Equilibrium constants acid ionization

Nernst distribution law constant Boiling point elevation constant Freezing point depression constant Equilibrium constant Acid ionization constant Michaelis-Menten constant... 

According to the Arrhenius definitions an acid ionizes m water to pro duce protons (H" ) and a base produces hydroxide ions (HO ) The strength of an acid is given by its equilibrium constant for ionization m aqueous solution... 

The carbon-metal bonds of organolithium and organomagnesium compounds have appreciable carbamomc character Carbanions rank among the strongest bases that we 11 see m this text Their conjugate acids are hydrocarbons—very weak acids indeed The equilibrium constants for ionization of hydrocarbons are much smaller than the s for water and alcohols thus hydrocarbons have much larger pA s... 

It is always important to keep in mind the relative nature of substituent effects. Thus, the effect of the chlorine atoms in the case of trichloroacetic acid is primarily to stabilize the dissociated anion. The acid is more highly dissociated than in the unsubstituted case because there is a more favorable energy difference between the parent acid and the anion. It is the energy differences, not the absolute energies, that determine the equilibrium constant for ionization. As we will discuss more fully in Chapter 4, there are other mechanisms by which substituents affect the energy of reactants and products. The detailed understanding of substituent effects will require that we separate polar effects fiom these other factors. 

Equilibrium constants for ionization reactions are usually called ionization or dissociation constants, often designated Ka. The dissociation constants of some acids are given in Figure 2-16. Stronger acids, such as phosphoric and carbonic acids, have larger dissociation constants weaker acids, such as monohydrogen phosphate (Ill Of ), have smaller dissociation constants. 

The underlying assumption in this concept is the absence of steric effects. Using the rate and equilibrium constants for ionization of benzoic acid as references, Hammett found that equilibrium constants for a variety of reactions showed a linear relationship with o and defined the LFER. Data for these equilibria are typically graphed as illustrated in Figure 5.4. 

There are some measurements of the rates of polymerization in systems with reversible formation of covalent species. The equilibrium constants of ionization can be calculated from these kinetic data according to the procedure outlined subsequently in Section IV.D.2.a. The ionization constant depends on the strength of the Lewis acid. For example, the propagating species are almost completely ionized in polymerizations of vinyl ethers with SbCL-, BCLt-, and SnCl5- counteranions, but only partially ionized when the counteranions are I3- or Zn3-. 

Because the reactivities of ions and ion pairs are similar and only weakly affected by the structure of the counteranions, kp + or kp determined by either stopped-flow studies or y-radiated systems (cf., Section IV. 13) can be used in Eq. (75). The equilibrium constant of ionization can then be estimated from the apparent rate constant of propagation and the rate constant of propagation by carbenium ions [Eq. (77)]. For example, Kf 10-s mol-,L in styrene polymerizations initiated by R-Cl/SnCl4 [148]. Kt for vinyl ether polymerization catalyzed by Lewis acids can also be estimated by using the available rate constant of ionic propagation (kp- = 104 mol Lsec-1 at 0° C) [217], The kinetic data in Ref. 258 yields Kj == 10 3 mol - l L in IBVE polymerizations initiated by HI/I2 in toluene at 0° C and Kf 10-1 mol- -L initiated by HI/ZnI2/acetone can be calculated from Eq. (76). 

The dynamics of exchange is also very important. Figure 13 shows a system nearly identical to the ideal system shown in Fig. 12. The only difference is the dynamics of ionization the concentrations of monomer, initiator, Lewis acid, and anion are the same, and the equilibrium constants of ionization and dissociation are also the same. 

Because ionization constants are equilibrium constants for ionization reactions, their values indicate the extents to which weak electrolytes ionize. At the same concentrations, acids with larger ionization constants ionize to greater extents (and are stronger acids) than acids with smaller ionization constants. From Table 18-4, we see that the order of decreasing acid strength for these five weak acids is... 

The acid dissociation constant is the equilibrium constant for ionization of an acid and is expressed in negative log units. 

Most acids are weak acids, which ionize only to a limited extent in water. At equilibrium, aqueous solutions of weak acids contain a mixture of nonionized acid molecules, ions, and the conjugate base. Examples of weak acids are hydrofluoric acid (HF), acetic acid (CH3COOH), and the ammonium ion (NH4). The limited ionization of weak acids is related to the equilibrium constant for ionization, which we will study in the next section. 

Because this reaction produces H ions, the pH of the solntion decreases. As you can see, the hydrolysis of the NHj ion is the same as the ionization of the NHj acid. The equilibrium constant (or ionization constant) for this process is given by... 

The strength of an acid is given by its equilibrium constant for ionization in aqueous solution ... 

The outstanding properly of each acid-base system is its pH curve the one shown in Fig. 2.12 is typical of a base neutralized by an acid. The shape of the curve is related to the equilibrium constants for ionization of the acid and base, and the concentrations of each of the ions. But the basis of the coordinate system is logarithmic, in that pH is defined as the negative logarithm of the hydrogen-ion concentration, in gram-ions/ liter ... 

The strength of an acid with the general formula HA is given by the equilibrium constant for ionization, which is obtained from the equation for ionization. 

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

The equilibrium constant for the overall reaction is related to an apparent equilibrium constant Ki for carbonic acid ionization by the expression... 

In a series of organic acids of similar type, not much tendency exists for one acid to be more reactive than another. For example, in the replacement of stearic acid in methyl stearate by acetic acid, the equilibrium constant is 1.0. However, acidolysis in formic acid is usually much faster than in acetic acid, due to higher acidity and better ionizing properties of the former (115). Branched-chain acids, and some aromatic acids, especially stericaHy hindered acids such as ortho-substituted benzoic acids, would be expected to be less active in replacing other acids. Mixtures of esters are obtained when acidolysis is carried out without forcing the replacement to completion by removing one of the products. The acidolysis equilibrium and mechanism are discussed in detail in Reference 115. 

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