, anions that are the conjugate bases of weak acids act themselves as weak bases in water They accept a proton from a water molecule, leaving an Oil ion that makes the solution basic. The reactions of the fluoride and carbonate ions are typical ... 

The typical strong acid of the water system is the hydrated proton H30+, and the role of the conjugate base is minor if it is a sufficiently weak base, e.g. Cl-, Br-, and C104. The conjugate bases have strengths that vary inversely as the strengths of the respective acids. It can easily be shown that the basic ionisation constant of the conjugate base KR canj is equal to Kw/KA conj, where Kw is the ionic product of water. 

On the above basis it is, in principle, unnecessary to treat the strength of bases separately from acids, since any protolytic reaction involving an acid must also involve its conjugate base. The basic properties of ammonia and various amines in water are readily understood on the Bronsted-Lowry concept. 

If p is the number of equivalent acidic protons in BH+, and q the number of equivalent basic sites in its conjugate base B, then a more precise form for the Brpnsted relation is... 

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 ... 

All anions that are the conjugate bases of weak acids produce basic solutions. For example, formic acid, HCOOH, the acid in ant venom, is a weak acid, and so the formate ion acts as a base in water ... 

To determine whether the solution of a salt will be acidic, basic, or neutral, we must consider both the cation and the anion. First we examine the anion to see whether it is the conjugate base of a weak acid. If the anion is neither acidic nor basic, we examine the cation to see whether it is an acidic metal ion or the conjugate acid of a weak base. If one ion is an acid and the other a base, as in NH4F, then the pH is affected by the reactions of both ions with water and both equilibria must be considered, as in Section 10.19. 

STRATEGY The CH3C02 ion is the conjugate base of a weak acid so the solution will be basic and we expect pH > 7. Use the procedure in Toolbox 10.2, taking the initial con-... 

Salts that contain the conjugate acids of weak bases produce acidic aqueous solutions so do salts that contain small, highly charged metal cations. Salts that contain the conjugate bases of weak acids produce basic aqueous solutions. 

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 ... 

One common indicator is phenolphthalein (Fig. 11.10). The acid form ol this large molecule (3) is colorless its conjugate base form (4) is pink. The structure of the base form of phenolphthalein allows electrons to be delocalized across all three of the benzenelike rings of carbon atoms, and the increase in delocalization is part of the reason for the change in color. The pFCIn of phenolphthalein is 9.4, and so the end point occurs in slightly basic solution. Litmus, another well-known indicator, has pkln = 6.5 it is red for pH < 5 and blue for pH > 8. 

The oxidation is slow in acidic solution but rapid in basic solution, where insoluble iron(III) hydroxide, Fe(OH)3, is precipitated. Although [Fe(H20)6]3+ ions are pale purple and Fe3 1 ions give amethyst its purple color, the colors of aqueous solutions of iron(III) salts are dominated by the conjugate base of [Fe(H20)g]3+, the yellow [Fe0H(H20)d2+ ion ... 

B (a) C032 is the conjugate base of HCO,, which is a weak acid therefore, the solution is basic. 

The second oxidation, which is normally slower than the first (which is why sulfoxides are so easily isolable), has the same mechanism in neutral or acid solution, but in basic solution it has been shown that the conjugate base of the peroxy compound (R 00 ) also attacks the SO group as a nucleophile " ... 

An aqueous solution of a soluble salt contains cations and anions. These ions often have acid-base properties. Anions that are conjugate bases of weak acids make a solution basic. For example, sodium fluoride dissolves in water to give Na, F, and H2 O as major species. The fluoride anion is the conjugate base of the weak acid HF. This anion establishes a proton transfer equilibrium with water ... 

It may surprise you that the pH at the stoichiometric point is not 7.0, but this is a reasonable outcome. The major species present at the stoichiometric point are water and a conjugate base, so the resulting solution is basic in nature. 

The solution of a salt derived from a strong base and weak acid is basic because the anion of a weak acid reacts with water (hydrolysis) to form hydroxide ions. Consider the soluble salt NaCIO found in chlorine bleaches prepared by reacting NaOH, a strong base, and HC10, a weak acid. The salt dissociates completely in water and the conjugate base of the weak acid, CIO-, hydrolyzes, producing OH- ions. 

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