Acidity functions Hammett

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 kinetics of reactions cataly2ed by very strong acids are often compHcated. The exact nature of the proton donor species is often not known, and typically the rate of the catalytic reaction does not have a simple dependence on the total concentration of the acid. However, sometimes there is a simple dependence of the catalytic reaction rate on some empirical measure of the acid strength of the solution, such as the Hammett acidity function Hq, which is a measure of the tendency of the solution to donate a proton to a neutral base. Sometimes the rate is proportional to (—log/ig)- Such a dependence may be expected when the slow step in the catalytic cycle is the donation of a proton by the solution to a neutral reactant, ie, base but it is not easy to predict when such a dependence may be found. 

Table 3.7 Hammett acidity functions for some anhydrous acids...

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). 

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. 

In the first instance, acidity influences the acid-base equilibria of the reactants. The amine is a Bronsted base. Aniline, a typical substrate, has pKa = 4.6, which means that the protonation shown in Scheme 3-11 is almost complete under normal conditions of diazotization (pH < 1). The base is definitely a much better reagent than the anilinium ion for nitrosation because the latter is an electrophilic substitution. One expects — simply on the basis of the equilibrium shown in Scheme 3-11 — that the rate of diazotization should decrease linearly with increasing acid concentration or, at higher acidities, with the Hammett acidity function h0 (for acidity functions see Rochester, 1970 Cox and Yates, 1983). 

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... 

In this, as in many catalysed reactions, the protonated substrate is postulated as an intermediate, and although the proposed reaction scheme in fact accords with all the known experimental facts it perhaps would be instructive to determine the dependence of the rate coefficient on the Hammett acidity function at high acid concentration and also to investigate the solvent isotope effect kD2JkH20. Both these criteria have been used successfully (see Sections 2.2-2.4) to confirm the intermediacy of the protonated substrate in other acid-catalysed aromatic rearrangements. 

The third approach is based on the correlation of rate coefficients with the Hammett acidity function, and is known as the Hammett-Zucker hypothesis. Although... 

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... 

Permanganic acid has a pA of —2.25 in perchloric acid and one of —4.6 in sulphuric acid when the Hammett acidity function, Hq, is used. Accordingly HMn04 is present to a significant extent only in strongly acidic solutions and comparatively few of the reactions which have been examined involve anything other than MnO. ... 

J(PQ) and J(PN)). Trends in J(P170) have been used to study the protonation of phosphoryl tribromide, phosphoryl trifluoride and dif1uorophosphoric acid. Nitric acid had a greater protonation power than Hammett acidity functions indicated.77 This coupling has been the subject of a semiempirical sum over states theoretical study.7 ... 

Canada T., Allain L., Beach D., Xue Z. High-acidity determination in salt-containing acids by optical sensors. The scope of a dual-transducer approach and the Hammett acidity function, Anal. Chem. 2002 74 2535-2540. 

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... 

Hammett acidity function, 5 207 14 9 Handbook 44 (National Institute of Standards and Technology), 26 237-238... 

According to this, there is a linear relationship between the logarithm of the partition coefficients, log (cah+/ca)> the Hammett acid function Ho- Plattner et al. (1949) were able to confirm this relationship for a substantial number of methyl-substituted azulenes. However, the further analysis of these measurements with a view to obtaining pAg proved difficult since the partition coefficient Pa of the free azulene between the two phases, which enters into equation (12c), is unknown. For this reason, the authors took the for which Cah+ = Ca as a charac-... 

Hammett acidity function chem An expression for the acidity of a medium, defined as ho = Kbh+ BH / B, where Kbh+ is the dissociation constant of the acid form of the Indicator, and BH+ and B are the concentrations of the protonated base and the unprotonated base respectively. ham at a sid ad e. faijk shan ) hand sugar refractometer analy chem Portable device to read refractive Indices of sugar solutions. Also known as protelnometer. hand shCig ar. re.frak tam-3d-3r1... 

The Hammett acidity function only applies to acidic solvents having a high dielectric constant, further requiring that the fi/fm+ ratio be independent of the nature of the indicator. Thus, the Hammett acidity function is applicable for uncharged indicator bases that are anhine derivatives. As mentioned above, other acidity functions have been suggested. No single formulation has been developed that satisfies different solvent systems or types of bases. 

A modification of the Bunnett-Olsen equation concerned with solvent acidity in which log([SH+]/[S]) - log[H+] = m X -h p/ sH where [S] and [SH+] are the solvent and protonated solvent concentrations, and X is the activity function log[(7s7H+/ysH+)] for an arbitrary reference base. In practice, X = - (Ho + log[H+]), called the excess acidity (where Ho is the Hammett acidity function, m = 1- (f), and 4> represents the response of the S + H+ SH+ equilibrium to changes in the acid concentration). See Acidity Function Bunnett-Olsen Equation... 

HALF-OF-THE-SITES REACTIVITY HAMMERHEAD RIBOZYME Hammett acidity function,... 

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