Entropy of ionization

It IS a curious fact that substituents affect the entropy of ionization more than they do the enthalpy term m the expression... 

Table 10 p bh+ values, m slopes, enthalpies and entropies of ionization for some amides in aqueous perchloric acid."... 

Free energies, enthalpies, and entropies of ionization for a number of reducing sugars have been determined by Christensen and co-workers (13). Their data, however, are for equilibrium mixtures of anomers rather than for individual anomeric forms. 

From our earlier discussion, you might expect that the dissociation of a proton from a carboxylic acid, which increases the number of independent particles, would lead to an increase in entropy. However, this effect is more than counterbalanced by solvation effects. The charged anion and proton both freeze out many of the surrounding molecules of water (fig. 2.4). Thus the ionization of a weak acid decreases the number of mobile molecules and so leads to a decrease in entropy. The entropy of ionization of a typical carboxylic acid in water is about —22 eu/mole. The entropy of dissociation of a proton from a quaternary ammonium... 

The entropies of ionization vary over a wide range with changes in R. Nevertheless current chemical theory has been very successful in relating the strengths of such acids to chemical structure. How can we account for this apparent anomaly ... 

The difference in the pKa values of m- and p-nitrophenol in aqueous medium has aroused interest. In earlier work (Hepler et al., 1965), the enthalpies of ionization were found to be almost identical, and the difference in pATa was attributed to an entropy effect. Since m-nitrophenol had the more negative entropy of ionization, this appeared to be consistent with a greater degree of solvent orientation around its conjugate base as expected since the negative... 

Solvent Effect on Enthalpies and Entropies of Ionization for 31a and 32a, and Ionization Barriers in CD2C1266... 

Equilibrium constants were obtained for each temperature from the temperature-dependent 29Si chemical shifts, leading to the reaction enthalpy and entropy of ionization for 66b —6.3 kcal mol-1 and —21 cal mol-1 deg-1, respectively. These values are in agreement with the reversible... 

Ionization is exothermic and favored at lower temperatures in systems such as trityl, alkoxycarbenium, and benzhydryl derivatives which generate stabilized carbenium ions due to their electron-donating substituents. The exothermicity of ionization of benzhydryl species with BCl3 in CH2CI2 decreases from AH = -62 kJ/mol for di(p-anisyl) to -22 kJ/ mol for less stabilized p-tolyl and phenyl derivatives, and to approximately -8 kJ/mol for unsubstituted benzhydryl chloride [193]. These values, based on the solvolysis rates, extrapolate to AH = -12 kJ/mol for cumyl chloride (HCl adduct of a-methylstyrene) and AH = 0 kJ/mol for 1-phenyl-ethyl chloride (styrene adduct) and ( -butyl chloride (isobutene adduct) [240]. The reported entropies of ionization do not vary significantly and... 

Larson, J.W., and Hepler, L.G. Heats and entropies of ionization, p, 1-44, jn Coetzee, J.F., and Ritchie, C.D., editors, "Solute-Solvent Interactions", Marcel Dekker,... 

The enthalpy change of some reactions can be measured directly, but for those that do not go to completion (as is common in acid-base reactions), thermodynamic data from reactions that do go to completion can be combined using Hess s law to obtain the needed data. For example, the enthalpy and entropy of ionization of a weak acid, HA, can be found by measming (1) the enthalpy of reaction of HA with NaOH, (2) the enthalpy of reaction of a strong acid (such as HCl) with NaOH, and (3) the equilibrium constant for dissociation of the acid (usually determined from the titration curve). 

How large a change in energy (or enthalpy) of reaction is needed for an acid ionization to lower the pfC by one unit at 25°C Assume that the entropy of ionization is unchanged. 

Enthalpies and entropies of ionization of water and hydrogen cyanide, Aust. J. Chem., 49, (1996), 651-657. Cited on page 435. 

Linear enthalpy-entropy compensation is well known to physical organic chemists and has been the subject of controversy since the relationship was first discovered experimentally. We have discussed the complications elsewhere and will only note here that the linearity found by Beetlestone et al. is statistically reliable for most of their examples. The most extensively studied set of small-solute compensation processes in water are the ionizations of weak acids. When acids such as acetic acid or benzoic acid are substituted in their nonpolar parts to form homologous series, the standard enthalpies and entropies of ionization are found to demonstrate compensation behavior with 7], values in the 280-290°K range but only after extraction of all the contributions to these quantities from the electronic rearrangements using methods developed by Hepler and Ives and their coworkers. The obvious conclusion is that this behavior in small-solute processes is due to solvation effects and thus a manifestation of some property of water. As a result of the comparison of their data with these small-solute examples, Beetlestone et al. suggested that bulk water also plays an important role in the protein processes they studied. 

Hitherto, the many studies of ionization processes in solution have been confined mainly to determination of dissociation constants, from which free energies of ionization can be calculated. Relatively few investigations have been concerned with heats and entropies of ionization. Some heats and entropies of ionization have been calculated from electrochemical data by Everett and Wynne-Jones and by Harned and Owen. > There have been few data available for members of homologous series of compounds. 

From the pk values f<3r these acids, the free energies, A O, may be calculated, and using the relationship AG AH TAS, the entropies of ionization, AS, may be derived. Such data for some aliphatic acids are shown in TabU 1. 

We turn now to the ionization process of a series of methyl mono-substituted ring systems pyridines, anilines, phenols, and substituted benzoic acids. Table 2 shows the free energies, A G, the heats, A H, and entropies of ionization, A S, for a number of these compounds. The compounds fall into two classes, according to the different ionic nature of the ionization process. The pyridine and aniline ionizations are of the type... 

Free energies, heats, avd entropies of ionization, methyl mono- substitnied compounds ... 

Fig. 1. Energy diagram of tiio heats and entropies of ionization of phenol and w-cresol...

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