Hammett acidity function, values

Lewis and Bigeleisen,71 who first determined the Hammett acidity function values for oleums, used a method to derive them from the vapor pressure measurements. Brand et al.,72 however, subsequently by the use of nitro compound indicators showed that the H0 values are not directly related to the vapor pressure. The most accurate Ho values for oleums so far have been published by Gillespie et al.7 (Table 2.4). 

Table 2. Hammett Acidity Function Values of Some Common Acids...

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). 

The spectral changes which occur in increasingly acid solutions of polyaza-heterocycles may indicate a second ionization. This event, however, can readily be distinguished from dehydration by measuring the spectra in anhydrous dichloroacetic acid, provided that the pKa value for the anhydrous species is above 1. Anhydrous dichloroacetic acid has a Hammett acidity function (Hq) of — 0.9 (as determined using o-nitroaniline as the solute), and the ultraviolet spectrum of a base with a p > 1 would be that of the anhydrous cation in this 2 A. Albert and W. L. F. Armarego, J. Chem. Soc. 4237 (1963). 

Table 98) correlated with the Hammett acidity function H0 rather than with log [ROH]/[R+]377. However, subsequent spectroscopic analysis and colligative property measurements showed that anils are present in strong sulphuric acid entirely as the monoprotonated species and since reaction is not instantaneous, it must be occurring on some other species, assumed to be diprotonated. For this to be true a plot of log rate versus the acidity function H+ would have to be linear the values of H+ were not known in this medium but were considered to parallel... 

Kresge and Chiang15 showed that the dependence of rate coefficient upon the Hammett acidity function was not a safe criterion of mechanism and also explained how different dependencies of rate upon acidity function for different compounds could arise. Rate coefficients for detritiation of [3H]-2,4,6-tri-methoxybenzene followed the acidity function h0 in perchloric acid up to 3 M, i.e. k = 0.484 (Ao)1 07. so that a plot of log k versus —H0 would have a slope of 1.07, which implies the A-l mechanism. (New acidity function values479 would make this slope 1.14.) However, the diagnostic value of such a correlation was... 

Hall and Robinson162 later reported pXBH+ values of various sulphones obtained from equation 27, where H is the Hammett acidity function and Kh is calculated from (28). 

When a solute is added to an acidic solvent it may become protonated by the solvent. If the solvent is water and the concentration of solute is not very great, then the pH of the solution is a good measure of the proton-donating ability of the solvent. Unfortunately, this is no longer true in concentrated solutions because activity coefficients are no longer unity. A measurement of solvent acidity is needed that works in concentrated solutions and applies to mixed solvents as well. The Hammett acidity function is a measurement that is used for acidic solvents of high dielectric constant. For any solvent, including mixtures of solvents (but the proportions of the mixture must be specified), a value Hq is defined as... 

Johnson and Stratton133 have compared pAbh+ values obtained using the Hammett acidity function method and the excess acidity method, and they consider that results obtained using the former may correspond more nearly to... 

Fig. 14. Values of Hammett acidity function (Wo) of H XW O o as a function of the negative charge of the polyanion. (From Ref. 63.)...

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). 

Values of the Hammett acidity function for these systems were first given by Lewis and Bigeleisen (1943) who proposed that for fuming sulphuric acid the acidity function could be determined from the vapour pressure of S03 above the solution. However, as pointed out by Paul and Long (1957), there is no particular reason to expect the acidity to be related to the S08 pressure. The acidity function measurements of Brand (1950) and Brand et al. (1952) confirmed, by the use of aromatic nitrocompound indicators, that the acidity does indeed increase with increasing concentration of S03 but Brand s H0 values do not appear to be related to the vapour pressure of S03. 

They then embarked on Hammett Acidity Function measurements [7] of several of the solutes previously studied by conductance measurements. Their results are summarised in Fig. 3. S03 is a very weak acid, increasing the acidity of HSO3F by less than 0.5 units beyond the H0 value of —15.07 for the pure acid. As in... 

Numbers in parentheses are values of the Hammett Acidity Function (Ho) calculated on the basis of the reported experimental preparative conditions and represent approximate acidity thresholds above which the particular species are stable in solution. 

In dilute aqueous solution, the acidity is measured using pH values. For concentrated acid solutions and non-aqueous acid solutions, pH values are no longer available. Hence, the Hammett acidity function Ho is used as a measure of the acidity of such media [130]. The proton donor ability of an acid in such media is measured by studying the equilibria of a series of indicator bases B (mostly nitroanilines), the UV/Vis absorption spectra of which are markedly different from those of their conjugate acids, so that the indicator concentration ratio I = [B]/[BH+] can be measured spectrophotometri-cally. The acidity function Ho is then given by Hq = p.Kbh+ + Ig f with the subscript zero indicating that the Hq function applies only to neutral bases B [130, 170], For dilute solutions, Ho corresponds exactly to pH in concentrated solutions, the two functions differ appreciably. 

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