Proton acidity, solvation effects

The accurate determination of gas-phase basicities and gas-phase acidities opened the way to analyses of the effect of solvation on proton acidities, and on hydrogen-bond acidities and basicities, as well as on substituents effects. 

We can appreciate that ionization of the carboxylie acid is affected by the electron-withdrawing inductive effect of the ammonium residue hence the increased acidity when compared with an alkanoic acid. Similarly, loss of a proton from the ammonium cation of the zwitterion is influenced by the electron-donating inductive effect from the carboxylate anion, which should make the amino group more basic than a typical amine. That this is not the case is thought to be a solvation effect (compare simple amines). 

The early work on gas-phase acidities and basicities (Brauman and Blair, 1970 Brauman et al., 1971), obtained by probing the preferred direction of proton transfer reactions in the gas phase, established a benchmark in physical organic chemistry. The results pointed out, for example, that polarizability arguments could satisfactorily account for the smooth trends observed within a homologous series, unlike the -values in condensed media where solvation effects may reverse the relative orders of acidities and basicities. 

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 SM2/AM1 model performed well in predicting free energies of hydrata-tion for the substituted benzenes [65], The use of the SM2/AM1 model to assess solvation effects allowed to propose a mechanism for the cis-trans isomerization of diazene, which proved consistent with experimental observations [66], The results obtained indicated that trace amounts of acid can rapidly protonate tram... 

The mechanistic subtypes presented throughout this book include those related to the acid-base properties of organic molecules. These are protonations, deprotonations, and proton transfers. Mechanistic types based on solvation effects include solvolysis reactions, SN1, and El processes. Additional mechanisms utilizing ionic interactions include SN2, SN2, E2, 1,2-additions, 1,4-additions, and addition-elimination processes. Finally, those mechanistic types dependent upon the presence of cationic species include alkyl shifts and hydride shifts. 

To understand differences in acidity of zeolites one has to consider not only changes in the bond strength of the OH bond, but also zeolite solvation effects on the positively charged carbenium or other ions generated by protonation. 

American 243, 148 (1980) Chemische Reaktionen ohne Ldsungsmittel, Spektrmn der Wissenschaft, January 1981, p. 27ff. [116] R. W. Taft Protonic Acidities and Basicities in the Gas Phase and in Solution Substituent and Solvent Effects, Progr. Phys. Org. Chem. 14, 247 (1983). [117] C. R. Moylan and J. 1. Brauman Gas-Phase Acid-Base Chemistry, Annu. Rev. Phys. Chem. 34, 187 (1983). [118] P. Kebarle Ion Thermochemistry and Solvation from Gas-Phase Ion Equilibrium, Annu. Rev. Phys. Chem. 28, 445 (1977). [119] D. K. Bohme, E. Lee-Ruff, and L. B. Young, J. Am. Chem. Soc. 94, 5153 (1972) D. K. Bohme in P. Ausloos (ed.) Interactions between Ions and Molecules, Plenum, New York, 1974, p. 489ff. [120] M. J. Pellerite and J. I. Brauman Gas-Phase Acidities of Carbon Acids, in E. Buncel and T. Durst (eds.) Comprehensive Carbanion Chemistry, Part A, p. 55ff., Elsevier, Amsterdam, 1980. 

The concept of acidity and basicity in mixed solvents is discussed and a method for analyzing differential solvation effects is described. This enables the free energy of transfer of the proton between water and the mixed solvent, AGt°(H+), to be calculated, and thereby AGt°fi) for i = X" and M+, using values for AGt°(HX) and AGt°(MX). The pKa values for acids are combined with AGt°(H+) to calculate proton affinities in mixed solvents, and these are used as measures of free energies of transfer of the charges on the molecular species, AGt°(i)e. Values of AGt°(i) and AGt°(i)e are compared for a range of co-solvents and the factors influencing the way these quantities vary with solvent composition are discussed. 

It is a serious drawback that it is not possible to determine the transfer activity coefficient of the proton (or of any other single-ion species) directly by thermodynamic methods, because only the values for both the proton and its counterion are obtained. Therefore, approximation methods are used to separate the medium effect on the proton. One is based on the simple sphere-in-continuum model of Born, calculating the electrostatic contribution of the Gibb s free energy of transfer. This approach is clearly too weak, because it does not consider solvation effects. Different ex-trathermodynamic approximation methods, unfortunately, lead not only to different values of the medium effect but also to different signs in some cases. Some examples are given in the following log yH+ for methanol -1-1.7 (standard deviation 0.4) ethanol -1-2.5 (1.8), n-butanol -t-2.3 (2.0), dimethyl sulfoxide -3.6 (2.0), acetonitrile -1-4.3 (1.5), formic acid -1-7.9 (1.7), NH3 -16. From these data, it can be seen that methanol has about the same basicity as water the other alcohols are less basic, as is acetonitrile. Di-... 

These equations factor all the acid-base types in a series to that where p = = 1. For example, if triethylamine and diaminoethane monocation are to be interpreted in the same correlation they need corrections because the former has p = 1 and q = 1 whereas the latter has p = 2 and q = 1. The assignment of p and q is not satisfactory for acid-base pairs with uncertain structures (such as H3O and HO ) or species with multiple centres of differing reactivity hydronium and hydroxide ions almost always show anomalous reactivities in corrected Bronsted correlations, but these are partly due to solvation effects. The protons attached to a single atom such as ammonium (R-NHj or oxonium ion (R-OH2 ) are regarded as having p = 1 rather than the number of identical protons. A similar convention selects the q value for hydroxide ion as unity even though there are three lone pairs free to accept a proton. The statistical... 

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