Acids and Bases in Solvents

The order of the boiling points NH3 HF H O shows that the number of possible bonds is more important than their strength. 

Hydrogen bonding may be viewed as a sort of tentative acid-base interaction. If an HBD molecule made an outright gift of a proton, instead of just sharing it, we should call it an acid. The receiving molecule would then be acting as a base. In the next sections, we take up the consequences of this and consider the Bipnsted-Lowry 

In this scheme, an acid is any species (not substance ) that can donate a proton to another species, and a base is a species that can receive a proton (see, e.g., Stewart, 1985). Since bare protons cannot exist for any appreciable time in a liquid, acids and bases always react in mutual fashion by proton transfer  

Solvents that are amphiprotic, that is, those that can both give and receive a proton, are capable of autoprotolysis or autoionization, for example, for water  

The activities (or, approximately, concentrations) of the solvo-acid and solvo-base (solvent minus a proton) are linked through the autoprotolysis or ion-product constant  

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Eaq and Caq are the tendency of acid A and base B to undergo ionic and covalent bonding, respectively. Equation (2) resembles that proposed by Drago et al. (18) to model heats of complex formation of acids and bases in solvents of low dielectric constant. Only Lewis acids of ionic radius greater than 1.0 A obey Eq. (2). For all smaller Lewis acids, a third pair of parameters has to be introduced ... 

The concepts and equations of acid-base dissociation have referred chiefly to aqueous solutions. Recently, interest in the behavior of acids and bases in solvents other than water has increased considerably. The classical definition of an acid and a base, which is satisfactory for water solutions, is too limited for other solvents. Because of the great importance of the general question of the acid-base equilibrium, the clear and fruitful views of Bronsted are exhaustively considered in a special (fourth) chapter. ... 

Aprotic solvents include those substances which may be considered to be chemically neutral and virtually unreactive under the conditions employed. Carbon tetrachloride and benzene come in this group, they possess low dielectric constants, do not cause ionisation in solutes and do not undergo reactions with acids and bases. Aprotic solvents are frequently used to dilute reaction mixtures while taking no part in the overall process. 

In the previous Sections, the properties of acids and bases in macrocycles and other concave structures have been compared. A number of factors have been recognized which influence the acidity or basicity of an acid or base (i) hydrogen bonds, (ii) hindered solvation (exclusion of solvent), (iii) formation of tight ion pairs (high microacidity but low overall acidity), and (iv) Coulomb forces when poly anions are formed. A fifth influence, (v) steric hindrance, still has to be discussed. 

The third group of solvents comprises the hydrocarbons and their halogen derivatives. They are not of interest for electrochemistry, since the solubilities and dissociation of salts, acids, and bases in them are low. Systems with protic or aprotic polar solvents are used in practice and have been investigated widely. 

One of the problems encountered when dealing with the interaction of Lewis acids and bases in a quantitative way is in evaluating the role of the solvent. Bond energies in molecules are values based on the molecule in the gas phase. However, it is not possible to study the interaction of many Lewis acids and bases in the gas phase because the adducts formed are not sufficiently stable to exist at the temperature necessary to convert the reactants to gases. For example, the reaction between pyridine and phenol takes place readily in solution as a result of hydrogen bonding ... 

In the case of non-HBD solvents, such as DMSO, the measured pK values are absolute (that is, free from ion pairing) and can be directly compared with gas-phase acidities6 in addition, knowledge of the heats of ionization in DMSO7 allows the evaluation of a possible entropy effect when the two phases are compared. The mechanism of proton transfer between oxygen and nitrogen acids and bases in aqueous solution has been reviewed8. 

When the temperature rises, polyphenylene sulfide can be attacked by hot strong acids and bases, chlorinated solvents, oxidizing agents, halogens, amines. For example, a given grade is unusable after a few months in 10% nitric acid at 90°C. After 1 year at 90°C in 37% hydrochloric acid, the retention of tensile strength is 29%. 

