Hammett acidity function superacid

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). 

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... 

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... 

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. 

Table 5. Hammett Acidity Function Values of the Most Important Superacids...

Up to a // value of — 10, all indicators are primary amines and are therefore suitable for the measurement of the Hammett H() function. For stronger acids, new indicators such as nitro compounds have to be used. Although the acidity function scale based upon nitro compounds as indicators may not be a satisfactory extension of the aniline indicator scale, Gillespie and Peel18 have shown that the most basic nitro compound indicator, para-nitrotoluene overlaps in a satisfactory manner with the weakest indicator in the aniline series, 2,4,6-trinitroaniline. Thus, the acidity measurements using the nitro compounds may be considered to give the best semiquantitative picture of the acidity of the various superacid systems. 

Acids which are stronger than 100% sulfuric acid are called superacids. Fluorosulfuric acid (HSO3F) is one of the strongest Brpnsted acids known, with//o (Hammett s acidity function) = -15.1. Its acidity is comparable to that of highly concentrated oleum, H2SO4 SO3 however, because of its stability, ease of purification, wide liquid range (m.p. = -89 °C, b.p. = 162 C) and relatively low viscosity (1.56 cP at 28 °C), and lesser oxidizing ability, it is more convenient to use. 

What makes Nafion so useful for catalytic purposes is the superacidity of the acidic form of the polymers. Because the sulfonic acid group is attached to a highly electron-withdrawing perfluoroalkyl backbone, a relatively high polarization of the -OH bond results. Because methods for measuring solution acidity cannot be directly applied to heterogeneous solid materials, determination of the acidity of Nafion-H is difficult. Nevertheless, studies indicate that the acidic character of Nafion-H resin is comparable with that of 100% sulfuric acid, because the estimated Hammett Hq acidity function value of Nafion-H, approximately —12, is comparable with that of 100% sulfuric acid [1]. 

The basic assumption of the Hammett method is that the activity coefficient ratio /bh+//b is independent of the nature of B at any given solvent composition. This seems to be a reasonably good assumption for a closely related series of bases, such as the primary anilines, but may not be true for bases of different structural types. This has led to the proposal of many different types of acidity function, and to much controversy as to the meaning and usefulness of the Hammett and related acidity functions. This subject has been extensively reviewed elsewhere [16]. Although it now appears that the concept of the acidity function is perhaps not as generally useful as was originally believed, at the present time we do not have any more reliable method of assessing the acidities of superacid media. 

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