Acid-base behavior solvents

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923, when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key process responsible for the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... 

Thermodynamics of complex formation of silver with several ligands such amines,368 hindered pyridine bases,369 nitrogen donor solvents,370 and azoles371 have been carried out. Other studies include the secondary-ion mass spectra of nonvolatile silver complexes,372 the relationship between Lewis acid-base behavior in the gas phase and the aqueous solution,373 or the rates of hydride abstraction from amines via reactions with ground-state Ag+.374... 

The use of organic solvents in nonaqueous capillary electrophoresis not only increases the solubility of the solutes, but also allows one to control important characteristics of the separation. For instance, the solvent properties affect the acid-base behavior of the analytes on a wider scale than... 

Organic solvents can also be classified according to their ability to accept or transfer protons (i.e., their acid-base behavior) [20,21]. Amphiprotic solvents possess donor as well as acceptor capabilities and can undergo autoprotolysis. They can be subdivided into neutral solvents that possess approximately equal donor and acceptor capabilities (water and alcohols), acidic solvents with predominantly proton donor properties (acetic acid, formic acid), and basic solvents with primarily proton acceptor characteristics (formamide, N-methylformamide, and N,N-dimethylformamide). Aprotic solvents are not capable of autoprotolysis but may be able to accept protons (ACN, DMSO, propylene carbonate). Inert solvents (hexane) neither accept nor donate protons nor are they capable of autoprotolysis. 

The "equilibrium boxes" for the solvents (Fig. 10-1) indicate the range over which differentiation occurs outside the range of a particular solvent, all species are leveled. For example, water can differentiate species (i.e., they are weak adds and bases) with pKa s from about 0 to 14 (such as acetic acid). Ammonia, on the other hand, behaves the same toward acetic acid and sulfuric acid because both lie below the differentiating limit of —12. The extent of these ranges is determined by the autoionization constant of the solvent (e.g, —14 units for water). The acid-base behavior of several species discussed previously may be seen to correlate with Fig. 10.1.11... 

The acid/base behavior of aromatic nitro compds in DMF, Me2CO, MeCOEt, and a mixt of solvents were studied using high frequency titration (Ref 42), Pr2CO/EtOH, MeEtCO/ MeOH and MeEtCO/EtOH are reported as suitable solvents for the potentiometric titration of TNT (Ref 43). Low concns of TNT in air were detected in their negatively charged state via electron transfer from ionized SF6. The charged... 

Davies, M.M., Acid-Base Behavior inAprotic Solvents, Nat. Bur. Stand.(U.S.) 105 Washington, D.C. 1968. 

Although there is a wealth of data in the literature on acid-base behavior in aprotic solvents,60 61 there are few examples of the use of buffers for polarography and voltammetry in aprotic solvents. This has occurred because most investigators have sought to keep all potential proton donors out of the system and thereby stabilize anion radicals. Although the picric acid-picrate system has been used as a buffer in a number of studies in aprotic solvents, its use in voltammetric work is limited because of the ease of reduction of picric acid. 

The major biological solvent is water, and the acid-base behavior of dissolved molecules is intimately linked with the dissociation of water. Water is a weak electrolyte capable of dissociating to a proton and a hydroxyl ion. In this process, the proton binds to an adjacent water molecule to which it is hydrogen-bonded (Chap. 4) to form a hydronium ion (H30+) ... 

Association constant — Solvents having a low dielectric constant (e.g., benzene er = 2.29) cannot split protons from acids. The acid/base behavior in these solvents is based on association reactions between acidic and basic components in the solution according to ... 

Just as the cation produced by dissociation of water (H30+) is the acidic species in aqueous solutions, the NH4+ ion is the acidic species in liquid ammonia. Similarly, the amide ion, NH2, is the base in liquid ammonia just as OH- is the basic species in water. Generalization to other nonaqueous solvents leads to the solvent concept of acid-base behavior. It can be stated simply as follows A substance that increases the concentration of the cation characteristic of the solvent is an acid, and a substance that increases the concentration of the anion characteristic of the solvent is a base. Consequently, NH4C1 is an acid in liquid ammonia, and NaNH2 is a base in that solvent. Neutralization becomes the reaction of the cation and anion characteristic of the particular solvent to produce unionized solvent. For example, in liquid ammonia the following is a neutralization ... 

Davies, M. M. Acid-base behavior in apiotic organic solvents, Nat. Bur. Standards Monograph 1968, 105. 

Some liquid covalent halides can act as nonaqueous solvents " based on Lewis acid-base behavior, according to the donor-acceptor definition. The self-dissociated ions consist of a cation formed by subtraction of a hahde ion from the neutral compound, while the anion is formed by its addition (equation 24). Salts derived from such covalent halides can be considered as titration products of the parent acidic and basic compounds (equations 25 and 26). In such cases, both the cation and the anion usually possess a stable coordination number with a high geometrical symmetry. 

Z. Pawlak, Solvent effects on acid-base behavior. Acidity constants of eight pro-tonated substituted pyridines in (acetonitrile + water), /. Chem. Thermodynam. 19 (1987), 443 47. 

The dielectric constant is a property of major concern in understanding acid-base behavior in various solvents. When the dielectric constant of a solvent is low, ion association and homoconjugation can take place, resulting in modification of otherwise simple proton transfer reactions. 

M. R. F. ASHWORTH Titrimetric Organic Analysis, Interscience, New York, 1964,1965. M. M. DAVIS Acid-Base Behavior in Aprotic Organic Solvents, National Bureau of Standards Monograph 105, NBS, Washington, D.C., 1968. 

Inert solvents, with neither acidic nor basic properties, allow a wider range of acid-base behavior. For example, hydrocarbon solvents do not limit acid or base strength because they do not form solvent acid or base species. In such solvents, the acid or base strengths of the solutes determine the reactivity and there is no leveling effect. Balancing the possible acid-base effects of a solvent with requirements for solubility, safety, and availability is one of the challenges for experimental chemists. 

Solvents can change the acid-base behavior of solutes. Compare the acid-base properties of dimethylamine in water, acetic acid, and 2-butanone. 

Chapters 10 and 11 describe the special properties of liquid water. Because of its substantial dipole moment, water is especially effective as a solvent, stabilizing both polar and ionic solutes. Water is not only the solvent, but also participates in acid-base reactions as a reactant. Water plays an integral role in virtually all biochemical reactions essential to the survival of living organisms these reactions involve acids, bases, and ionic species. In view of the wide-ranging importance of these reactions, we devote the remainder of this chapter to acid-base behavior and related ionic reactions in aqueous solution. The Bronsted-Lowry definition of acids and bases is especially well suited to describe these reactions. 

Protonic Solvents monia, and sulfuric acid) share a similarity in their acid-base behavior. All are... 

Transition metal hydrides, which are weakly basic as isolated molecules, are expected to display acidic properties in solution. With an appropriate choice of solvent we are thus able to induce Umpolung of the acid-base behavior of certain transition metal hydrides. The break-even point of a TMH in water would be reached with 3.7. This relatively low value indicates that most transition metal hydrides will dissociate protons in water. 

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