Amphiprotic bases

Fig. 7. Acidic pKa vs. basic pKa for amphiprotic bases 1. berumic acid 2. acetic acid 3. phenol 4. methyl mercaptan 5. methanol, 6. acetamide.

Amphiprotic Species Some species can behave as either an acid or a base. For example, the following two reactions show the chemical reactivity of the bicarbonate ion, HC03, in water. 

A species that can serve as both a proton donor and a proton acceptor is called amphiprotic. Whether an amphiprotic species behaves as an acid or as a base depends on the equilibrium constants for the two competing reactions. For bicarbonate, the acid dissociation constant for reaction 6.8... 

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. 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

Amphiprotic (Section 26.1) Capable of acting either as an acid or as a base. Amino acids are amphiprotic. 

The acidic and basic properties of aqueous solutions are dependent on an equilibrium that involves the solvent, water. The reaction involved can be regarded as a Bransted-Lowry acid-base reaction in which the H20 molecule shows its amphiprotic nature ... 

For the acid-base properties of amphiprotic anions such as HCO3 otHJPOt. see the discussion at the end of this section. 

A similar argument can be used to decide whether an amphiprotic anion such as HC03 or H2PO4- is acidic or basic. In principle, these ions can act as either weak acids or weak bases ... 

Both the zwitterion and the HC03 ion are amphiprotic, capable of acting as either a Bron-sted acid or a Bronsted base. 

Amphiprotic species Capable of acting as either a Brensted-Lowry acid or base, 354,372-373,623... 

An important implication of the Bronsted definitions of acids and bases is that the same substance may be able to function as both an acid and a base. For example, we have seen that a water molecule accepts a proton from an acid molecule (such as HC1 or HCN) to form an H30+ ion. So water is a base. I lowever, a water molecule can donate a proton to a base (such as O2- or NH3) and become an OH ion. So water is also an acid. We describe water as amphiprotic, meaning that an H20 molecule can act both as a proton donor and as a proton acceptor. 

A note on good practice Distinguish between amphoteric and amphiprotic. Aluminum metal is amphoteric (it reacts with both acids and bases), but it has no hydrogen atoms to donate as protons, and is not amphiprotic. 

Because water is amphiprotic—because it is both a Bronsted acid and a Bronsted base—proton transfer between water molecules occurs even in pure water, with one molecule acting as a proton donor and a neighboring molecule acting as a base ... 

The conjugate base of a polyprotic acid is amphiprotic it can act as either an acid or a base because it can either donate its remaining acidic hydrogen atom or accept an acidic hydrogen atom and revert to the original acid. For example, a hydrogen sulfide ion, HS-, in water acts as both an acid and a base ... 

The pH of the aqueous solution of an amphiprotic salt is equal to the average of the pKlts of the salt and its conjugate acid. The pH of a solution of a salt of the final conjugate base of a polyprotic acid is found from the reaction of the anion with water. 

Write the two proton transfer equilibria that show the amphiprotic character of (a) H2P04 (b) HC204, hydrogen oxalate ion. Identify the conjugate acid-base pairs in each equilibrium. 

The two strands of the nucleic acid DNA are held together by four organic bases. The structure of one of these bases, thymine, is shown below, (a) How many protons can this base accept (b) Draw the structure of each conjugate acid that can be formed, (c) Mark with an asterisk any structure that can show amphiprotic behavior in aqueous solution. 

These examples of acid-base reactions show that water can act as either an acid or a base Water accepts a proton fixtm an HCl molecule, but it donates a proton to a PO4 anion. As an acid, water donates a proton to a base and becomes a hydroxide anion. As a base, water accepts a proton from an acid and becomes a hydronium cation. A chemical species that can both donate and accept protons is said to be amphiprotic. Water is an amphiprotic molecule. 

The water equilibrium illustrates the amphiprotic nature of H2 O. In this reaction, one water molecule acts as a proton donor (acid), and another acts as a proton acceptor (base). 

C17-0128. Pure sulfuric acid (H2 SO4) is a viscous liquid that causes severe bums when it contacts the skin. Like water, sulfuric acid is amphiprotic, so a proton transfer equilibrium exists in pure sulfuric acid, (a) Write this proton transfer equilibrium reaction, (b) Construct the Lewis stmcture of sulfuric acid and identify the features that allow this compound to function as a base, (c) Perchloric acid (HCIO4) is a stronger acid than sulfuric acid. Write the proton transfer reaction that takes place when perchloric acid dissolves in pure sulfiaric acid. 

At the first stoichiometric point of the titration, aii the diprotic acid has been converted to its conjugate base, H A. This amphiprotic anion can react with itseif, analogous to the self-ionization of water ... 

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. 

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 ... 

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. 

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 ... 

We have labeled water as both an acid and a base. It is useful to think of water as either, because it really has no more properties of one than the other. Water is sometimes referred to as amphiprotic. It reacts as an acid in the presence of bases and it reacts as a base in the presence of acids. 

This possible mechanism should be evaluated in relation to the catalysts. If the catalytic action is to be ascribed to the acid character of the catalysts, the condensation under consideration may differ from the ordinary aldol condensation, which is catalyzed preferentially by basic agents. Nevertheless, many condensations of the aldol type are effected with the aid of acidic reagents. Moreover, the condensation of sugars with dicarbonyl compounds has been carried out in aqueous alcoholic media which are non-acidic hence, there also exists the possibility of a mechanism catalyzed simultaneously by acid and by base, somewhat like that suggested by Lowry46 in another connection. A transition state with an amphiprotic structure has been postulated. Its formation can be catalyzed by either acids or bases. 

Notice that HC03 is the amphiprotic species, acting as both an acid and as a base. 

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. 

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