Superacids definition

In the 1960s Gillespie suggested calling protic acids stronger than 100% sulfuric acid superacids. This arbitrary but most useful definition is now generally accepted. It should be mentioned, however, that... 

Paul and Long35 have tabulated pATBH+ values for indicators, which were used to establish Hammett acidity functions for aqueous acids between the years 1932 and 1957. The data were summarized as a set of best values of pAiBH+ f°r the bases. Since then, subsequent work seems to suggest that some of these values are incorrect. This is particularly the case for some of the weaker bases whose quoted pAfBH+ were based on a stepwise extrapolation of results of some indicators that have since been proven to be unsatisfactory based on the strict definition of H0- These data, as well as those for weaker bases that have been studied since, covering the whole acidity range from dilute acid to the superacid media are collected in Table 1.1. 

It appears that despite the lack of reliability of acidity determination of solid acids by spectroscopic means and in the absence of knowledge of the nature of the initial step in alkane activation by solid acids the qualification of superacid solids has been and continues to be used, despite the absence of a clear definition of solid superacidity. 

Considering the impressive amount of literature on sulfated zirconia and solid superacids,125 134-139 it will be difficult to impose a definition a posteriori. On the other hand, due to the large difference in acidity and in structure between various liquid superacids, there is no unique chemistry of hydrocarbons in liquid superacids. For this reason it is not possible to suggest a unequivocal definition of solid superacidity at the present stage. Nevertheless, it seems clear from all the data presently available that at high temperatures the chemical reactivity of the proton bound to the surface shows a close resemblance to the one observed at low temperature in liquid superacidic media as will be seen in Chapter 5. 

Using Gillespie s arbitrary definition, Br0nsted superacids are those whose acidity exceeds that of 100% sulfuric acid (//, = —12). The physical properties of the most commonly used Br0nsted superacids are summarized in Table 2.1. 

As already pointed out earlier (see Section 1.4.8), a clear definition of solid superacidity is needed. On the other hand, for catalysts to be able to activate alkanes at low temperatures, such as sulfated zirconias and heteropoly acids, the redox properties should not be neglected in the activation step.142,143... 

This definition is principally concerned with ionic solvents. A few important organic species may be synthesised in the superacid solvents to which this definition has most application. 

Another necessary condition for superacidity in these systems is presence of WO3 clusters of definite optimal size on the surface of tetragonal Zr02 [2]. Such optimal clusters are characterized by the width of forbidden zone Eo = 3.0-3.2 eV. The size of WO3 clusters... 

The widely accepted definition of a superacid is an acid system that is stronger than 100% H2SO4. R. J. Gillespie and T. E. Peel, Adv. Phys. Org. Chem., 9, 1 (1972). 

Many/most of the anions used for electrolyte lithium salts were initially developed in efforts to generate stronger superacids. The term superacid generally refers to acids which are more acidic than mineral Brpnsted acids [20-27]. In particular, a widely used definition for superacids was given by Gillespie who defined this term to be applicable to acids which are more acidic than sulfuric acid [28-30]. The stronger the acidity, the weaker the coordination of the anions is with the associated protons (H cations). The gas-phase acidity is given by ... 

Another area of difficulty is measuring the acid strength of solid superacids. Since solid superacid catalysts are used extensively in the chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have acidic properties widely different from those of the bulk material this is especially true for well-structured solids such as zeolites. 

Addition of alkali metals to certain types of oxides resulted in the formation of very strong base sites. Materials which possess base sites stronger than H- = —26 are caUed superbases. The H- value 26 proposed to be set in conformity vrith the definition of superacid. The critical Ho value for superacid is ca. —12, whidi differs by 19 Ho units from Ho=7, the neutral acid-base strength. The H- value 26 differs from H- = 7 for neutral acid-base strength by 19 H- units. Althou alkali and alkaline earth oxides show superbasicity without the addition of alkali metals, as described in Sections 3.1.1 and 3.1.2, this section deals only with alkali metal-added materials showing strong basicity. The materials which show superbasicity by the addition of alkali metals are limited to alkaline earth oxides and alumina. 

Although the terms acid and base have been in use since the early days of chemistry, the meaning of these terms has undergone considerable modification through the centuries and continues even today with the recent introduction of new terms such as hard acid, soft acid and superacid. It is the purpose of this chapter to consider some of the classic definitions of the terms acid and base and also some of the more recent developments of our ideas concerning acids and bases [1]. 

Azido group in hydrazoic acid is known to be protonated in superacid solutions with formation of aminodiazonimn ion, H2N-N2 in the gas-phase protonation, the iminodiazenium ion, HNNNH, can be also formed [34], The protonation of azidopyridine and azidoquinoline takes place definitely at endocyclic ("azine") nitrogen atom the spectra of protonated azidoazines coincide with those of N-methylated ones. 

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