Hammett acidity, superacids

For anhydrous hydrogen fluoride, the Hammett acidity function Hq approaches —11. The high negative value of Hq shows anhydrous hydrogen fluoride to be in the class of superacids. Addition of antimony pentafluoride to make a 3 Af solution in anhydrous hydrogen fluoride raises the Hammett function to —15.2, nearly the strongest of all acids (34). 

It has been shown, however, that such catalysts may contain protons, either by design or because of the difficulty in removing all traces of moisture, and these protons have been shown to be superacidic with Hammett acidities up to —18. These protons will also play some role in the catalytic activity of these ionic liquids in practical situations. Ionic liquids in which superacidic protons have deliberately been generated by addition of small amounts of water, HCl or H2SO4 have been used to catalytically crack polyethene under relatively mild conditions. The main products are mixed C3-C5 alkanes, which would be a useful feedstock from waste polyethene recycling. In contrast to other cracking procedures no aromatics or alkenes are produced, although small amounts of polycyclic compounds are obtained. 

Ionic liquids containing chloroaluminate ions are strong Lewis, Franklin and Bronsted acids. Protons present in [emim][AlCl4] have been shown to be superacidic with Hammett acidities up to —18. Such highly acidic ionic liquids are, nonetheless, easily handled and offer potential as non-volatile replacements for hazardous acids such as HF in several acid-catalyzed reactions. 

The typical acids used in solution studies of carbenium ions are, by thermodynamically rigorous methodology, stronger than 100% sulfuric acid. Table I (19, 20) lists some of the common superacids as well as their approximate Hammett acidities. NMR studies of the products from organic... 

Hammett Acidity Values (H0) for Selected Superacidsa b (100% H2SO4, the Threshold of Superacidity,... 

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. 

There are numerous ways to determine experimentally pK values of chemical compounds (205). Classical methods are potentiometric titration and ultraviolet (UV) spectroscopy, among others. These techniques have been widely applied for nucleobases and also for metal-nucleobase complexes. For the extremes such as negative pK values (pK < —2) of singly or multiply protonated nucleobases, or very high pK values (pK >15) for deprotonation of exocyclic amino groups of nucleobases (C, G, A), modifications have to be employed. These include the consideration of the Hammett acidity function in superacidic solvents or solvent mixtures (206), as well as extrapolative techniques according to Bunnett-Olsen and Marziano-Cimino-Passerini to be applied in polar, aprotic solvents (45, 207). 

Compounds which are vanishingly weakly basic in H20, such as nitro-anilines, nitro-aromatic or halo-nitro-aromatic compounds are protonated to a greater or less extent in superacids, allowing them to be used as indicators in Hammett Acidity Function measurements. Cl- which has essentially zero base strength in H20 is protonated by the superacids to HC1, which being monomolecular is expelled as a gas from the highly associated solvents. This provides a commonly-used important route to synthesis of anhydrous fluorides, fluorosulfates and triflates. 

Acid solutions more acidic than sulfuric acid are ealled superacids, for which George Olah won the Nobel Prize in Chemistry in 1994. The acidity of such solutions is frequently measured by the Hammett acidity function ... 

According to Arata and Hino [3], SZ exhibits an acidity 10 times stronger than 100% sulfuric acid, as determined by Hammett acid function, therefore it is recognised as a superacid. Even if the presence of superacidity can be debated [4], several studies have been devoted to the synthesis of SZ with the aim to study new catalytic applications. 

Many types of solid acid and base catalysts are known.11 Superacids are those that are at least as strong as 100% sulfuric acid.12 The acid strengths are measured using basic indicators and are assigned a Hammett acidity function, H0- Table 6.1 lists some superacids, with the strongest at the top. 

Pure chloroaluminate based ionic liquids have no proton donating ability. However, these liquids can demonstrate Bronsted acidity as a result of intentional or unintentional protonic species. These protons behave as superacids with Hammett acidity functions ranging from —12.6 to —18. The proton speciation in acidic... 

The synthesis of a superacid clay, with Hammett acidity ranging from -13,2 to — 12,7 has also recently been reported [40], The surface acidity decreases as the amount of residual water in the clay increases, with an extension related to the nature of the exchangeable cations (significant for Ca and Mg ions or almost negligible for K " and Al ) and activation conditions adopted [9,11,24,38],... 

There are acids with values of the Hammett acidity function of —20 or less these are called superacids because the Hq value for pure H2SO4 is only -12. These... 

The Hammett acidity function has been evaluated for a number of superacid systems, including HSOgF-AsFs and HSOgF-SbEg. The structure of the AsgFjoO- ion in the dicaesium salt has been determined by X-ray... 

Table 1 Hammett acidity functions of selected superacids.

Superacid acidity is measured by the Hammett acidity function ... 

Measuring super acids requires something beyond the normal pH scale from 0 - 14. The superacid scale, or Hammett acidity function, is based on experimental evidence. For example, in this equation... 

Figure 8.1 Comparison of the acidity of selected liquid and solid acids on the Hammett acidity scale. Ho = — 2 for a superacid. (See text for an explanation of Ho) [Reproduced from reference 1 with permission. Copyright 2002 American Chemical Society.]...

Superacids, which can be 10 to 10 ° times stronger than 100% sulfuric acid, have attracted considerable interest in the past several decades due to their increasing use as catalysts for synthesis and their fundamental importance in organic research [441]. The term superacid was first employed by Hall and Conant in 1927, referring to highly acidic solutions in nonaqueous media [442]. Later the term came to be applied to solutions more acidic than 100% sulfuric acid (Hammett acidity function Hq = -12) [443,444]. Examples of Br0nsted acids more acidic than sulfuric acid include flu orosulfuric acid (HSO3F, Hq = -15.1) and trifluoromethanesulfonic acid rCF SO H. Hq == -14.11 [441.445]. With Lewis acids. Olah, Prakash. and... 

The above synthesis of molecular fluorine is not useful in a practical sense. Much more important is the application of SbFj in the preparation of superacids, which are deflned as acids that are stronger (i.e., have greater protonating power) than 100% sulfuric acid. The acidity of such strong acids is typically quantified by the Hammett acidity function, which we encourage you to read up on at your convenience. 

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