Amphoteric character

As already noted by Sprague and Land (46), no accurate data are available for 2-thiazolecarboxylic acid, but it appears to be a considerably stronger acid than either 4- or 5-thiazolecarboxylic acid (7). It is difficult to study experimentally the acidity of these compounds because of their amphoteric character. 

A water molecule has amphoteric character. This means it can act as both an acid and a base. The autoionization equilibrium process in water. 

Crystallisation is often used as a method to recover tire amino add. Because of tire amphoteric character (contains both addic and basic groups) of amino adds, their solubility is greatly influenced by the pH of the solution and usually show minima at the isoelectric point (zero net charge). 

The reactions discussed above show that arsenic(fV) is of redox amphoteric character and a stronger reducing agent than arsenic(in), but at the same time it is a stronger oxidant than arsenic(V). Partners of the oxidation-reduction reactions of arsenic(fV) known so far can be seen in Table 13. It follows from the redox amphoteric character that the oxidation potentials of couples involving arsenic species are in the order... 

According to Bielski and Allen , since the HO2 radical is of amphoteric character, it will be either protonated... 

The NMR spectra of the two-coordinate stannylenes in solution show values of Sn ranging from about 1150 (e.g., in ArSnl) to 3750 (in (Ar3Sn)Sn ), with a large anisotropy. The stannylenes behave as Lewis acids, for example, in the three- or four-coordinate complexes (e.g., 78, 79, and 80), which are formed when the molecule carries an intramolecular ligand, and as Lewis bases, particularly in complexing to transition metals (e.g., 81, 82, and 83). The dimerization of stannylenes to give distannenes can be regarded as a result of this amphoteric character (Equation (179)). 

The amphoteric character of water (i.e., the ability to act either as an acid or as a base) makes water so special. While this renders the use of water as a solvent in acid-or base-sensitive reactions problematic, the possibility to have the solvent as a reactant in acid- or base-initiated reactions is often desirable. These qualities led chemists to rediscover water as a solvent in organic chemistry. Unfortunately, from a chemical point of view, not all transformations are feasible in aqueous solvent systems. Many reagents decompose when brought into contact with water while many others are insoluble. Consequently, it is not surprising that water has not been a very popular solvent in organic chemistry in the past, but this picture is changing rapidly. 

The point of interest is the "amphoteric" character of the allyl anion in this complex. On the one hand it may react as an anion, but on the other hand it is susceptible to nucleophilic attack by, for example, carbon centred anions. This has found widespread use in organic synthesis. The reaction with the anion releases a palladium zero complex and in this manner palladium can be employed as a catalyst. 

Astatine, regardless of its electronic state vide infra), appears to possess many physiobiochemical properties similar to those of its nearest homolog, iodine 34, 57, 60, 119, 176). However, astatine also exhibits a proclivity to accumulate in macrophage-laden tissue, such as lungs, liver, and spleen 37, 60, 119) this has been attributed to its amphoteric character. 

Chloryl fluoride, like most of the other known chlorine fluorides and oxyfluorides, possesses amphoteric character. Owing to its weak and polar (p—jr )a Cl—F bond (see Section II, C), it exhibits a much stronger tendency to form adducts with Lewis acids than with Lewis bases. The adducts with Lewis acids result in salts containing ClOg" cations, and those with bases result in ClOgFj" salts. Both ions are discussed in detail in Sections III, G and H, respectively. 

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [166]. The most common acidic probe molecules used are CO2 (p/fa = 6.37) and SO2 (p/fa = 1.89). Carboxylic acids such as acetic acid can also be used but dimmers can be formed, particularly at high coverage. Pyrrole may also be used, particularly at low adsorption temperature, but has sometimes shown some amphoteric character [103]. Hexafluoroisopropanol has also been used to characterize the surface basicity of some solids [145]. 

Bulk alumina and india are isostructural, with a linear structure OMOMO, while B2O3 molecule is V-shaped. The Ga203 can present the both types of isomers, the V-shaped structure being a little more stable than the linear one [1], These very different structural features (shape, electronegativities, etc.) of group lllA oxides may help explain their specific properties that fail to strictly follow any simple rule. Their amphoteric character (except for boria) that is not easy to evaluate, has been confirmed and quantified by the experimental microcalorimetric results. 

Another study examined the acidity and basicity of bulk Ga203 by NH3 and SO2 adsorptions microcalorimetry performed at 150°C. As alumina, Ga203 is amphoteric, with heats higher than 100 kJ/mol for both NH3 and SO2 adsorption, respectively [186]. The amphoteric character of bulk gallium oxides and strong heterogeneity of the surface acidic and basic sites were proved also by Petre et al. [179] using microcalorimetry of pyridine adsorption at 150°C and CO2 adsorption at 30°C. 

The snrface acid-base properties of supported oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. As shown, acidic and basic sites are often present simultaneously on solid surfaces. The knowledge of the detailed amphoteric character of supported metal oxides is of extreme interest due to the possibility of using them as catalysts in different reactions in which acidity governs the reaction mechanism. 

Drying, the removal of water, is a special problem because water is a unique compound and there is a lot of it around. The features which together make water unique include a high dipole moment and polarisability, its amphoteric character, its ability to act as donor and acceptor in hydrogen bond formation, and its smallness. 

J. Kendall, J. E. Booze, and J. C. Andrews 3 have shown that the formation of hydrates, in the sense of water of crystallization, with the weak acids very seldom occurs, and when hydrates are formed, the acid has the amphoteric character of a phenol. There is also a regular increase in the tendency of an acid to form hydrates, as the strength of the acid increases, until, with the strong acids, well-defined stable hydrates appear. The complexity and stability of the hydrates increase with the strength of the acid. These facts are in harmony with the weak acid nature of water. 

The purely hydrogen-bonded structures range from the acids to the polyhydroxyl compounds such as carbohydrates. In contrast the alkali h3rdroxides are not hydrogen bonded to any extent, but resemble more closely ionic compounds with polarizable ions. Even in alkaline earth hydroxides, except those of beryllium, no true hydrogen bonds are formed, and it is only in the hydr oxides of the third group, which are of amphoteric character, that the hydrogen bonds reassert themselves [I]. 

In addition to dissolving in acids, freshly precipitated niobium pentoxide dissolves in caustic soda and in caustic potash, and hence it appears to possess weakly amphoteric character. Its colloidal state in solution has hitherto prevented any direct determination of its basicity or acidity. According to Weinland and Storz 6 it is comparable to silicic acid, and is more strongly acid than titanic add. That niobic add is a very weak add is shown by the readiness with which the niobates are (a) hydrolysed and (b) decomposed by mineral acids,... 

Arsenious oxide in solution exhibits a slightly amphoteric character, but its basic nature is extremely feeble. By determining the solubility... 

Zinc(II) oxide is insoluble in neutral water, but soluble in dilute acids Zn2+(aq) is stable at low pH values. If an acid solution containing Zn2+ is treated with an alkali, Zn(OH)2 precipitates around neutral pH but redissolves at a pH of about 14, with formation of Zn(OH)J(aq) and Zn(OH)4 (aq). The fact that Zn(II) forms cationic aqueous species in acid solution and anionic species at alkaline pH determines the amphoteric character of ZnO. 

Lautens, M. Ouellet, S. G. Raeppel, S. Amphoteric character of 2-vinyloxiranes synthetic equivalents of /3,y-unsaturated aldehydes and a vinylogous enolate. Angew. Chem. Int. Ed. 2000, 39, 4079 1082. 

The amphoteric character of alumina leads to a more complex reten-... 

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