Sulfuric acid: Hammett acidity function

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... 

A useful diagnostic tool for investigating possible hydration of cations of bases for which pA is greater than about one is the measurement of their ultraviolet spectra in aqueous acid solutions and also in an anhydrous acidic solvent such as dichloroacetic acid (for which the Hammett acidity function, Hq, is — 0.9, and in which hydration of the cation cannot occur). This technique has been used with quinazoline to obtain spectra approximating those of the hydrated and anhydrous cations, respectively. For weaker bases, spectral measurements in sulfuric acid-water mixtures of increasing acid content may be used to reveal a progressive conversion of hydrated into anhydrous species as the thermodynamic activity of the water decreases. 

Figure 3.1 Values of various acidity functions in mixtures of water and sulfuric acid. Data for H0, Hm, Hi, and H n are from L. P. Hammett, Physical Organic Chemistry, 2nd ed., McGraw-Hill, New York, 1970, p. 271 data for Hc are from M. T. Reagan, J. Amer. Chem. Soc., 91,5506 (1969). Adapted by permission of McGraw-Hill.

A kinetics study has been performed on the formation of 1,3,6-trioxocane in the interaction of formaldehyde with diethylene glycol in the presence of 0-0.5moll-1 of sulfuric acid <1998ZFK1031>. The rate of the reaction was found to be determined by the equilibrium of cyclization of half-acetal into eight-membered ring and of reverse process of hydrolysis. Rate constants demonstrated linear dependence on Hammett acidity function. Activation energies were 20.9 and 14.0 kcal mol-1 for the formation of cyclic acetal and its hydrolysis, correspondingly. 

Acid catalysis is an important kinetic phenomenon, and its study often requires the use of concentrated acid solutions, in which the conventional pH scale is not applicable. In such solutions (e.g., sulfuric acid-water mixtures covering the full range of compositions) the acid component simultaneously functions both as an acid and as a solvent thus, a medium effect is superimposed on the acidity effect. In this section we briefly describe the acidity function approach to coping with this problem. (A comparable approach can be taken to the study of highly basic solutions.) For more detailed reviews see Hammett,Rochester, and Stewart. ... 

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 ... 

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. 

Several of the values for acidity function Ho measured by Hammett and Deyrup for water-sulfuric acid mixtures are recorded below. The acidity function of concentrated sulfuric acid is seen to be 10.6 and therefore this compound is (10.6 - - 7.0) = 17.6 units more acid than water (pH of water at 25° is 7.0). 

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. 

Nitric acid ionizes as a base, toward the sulfuric acid, and the maximum rate of nitration occurs at a definite acidity defined by Hammett s Ha function, a logarithmic measure which is equivalent to pH in dilute acids but which acquires a negative value in super-acid solutions. For the nitration of aromatic compounds, on which these rate studies were made, the maximum was reached in sulfuric acid of 90 % concentration. So the simple reaction... 

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. 

Although the ultraviolet spectrum of benzofuroxan undergoes an extensive, and reversible, change on solution in fairly concentrated sulfuric acid, suggesting that protonation is occurring, the site of proton attachment is not known. Benzofuroxans behave approximately as Hammett bases (Gr. (logio[BH+] — logio [B])/Hq= 1), with pK s —8.3+ 0.1 (unsubst.), —8.4+ 0.1 (4-Me), —8.2+ 0.1 (5-Me) (cf. — 8.4 + 0.1 for benzofurazan). C-Protonation (as 35) would probably require the protonation to follow a steeper acidity function (cf. azulenes ), and therefore N- (36) or 0- (37) protonation is in-... 

Melting points of monocyclic triazines are given in Table 4. Basicity estimated by the theoretical method is also given in the previous section. The observed pA s of 4,6-dimethyl- (le), 4-methyl-6-phenyl- (li), and 4,5,6-trimethyltriazine (If) are —0.58, —0.79, and 0.07, respectively. The data were obtained from measurement of the UV absorption in sulfuric acid at room temperature using Hammett s acidity function values <89UP 6l3-0l>. The pA determination of the parent triazine (la) failed because of its instability in acidic media. 

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 decomposition of formic acid in the liquid phase influenced by acids was intensively studied by Hammett. He found a beautiful correlation between the rate of decomposition of formic acid in concentrated sulfuric acid and the acid function [Hammett (101), Long and Paul (103)] ... 

Louis P. Hammett (1894-1987) made two major contributions to physical-organic chemistry the first one (on which wc will focus) is the concept of supcracidity and his acidity function. The. second one is the so-called llammeti equation wiiich concents the correlation of equilibria and rales for reactions of substituted aromatic compounds. With A.H. Deynip. Hammett set up an acidity scale based on the indicator properties of aromatic amines, a. scale that measures 100% sulfuric acid as lo limes as strong as 10% acid, and then showed that this acidity scale was relevant to chemisir> ... 

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. 

Hammett, L. P. and Deyrup, A. J. A series of simple basic indicators. I. The acidity functions of mixtures of sulfuric and perchloric acids with water. J. Am. Chem. Soc. 54 2721, 1932. 

Nafion resins have high chemical and thermal stability. The maucimum continuous operation temperature of Nafion-H is about 430 K in anhydrous systems. The maximum temperature in aqueous systems is 420 — 510 K. Besides its thermal and chemical stability, the feature that makes Nafion type polymers catalytically useful is their high acid strength of the acid form of polymers (Nafion-H ). Since the sulfo group is attached to a highly electron-withdrawn perfluoroalkyl backbone, a relatively high polarization of O-H bond results. Thus, Hammett Ho acidity function value of Nafion-H was estimated to be -10 —12, which is comparable to or stronger than that of 96-100% sulfuric acid. ... 

The acidity of hydrofluoric acid solutions vary with concentration owing to hydrogen-bond interactions of the fluoride ion. Dilute solutions are weakly acidic with an acid ionization constant Ka = 6.6 x 10 " (or pKa = 3.18), in contrast to corresponding solutions of the other hydrogen halides which are strong acids. Concentrated solutions of hydrogen fluoride are much more strongly acid than implied by this value, as shown by measurements of the Hammett acidity function Ho (or effective pH ). For 100%, HF has an Ho, estimated to be between -10.2 and -11, which is comparable to the value -12 for sulfuric acid. 

The catalytic activity of sulfuric acid seems especially interesting. Depending on the concentration of water or of sulfur trioxide, either sulfur trioxide or the H-acid or both may be responsible for the catalysis. Lewis and Bigeleisen have shown that the very rapid increase in the acidity function of Hammett and Deyrup as 100 per cent sulfuric acid is approached is probably due to sulfur trioxide. This might indicate that in the sulfonation of benzene the sulfur trioxide is the principal agent in a direct electrophilic attack. 

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