Solvent Effects on Acid-Base Reactions

Salomaa, P., Hakala, R., Vesala, S., and Aalto, T., 1969, Solvent deuterium isotope effects on acid-base reactions. Part III. Relative acidity constants of inorganic oxyacids in light and heavy water. Kinetic applications Acta Chemica Scandinavica, v. 23, p. 2116-2126. 

Solvent effects on acid-base equilibria are naturally most marked when the solvent itself enters into the equilibrium, as is the case for the conventional definition of acid strength by means of the equilibrium A-fSH B4-SH2 (where SH is the solvent). The existence of such an equilibrium implies that the solvent has some basic properties. Similarly, the occurrence of the reaction B + SH A-f S (where S is the anion derived by abstracting a proton from the solvent) implies that the solvent is acidic. The most important factor determining qualitative behaviour in a wide range of solvents is the acidic or basic nature of the solvent, as determined by its chemical nature. In a preliminary classification we can neglect other factors, notably the effect of dielectric constant on the association of ions or the forces between them. 

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

Reaction of CO2 with bases - either as solvent or solute - is by far the most significant effect on CO2 solubility in aqueous media. The equilibria are well-known for aqueous solutions (1, 2, 3, 7), but little data has been systematically compiled on acid-base reactions of CO2 in nonaqueous solutions (see Introduction). 

The solvent effect on proton-transfer reactions is determined by two effects In cases of slow proton transfers, which are not diffusion-controlled, e.g. reactions between carbon acids and weak bases the solvent effect is determined by hydrogen-bonding and formation of ion pairs whereas tte viscosity of the solvent prevails in very fast diffusion-controll l reactions. 

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

Ethers are unreactive to many reagents used in organic chemistry, a property that accounts for their wide use as reaction solvents. Halogens, dilute acids, bases, and nucleophiles have no effect on most ethers. In fact, ethers undergo only one reaction of general use—they are cleaved by strong acids. Aqueous HBr and HI both work well, but HCl does not cleave ethers. 

The major disadvantage of the HSAB principle is its qualitative nature. Several models of acid-base reactions have been developed on a quantitative basis and have application to solvent extraction. Once such model uses donor numbers [8], which were proposed to correlate the effect of an adduct on an acidic solute with the basicity of the adduct (i.e., its ability to donate an electron pair to the acidic solute). The reference scale of donor numbers of the adduct bases is based on the enthalpy of reaction. A//, of the donor (designated as B) with SbCb when they are dissolved in 1,2-dichloroethane solvent. The donor numbers, designated DN, are a measure of the strength of the B—SbCb bond. It is further assumed that the order of DN values for the SbCb interaction remains constant for the interaction of the donor bases with all other solute acids. Thus, for any donor base B and any acceptor acid A, the enthalpy of reaction to form B A is ... 

For nonquantitative -C-NMR techniques, this could be the extent of the analysis possible. However, the combination of F-NMR with C-NMR allowed us to quantitatively calculate the isomer composition and to investigate solvent effects on isomer formation. Figure 17.5 illustrates these concepts. Two possible isomers (structures in Figure 17.5) can be formed from the reaction of 3-fluorophthalic anhydride with 4-fluoroaniline. Upon formation of the amic acid based on 3-fluorophthalic anhydride with 4-fluoroaniline, two isomers were found in both NMP and chloroform reactions as shown by the F-NMR spectra in Figure 17.5a and b, respectively. Two signals were observed for each type of fluorine atom, labeled as Fi and F2 for the anhydride and amine fluorine atoms respectively. Ortho and meta isomers were formed in a ratio of 4.75 1 in solution in NMP, while the same ratio was 1.04 1 in chloroform, where the product precipitated. The major isomer was the ortho in each case as determined by C-NMR of the chloroform prepared amic acid (Table 17.1). 

This book was written to provide readers with some knowledge of electrochemistry in non-aqueous solutions, from its fundamentals to the latest developments, including the current situation concerning hazardous solvents. The book is divided into two parts. Part I (Chapters 1 to 4) contains a discussion of solvent properties and then deals with solvent effects on chemical processes such as ion solvation, ion complexation, electrolyte dissociation, acid-base reactions and redox reactions. Such solvent effects are of fundamental importance in understanding chem-... 

The classification of solvents has been dealt with in various books on non-aque-ous solvents [25, 26]. In the classification of solvents, it is usual to use some solvent properties as criteria. In order to discuss solvent effects on chemical reactions, it is convenient to use relative permittivities and acid-base properties as the criteria. 

The characteristics of acid-base reactions in dipolar aprotic solvents, compared to those in dipolar amphiprotic solvents, are the easy occurrence of homo- and heteroconjugation reactions [2, 3, 5]. However, before discussing the homo- and heteroconjugations, we first discuss the solvent effects on the acid dissociation constants in dipolar aprotic solvents. 

A treatment partially based on the Bunnett-Olsen one is that of Bagno, Scorrano, and More O Ferrall,95 which formulates medium effects (changes in acidity of solvent) on acid-base equilibria. An appropriate equilibrium is chosen as reference, and the acidity dependence of other reactions compared with it, by use of the linear free-energy equation... 

We are concerned here with the solvent effects on the equilibrium behavior of acids and bases in dipolar aprotic solvents, such as monoglyme in x = 10, 30, and 50 mass percent mixed solvent compositions. The following reaction is of particular interest in such a study ... 

Water has a limiting effect on the strength of acids and bases. All strong acids behave the same in water 1—M solutions of the strong acids all behave as 1 M solutions of the HsO+ ion and very weak acids cannot act as acids in water. Acid-base reactions don t have to occur in water, however. When other solvents are used, the full range of acid-base strength shown in the table below can be observed. 

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