Solvent amphiprotic

Dissociation of Water Water is an amphiprotic solvent in that it can serve as an acid or a base. An interesting feature of an amphiprotic solvent is that it is capable of reacting with itself as an acid and a base. 

Amphiprotic solvents consist of liquids, such as water, alcohols and weak organic acids, which are slightly ionised and combine both protogenic and protophilic properties in being able to donate and to accept protons. 

For all the amphiprotic solvents in the pure state the cation (or acid) concentration is by definition equal to the anion (or base) concentration in other words, the pure solvent is neutral therefore, adding a solute such as HC1 makes the solution acidic, whereas a solute such as NaOH makes it basic. 

For all amphiprotic solvents, reactions like the examples in the overall reactions 4.4-4.8 apply, often expressed by the equilibrium equation ... 

HS r-——— H2S + + S , while the electroneutrality of pure solvents lies at An extensive list of pi(Ts values (mainly at 25° C) for various amphiprotic solvents and for mixtures of water and organic solvents (together with relevant literature data) has been given by Tremillon4. 

Starting from the foregoing theory of amphiprotic solvents, we shall next consider their interaction with some reactive (inorganic or ionophoric) solutes as illustrated below. 

As in water, neutralization in all amphiprotic solvents represents the backward reaction of self-dissociation down to the equilibrium level of the ionic product in the pure solvent. 

Comparison of reactions 4.9, 4.10, 4.12, 4.13 and 4.15 leads to another important conclusion, viz., in an amphiprotic solvent its own solvonium cation represents the strongest acid possible, and its own anion the strongest base. Even when a very strong foreign acid or base is dissolved, excessive proton donation to and proton abstraction from the solvent molecule yield the respective acid or base this phenomenon is generally known as the levelling effect, which in an amphiprotic solvent takes place on both the acid and the basic... 

This example shows also that the proton theory, in addition to being valid for aprotic solvents, also works for amphiprotic solvents, and so represents a more general theory. How in an acid-base titration the theory works out can be followed from the titration of a certain amount of HC1 gas introduced into pyridine as an aprotic solvent ... 

The above solvents theory (A) and proton theory (B) have shown that in theory the neutrality point (of the pure solvent) lies for the amphiprotic solvents at pH = pKs and for the aprotic protophilic solvents at a pH somewhere between the highest acidity (of the protonated solvent) and an infinitely high pH. However, the true pH of the neutrality point of the solvent can only be obtained from a reliable pH measurement and the problem is whether and how this can be achieved. For water as a solvent, the true pH = - logaH+ = colog aH+ is fixed by the internationally adopted convention E°m ( H2(latm) = 0... 

As would be expected, the larger titration potential ranges offer much more scope for mutually distinguishing between individual acids or bases in amphiprotic solvents, as a consequence of self-dissociation, the potential ranges are rather limited, whereas in the aprotic protophilic solvents and "aprotic inert solvents these ranges are considerably more extensive. 

The molecules of amphiprotic solvents which are the most important will be designated as SH. Self-ionization occurs to a small degree in these solvents according to the equation... 

In amphiprotic solvents protolysis of acids and bases takes place. This reaction is also termed solvolysis, classically dissociation. The degree of solvolysis depends on the nature of both the dissolved acid or base and on the solvent. 

If an amphiprotic solvent contains an acid and base that are neither mutually conjugate nor are conjugated with the solvent, a protolytic reaction occurs between these dissolved components. Four possible situations can arise. If both the acid and base are strong, then neutralization occurs between the lyonium ions and the lyate ions. If the acid is weak and the base strong, the acid reacts with the lyate ions produced by the strong base. The opposite case is analogous. A weak acid and a weak base exchange protons ... 

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

In cases where both types of compounds are present in significant amount, the use of a mixture of solvents, nucleophilic and electrophilic, was suggested. In such cases, an amphiprotic solvent could also be suitable (an amphiprotic solvent contains simultaneously an electron donating and electron accepting groups in a single molecule), for instance ethanolamine. 

Abstract Titration of weak bases in non-aqueous solvents can provide valuable information about these weak bases. Some primary amines 1-aminobutane, 1-aminopropane, 2-aminoheptane, aminocyclohexane, 3-amino-l-phenylbutane were titrated with hydrochloric acid in toluene solvent. All the primary amines gave very well-shaped potentiometric titration curves. The same titrations were done with hydrochloric acid in methanol solvent to show the effect of amphiprotic solvent in the titrations with hydrochloric acid. 

Keywords Amphiprotic solvent, potentiometric titration, primary amines... 

As a result, inert and aprotic solvent toluene is suitable for the titration of weak bases in non-aqueous media as solvent, although benzene which is more carcino-genic aromatic hydrocarbon used widely in literature for non-aqueous titrations. The major advantage of toluene is tliat it does not compete for protons with the reactant in the titrations because of its autoprotolysis constant approaching zero. The major disadvantages of solubility can be removed by using small amount of amphiprotic solvents. 

Autoprotolysis constants exist for any amphiprotic solvent and can be determined from electric conductivity studies of the solvent and solutions. A few examples include ... 

Amphiprotic molecular entities, AUTOPROTOLYSIS Amphiprotic solvent,... 

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. 

Water is the prototype of an amphiprotic solvent and all other solvents with similar acid-base properties are called neutral solvents. Solvents which are much stronger acids and much weaker bases than water are called protogenic solvents, while those which are much stronger bases and much weaker acids than water are designated protophilic solvents. 

DMSO is an amphiprotic solvent because its autoprotolysis occurs slightly (pKsH 33) and the lyate ion (CH3SOCH2) is somewhat stable. However, DMSO is classified as an aprotic solvent. The rough criteria for aprotic solvents are pKsH>22 and AN<20. 

Amphiprotic solvents have both acidic and basic properties in terms of the Bransted acid-base concept. If we denote an amphiprotic solvent by SH, it donates a proton by SH S +H+ and accepts a proton by SH + H+ = SH2. Overall, the autoprotolysis (autoionization) occurs by 2SH = SH2 + ST The extent of autoprotolysis is expressed by the autoprotolysis constant, Kjh = aSH2 aS, the values of which are also included in Table 1.5 as pKSH values (for more details, see Table 6.6). 

Using water as reference, an amphiprotic solvent having an acidity and a basicity comparable to those of water is called a neutral solvent, one with a stronger acidity and a weaker basicity than water is called a protogenic solvent, and one with a weaker acidity and a stronger basicity than water is called a protophilic solvent. The solvent with relatively strong acidity usually has in its molecule a hydrogen atom... 

High-permittivity amphiprotic solvents easy easy... 

Low-permittivity amphiprotic solvents a little difficult or easy difficult... 

In order that the acid HA dissociates almost completely into H+ and A-, i.e. in order to get a large Ka value, it is essential that both the ionization and the dissociation processes occur easily. This means that both K, and KD should be large enough, as is often the case in amphiprotic solvents of high pennittivities. In the following sections, we discuss the characteristics of acid-base reactions in each of the four classes of solvents. 

Acid-base Reactions in Amphiprotic Solvents of High Permittivity... 

Among amphiprotic solvents of high permittivities, there are water-like neutral solvents (e.g. methanol and ethanol), more acidic protogenic solvents (e.g. formic acid), and more basic protophilic solvents (e.g. 2-aminoethanol). There are also amphiprotic mixed solvents, such as mixtures of water and alcohols and water and 1,4-dioxane. The acid-base equilibria in amphiprotic solvents of high permittivity can be treated by methods similar to those in aqueous solutions. If the solvent is expressed by SH, the acid HA or BH+ will dissociate as follows ... 

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. 

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