Amphiprotic compounds

In pure water, we have seen that the hydronium ion concentration is 1 X 10-7 M. The logarithm of this is -7 and, thus, the pH of pure water is 7. Since water is an amphiprotic compound, pH 7 means a neutral solution. On the other hand, in a 0.01 M HC1 solution (dissociating completely), we have [H30+] = 1 X 1CT2 M. Thus its pH is 2. The pH scale shows that acidic solutions have a pH less than 7 and basic solutions have a pH greater than 7. 

The simple amino acids are an important class of amphiprotic compounds that contain both a weak acid and a weak base functional group. When dissolved in water, an amino acid such as glycine undergoes a kind of internal acid/base reaction to produce a zwitterion—a species that bears both a positive and a negative charge. Thus,... 

Properties of Amphiprotic Compounds An amphiprotic compound is a species that in solution is capable of both donating and accepting a proton. A typical amino acid, such as glycine, is an amphiprotic compound. When glycine is dissolved in water, three important equilibria operate ... 

All amino acids are amphiprotic compounds and can be denoted by the general formula... 

An amphiprotic species is a molecule or ion that can both accept and donate protons. For instance, HCOj can act as an acid (to form COf") and as a base (to form H2CO3). Among the most important amphiprotic compounds are the amino acids, which can act as proton donors by virtue of their carboxyl groups and as bases by virtue of their amino groups. 

VII. Relationships Between Acidity and Basicity of Amphiprotic Compounds. 322... 

A recurrent question in structure-reactivity studies is what relation, if any, exists between the base strengths of a series of amphiprotic compounds and their strengths as acids. Until very recently there were so few reliable basicity data for acidic compounds that almost nothing could be said on this score. On Figure 7 are plotted the acid... 

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. 

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. 

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. 

The same titrations were done with hydrochloric acid in methanol solvent to show the effect of amphiprotic solvent in the titrations with hydrochloric acid. But, some of the compounds did not show expected results, while most of them gave well shaped curves. 

The isoelectric pH of a polyprotic compound is that pH at which the average charge of all species is 0. For a diprotic amino acid whose amphiprotic form is neutral, the isoelectric pH is given by pH = (pK + pK2). The isoionic pH of a polyprotic species is the pH that would exist in a solution containing only the ions derived from the neutral polyprotic species and from H20. For a diprotic amino acid whose amphiprotic form is neutral, the isoionic pH is found from [H + ] = VC / F + K K )l(Ki + F), where F is the formal concentration of the amino acid. 

Bismuth(III) oxide, Bi O is the compound produced by heating the metal, or its carbonate, in air. It is definitely a basic oxide, dissolving readily 111 acid solutions, and unlike the arsenic or antimony compounds, not amphiprotic in solution, although it forms stoichiometric addition compounds on heating with oxides of a number of other metals. It exists in three modifications, white rhombohedral, yellow rhombohedral, and gray-black cubical, Bismuth(II) oxide. BiO, has been produced by heating die basic oxalate. 

The dioxide. PbiO. has rutile structure, and the compound is a strong oxidizing agent. It is also amphiprotic. giving unstable lcad(IV) salts with acids, and orthoplnmbates. M PbOa, or metuplumbates. M PbO.. upon fusion with alkalies. Lead dioxide dissolves in aqueous alkali with formation of the ion Pb(OH)6. the alkali salts of which are isomorphous with the corresponding stannates and platinates. Lead sesquioxide, PbiO,. has been shown not to exist as a stable phase. 

Tin forms two series of compounds tin(II) or stannous compounds and tint IV1 or stannic compounds. Tin(ll) oxide, SnO, insoluble in water, is formed by precipitation of an SnO hydrate from an S11CL solution with alkali and later treatment in water (near the boiling point and at constant pH). It is amphiprotic, but only slightly acid, forming stannites slowly with strong alkalis. Sodium stannite is conveniently prepared from... 

Zirconium oxide, Zr02 is widely known, both as a mineral, baddeleyite, and as an industrial product obtained horn zircon, Z1SO4. Moreover, the precipitate obtained by action of alkali hydroxides upon solutions of tetravalent zirconium is a hydrated oxide. The latter is readily soluble in acids to form oxysalts, which arc usually formulated in terms of the Zr02+ ion, without including its water of hydration, e.g., as Zr0(H2P04)2. The hydrated Zr02+ ion is not amphiprotic it does not dissolve in alkali hydroxides, While it does react on alkali carbonate fusions, the compounds formed have been shown to be mixed oxides rather than zirconates. 

When amphiprotic groups as NH2 and OH are placed in 6a position, the possibility of a third hydrogen bond appears, and as a result, the tendency is reversed, the homochiral complexes being more stable than their heterochiral counterparts, due to the third hydrogen bond formed in the homodimer. In the amino substituted compound, the discrimination energy favors the homochiral complex in 11.3 kJ mol-1. 

Amphiprotic species A compound that can both donate and accept protons. 

Thus the compound HC03 is a conjugate base in the first reaction and an acid in the second reaction. A compound that can act either as an acid or a base is called amphiprotic. In the self-ionization reaction... 

The extent of the ionization step depends on the relative strength of the conjugate acid-conjugate base pairs. The amphiprotic properties of the solvent have an essential effect on the equilibrium constant of this reaction step. The extent of the dissociation step is influenced by the polarity of the solvent, increasing with the dielectric constant of the solvent. In water, all products of acid-base reactions of moderate to low concentrations are essentially completely dissociated into solvated ions (Pecsok et al., 1976). The dissociation step is suppressed by addition or substitution with cosolvents of lower polarity, e.g., alcohols in aqueous formulations. The ion-pair aggregates may have absorption spectra different from the dissociated species. Thus, the amphiprotic properties and polarity (expressed as the dielectric constant) of the solvent are essential for the acid-base equilibrium of the drug and thus the absorption spectrum of the compound. This subject is further discussed in Section 14.2.3. 

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