Sarmini, K., and Kenndler, E. (1999). Ionization constants of weak acids and bases in organic solvents. /. Biochem. Biophys. Methods 38, 123—137. 

Table 1. Acids and Bases in the Bromine Trifluoride Solvent System.

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 ... 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

Illustrative Example 8.3 Assessing the Air-Water Distribution of Organic Acids and Bases in a Cloud Organic Solvent-Water Partitioning... 

Consider the organic solvent-water partitioning of organic acids and bases. In which cases and/or under which conditions can you neglect the partitioning of the charged species into the organic phase ... 

Although it is unlikely that S02 undergoes ionization as shown in Eq. (5.98), some reactions take place as though these ions were present. For example, because according to the solvent concept S02+ would be the acidic species in liquid S02, a compound such as SOCl2 would be expected to react as an acid. Similarly, K2S03 would be a base because it would provide the basic S032- ions. Therefore, the reaction between the acid and base in liquid S02 would be... 

Regarding hydrochloric acid, in a concentration range of 30.10 4 to 300.10 4 mol/L, equivalent conductance assumes an extremely low and constant value of 0.03 S cm2/mol, as seen in Figure 3. This behavior certainly cannot be explained on the basis of simple dissociation phenomena. Thus we have interpreted these results on the basis of theoretical work by Caruso and co-workers (31) who consider the conductometric, potentiometric, and spectrophotometric behavior of weak acids and bases in nonaqueous solvents. In these solvents a weak acid, HA, besides undergoing simple ionic dissociation, also may undergo conjugation phenomena by the H+ and A" ions which lead to the formation of ionic complex species A(HA)/ or H(HA)/. Caruso shows that the... 

Neutralization Curves.—The variation of the pH of a solution of acid or base during the course of neutralization, and especially in the vicinity of the equivalence-point, i.e., when equivalent amounts of acid and base are present, is of great practical importance in connection with analytical and other problems. It is, of course, feasible to measure the pH experimentally at various points of the neutralization process, but a theoretical study of the subject is possible and the results are of considerable interest. For this purpose it is convenient to consider the behavior of different types of acid, viz., strong and weak, with different bases, viz., strong and weak. For the present the discussion will be restricted to neutralization involving a conventional acid and base in aqueous solution, but it will be shown that the results can be extended to all forms of acids and bases in aqueous as well as non-aqueous solvents. 

In the third article of the series the authors set out to determine the kinetic initiation parameters and the lifetime of the ionic chain carriers. The values of kj were cmnputed frmn the measured rates of carbenium icm formation assuming bimolecular initiation. This assumption is unacceptable a priori since the interaction between Brjinsted acids and olefins in solvents like the one used in this work has been shown to involve kinetic patterns whidi are almost always more complicated than a simple first order in each reactant (see Sect. III-B). As for the calculation of the mean lifetime of the active species based on the expression... 

Caution is needed in interpreting these reactions. For example, SOCI2 and S03 react as acid and base in SO2 solvent, with the reaction apparently occurring as... 

Hydrocarbons and their halogen derivatives have no tendency to gain or lose protons they are inert and exhibit no levelling effect. The dielectric constants are very low (2 to 6) and the ions associate, making conductance measurements of no value for determining the extent of protolysis. Proto-lytic equilibria are also complicated by association of the uncharged molecules themselves carboxylic acids, for example, exist as dimers in benzene. The same factors reduce greatly the solubilities of acids and bases in these solvents. 

MacFarlane DR, Forsyth SA (2003) Acids and bases in ionic liquids. In Rogers RD, Seddon KR (eds) Ionic liquids as green solvents progress and prospects. American Chemical Society, Washington, pp 264-276... 

Our discussion has emphasized water solutions of these acids. Many organic compounds are soluble in numerous other solvents. The properties of acids and bases in other solvents depend on solvent properties such as polarity, acidity or basicity, and polarizability. 

As yet but little quantitative information is available concerning the strengths of acids and bases in various solvents. A number of laboratories are studying the problem, and it is expected... 

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