Hammett’s Acidity Function

In 1988 Masoud and Ishak demonstrated that ( -arenediazo methyl ethers do not react with 2-naphthol in dry organic solvents such as dioxan, ethanol, 2-propanol, but only in the presence of water. The reactions are catalyzed by hydrochloric acid (even in the absence of water). Under such conditions almost quantitative yields of azo compounds were obtained. A careful and extensive kinetic investigation of the HCl-catalyzed dediazoniation of substituted benzenediazo methyl ethers, varying the HC1 concentration and the diazo ether/2-naphthol ratio (the latter either absent or in large excess), and comparing the observed rate constants with Hammett s acidity functions for dioxane and ethanol (see Rochester, 1970) indicated the mechanism shown in Schemes 12-8 to 12-10 (DE = diazo methyl ether, D+ = diazonium ion). 

Certain expressions describing a solvent acidity function, where S is a base that is protonated by an aqueous mineral acid solution. The equations describe a linear free-energy relationship between log([SH+]/[S]) + Ho and Ho + log[H ], where Ho is Hammett s acidity function and where Ho + log[H+] represents the activity function log(7s7H+/ysH ) for the nitroaniline reference bases to build Ho. Thus, log([SH+]/[S]) log[H+] = ( 1)... 

Only at pH of —7.3 expressed as Hammett s acidity function H0. Butin the excited state, protonation occurs at pH zero suggesting that benzoic add is more basic or less acidic in the excited state. 

Lane38 measured the rate of hydrolysis of ethyl acetate at 25°C in 11 -79% sulphuric acid by both spectrophotometric and dilatometric techniques. He also measured spectrophotometrically the concentration of the conjugate acid of the ester in solution by measuring the absorptivity at 190 nm, and extrapolating to zero time, and by the change in the chemical shift of the acetyl protons. He found a linear relation between the ionization ratio and Hammett s acidity function... 

The Hr function is also known as the C function (22) and is closely related to the J function (23) ancT the well known Hammett s acidity°function (2JT-It is reasonable to assume... 

H0 = Hammett s Acidity Function B = p-nitroaniline BH+ = protonated p-nitroaniline... 

Although Dahn and Gold accepted Lane and Feller s diagnosis of the mechanism in acetic acid, they argued that a change of mechanism occurs in the more polar aqueous medium with its weaker nucleophiles (water as opposed to acetate), and that reaction of the diazonium ion is now unimolecular. The more positive entropy of activation and the correlation with Hammett s acidity function were cited in support of their view. However, neither of these criteria can now be considered definitive, and the most striking fact is the similarity of the results to those for diazoacetic ester. Careful application of the Swain-Soott relation would appear to provide the best criterion of the molecularity of nitrogen loss, but at present suitable results are not available. 

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. 

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. 

Again the reference point for discussion is Hammett s acidity function treatment (Hammett and Deyrup, 1932 Hammett, 1970). We write the Bronsted acid-base equilibrium in terms of proton loss from BH+ as in eqn (2), for which the thermodynamic equilibrium... 

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