Hammett acidity scale

The exact p R+ value for 20+ could also not be determined by the earlier method. Therefore, dimethyl sulfoxide (DMSO)-water-tetramethylammonium hydroxide (0.011 M) system was used for the measurement. The H scale (Hammett acidity scale) for the system ranges from 12 in water up to 26.2 in... 

Figure 8.1 Comparison of the acidity of selected liquid and solid acids on the Hammett acidity scale. Ho = — 2 for a superacid. (See text for an explanation of Ho) [Reproduced from reference 1 with permission. Copyright 2002 American Chemical Society.]...

Finally, it should be pointed out that the question of connecting the p.K g-values measured in the system anhydrous hydrofluoric acid+NaF and/or BFg, to the Hammett Ho-scale still remains. In order to answer this question, Hyman and collaborators (1957) determined the function for the systems HF-1-H2O and HF+NaF. For the latter system, these authors obtained the following values ... 

Titration method (consisting in the stndy of the interaction of indicator dyes with solids from solntions) is a techniqne for both qnalitative and qnantitative characterization of solid snrfaces. If a basic indicator B is nsed, the proton acidity of the snrface is expressed by the Hammett acidity fnnction. Similarly, the basicity can be defined when an acid is converted by its conjngated base. This allows defining acidity and basicity in the same scale. 

Sulfated tin oxide (STO) is classified as one of the strongest solid acids (STO calcined at 550°C ranks first among sulfated metal oxides according to the Hammett function scale, Hq value = 18). However, the use of STO has been more limited than that of SZ (calcined at 650°C, Hq value = —16.1) due to preparation difficulties and poor yields. However, new preparation routes are making this catalyst more accessible, and recently its use has become more widespread. In a recent study by Furuta et al., STO was compared to SZ in the esterification of n-octanoic acid with methanol. The STO catalyst showed superior activity compared to SZ at temperatures below 150°C. For instance, STO approached a 100% ester yield at 100°C, while SZ required temperatures as high as 150°C to reached similar yields. 

The addition of hydroxide ions to substituted benzaldehydes (ArCHO + OH <=> ArCH(0H)0 ) is used to establish J-acidity scales in water-ethanol and water-DMSO mixtures containing sodium hydroxide as a base. The pK-values in such mixtures are linearly correlated with Hammett substituent constants. The independence of reaction constant p of solvent composition confirms that substituted benzaldehydes are suitable J- indicators for hydroxide solutions in water-ethanol and water-DMSO mixtures. Dependence of J- values on sodium hydroxide concentration is only slightly affected by ethanol up to 90 % and at a constant sodium hydroxide concentration shows only small increase between 90 and 98 % ethanol. J- increases more with increasing DMSO concentration, but the effect is much smaller than that of DMSO on H- values based on proton abstraction from aniline. 

H0 is known as the Hammett acidity function, and the series of substituted anilines used to establish the scale are called Hammett indicators. 

The original scheme of Hammett (8) was to define an acidity scale for non-aqueous media (He) such that... 

At this point, we can only refer to the various acidity scales for different series of solvents, such as those of Hammett [15] and Grunwald [16]. The acidity functions are introduced in order to obtain expressions that are not affected by the relative permittivity and that allow a quantitative comparison of acidity in different solvents. It should be stated, however, that there exists no single scale of acidity or basicity that is universally valid in all types of solvents and appHcable to both equilibrium and kinetic situations [17, 109]. 

When an acid solution is highly diluted in water, the pH acidity scale is used, which becomes problematic when the acid concentration increases, and more so, when non-aqueous media are employed. Considering the limited application of the pH scale, a quantitative scale was provided by Hammett and Deyrup to express the acidity of more concentrated or non-aqueous solutions that is, Hammett acidity function, Hq [1]. 

Rate data for acid catalyzed reactions in moderately concentrated aqueous solutions of strong acids may be correlated with the Hammett H0 acidity scale [83]. The definition of the H0 function (see Vol. 2, p. 358) is given by... 

Similar acidity functions /f and for anionic and cationic bases have been proposed. As might be predicted, the specific interaction effects with cationic, anionic, and neutral indicators are different, and the acidity values and may be either higher or lower than Hq. The nitroanilines apparently are exceptionally well-behaved substances in terms of setting up an acidity scale, and Hammett later stated that the choice was fortunate. ... 

Acidity scales for highly acidic solutions in media of low dielectric constant have been proposed, such as for ethanol-water mixtures and glacial acetic acid-acetic anhydride. Ion pairing is a complicating factor in these solvents. The extent of formation of ion pairs and of free ions differs for various indicators and salts in particular, dissociation constants are sensitive to the size of the anion. The Hammett postulate therefore cannot easily be extended to include media of low dielectric constant. 

The normal pH concept introduced above is restricted to dilute solutions. The pH scale may also be expanded by accounting for the changes in the activity coefficients of the acid and the conjugated base. The Hammett acidity function for solutions is related, e.g., to Equations 8.30 and 8.31 for the first deprotonation step in the following way °... 

It is obvious that the expression enclosed in the brackets by the author of the present book is nothing but the primary medium effect of O2- expressed via the difference in the values of the equilibrium constants of equation (1.3.6) for the media compared the molten equimolar KCl-NaCl mixture, which was chosen as a reference melt, and for which pKHa/H20 was found to be 14 at 700 °C, and the melt studied. As to the physical sense of the common acidity function Cl, this is equal to the pO of the solution in the molten equimolar KCl-NaCl mixture, whose acidic properties (oxide ion activity) are similar to those of the solution studied. Moreover, from equation (1.3.7) it follows that solutions in different melts possess the same acidic properties (f ) if they are in equilibrium with the atmosphere containing HC1 and H20 and Phc/Ph2o — constant. This explanation confirms that the f function is similar to the Hammett function. Therefore, Cl values measured for standard solutions of strong bases in molten salts allow the prediction of the equilibrium constants on the background of other ionic solvents from the known shift of the acidity scales or the f value for the standard solution of a strong Lux base in the solvent in question. According to the assumption made in Refs. [169, 170] this value may be obtained if we know the equilibrium constant of the acid-base reaction (1.3.6) in the solvent studied. 

The concepts of hydrogen ion concentration and pH discussed above are meaningful only for dilute aqueous solutions of acids. In water-like solvents such as methanol, similar concepts may be developed, but in most organic solvents, in concentrated aqueous solutions and in the anhydrous state the concept is meaningless and some other scale of acidity is required. There are a number of different acidity scales but the one most commonly used is the Hammett acidity function, which allows comparison of the same acid in different media as well as intercomparisons of acids. The function H0 pertains to the equilibrium between a base, B, its conjugate acid, BH+, and the proton, H+ n+H+ RH+... 

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 various species present in nitration media of different acidities are presented in Figure 1. The forms of the oxy-aclds of the N(III) and N(V) oxidation states of nitrogen are shown as a function of acidity also shown is the point of significant protonation of 2,4-DNT. The scale used at the bottom of the figure is the Hammett acidity function, used here for convenience, since the function spreads the acid regions of interest, even though the species involved are not necessarily Hammett bases. 

There are indications that cyclohexadienones are not Hammett bases 3M-226) that in many cases their protonation is better described by the amide acidity function Therefore the basicity of cyclohexadienones is generally characterized either by the value of pK in the scale or by that of (H )ip f e value of the Hammett acidity function at which the cyclohexadienone is half converted into a conjugate acid — the hydroxybenzenium ion. The available data on the basicity of cyclohexa-2,5-dienones are summarized in Table 4. In a comparison is drawn between the baadty of cyclohexa-2,5-dienones and that of cyclohexa-2-enones. 

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