Hammett functions

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). 

It was at Columbia that Hammett carried out his research on the physical properties of organic compounds that made him famous. His first accomplishment was his development of a concept now known as the acidity function, a new interpretation of the behavior of acids in concentrated and dilute solutions. He also derived the Hammett equation and Hammett function for organic substances, accomplishments for which he is perhaps best known today. In 1940, he published Uistextbook Physical-Organic Chemistry, which some chemists have called "one of the great textbooks in the history of chemistry."... 

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 introduction of substituent groups into the benzo rings on dibenzo-18-crown-6 ethers leads to marked effects on their selectivity. A reversal of the usual selectivity sequences occurs when strong, electron-withdrawing substituents are present.490 Similar effects are found for benzo-15-crown-5, and a good correlation is found with the Hammett functions of the substituents.491 In contrast for substituted benzo-18-crown-6 systems, no such correlation was found, indicating that caution must be exercised on extrapolating from one crown ether system to another. 

Hammett Function Ho Melting Point (°C) Boiling Point (°C)... 

Hansch and Fujita used a Gaussian probability function to characterize the partitioning Step 1 and the Hammett function (log(k/k0) = pa) to describe the rate Step 2 in their model (10,12,13). By appropriate mathematical treatment, they arrived at the following general structure-activity relationship which has come to be termed the Hansch Equation. 

Hydrohalides dissolve easily in water to form hydrohalic acids and undergo virtually complete ionic dissociation (equation 84). All the hydrohalic acids, except HF, are very strong acids. Although AHF is one of the most acidic liquids, as measured by its Hammett function, hydrofluoric acid is a weak acid. 

PMR studies of the corrinoids 90, 93) have shown that the t value of the proton on the methene bridge Cio depends on the nature of the axial ligands, and that there is some correlation between the t value and the energy of the first electronic transition of the corrin ring. When Y = Bz the resonance moves to higher field in the order X = HgO < HO < CH2=CH < CHs . Finally Hill et al. (91) have shown that there is a correlation between the t value of the methyl protons in [Co(DMG)2 P 3-X] and the Hammett function of X, the t values increasing in the order X = N02 < CN < Cl Br 1 ONO < alkyls. 

This stereochemically unfavorable reaction permits the conclusion that the formation of the product depends on electronic factors. In the course of the reaction the C-0 bond is opened more quickly than the new C-C bond is formed. A carbonium ion results, which is stabilized by interactions with the neighboring groups. Boireau et al. comprehensively examined every detail of the reaction. l-Phenyl-2,3-epoxypropane and derivatives substituted on the phenyl group give alcohols of the same type, but the substituents exert considerable effects on the reaction. The value of p calculated from the Hammett function is 5.8. The results for 1,2-diphenyloxirane derivatives are presented in Eqs. 249 and 250. ... 

A topological Hammett function a, was also proposed and is defined by the most significant differences between the connectivity indices as ... 

The starting reactant of this second process was suggested to be the cation 118, which is initially oxidized in a two-electron step, and the final product formed in the time scale of CV experiments (v = 0.010 — 0.100 V s-1) undergoes cathodic reduction at a potential more positive by about 0.15 V than the reduction of 122. With the increase of acid concentration this new cathodic peak increases at the cost of the peak corresponding to 122. This electroactive product was not identified, but from the dependence of halfwave and peak potentials of the second oxidation process on the Hammett function it... 

Acidities of montmorillonites can be evaluated according to their Hammett functions (Hq) (Table 4). From these values, the acidity of clay minerals can be compared to that of HNO3 (-5) or H2SO4 (-12). 

On the other hand, dipolar solvent molecules may also compete with the aquo ligands in the coordination sphere of the metal cation, e.g., in the nonprotonated acid form (HX). Adduct formation in the absence of proton exchange or ion association (e.g., in nonaqueous solvents) is denoted as an Lewis acid-base interaction. The adduct formation may be described and evaluated by using the Hammett function introduced previously (Equations 8.54 and 8.55) according to - °... 

The Hammett function may also be used for the determination of the acidity of surface sites. For solids dispersed in nonaqueous liquids, Walling defined the acid strength as the ability of the surface sites to convert an adsorbed neutral base into its conjngate acid - °... 

The same equation has also been used to quantify the amount of specifically adsorbed cations. For both Lewis and Brpnsted acids and bases this adsorption isotherm relates to the Hammett function (Equations 8.80, 8.109, and 8.110). 

Addition of HCl can enhance acidity the chloride ion substitutes for an oxygen ion. The catalytic Brpnsted acid in silica-alumina and related catalysts can be very strong. It can have a strong tendency to donate protons (up to a Hammett function of Hq = —13.3). 

If we consider the studied solution to be infinitely diluted we can neglect the term connected with the activity coefficients of the charged particles in the solution and obtain finally that the Hammett function is dependent only on the proton activity and the primary medium effects for the conjugate indicator pair ... 

Fig. 1.2.1. The dependence of the Hammett functions (2), the optical basicities A, obtained with the use of Bi(III) (3) and T1(I) metallo-indicators of some melts and superacids from the optical basicity obtained with the use of Pb(II) metallo-indicator (1).

Tremillon introduced the common oxoacidity function, ft, as a measure of the relative acidity (basicity) of solutions of various substances in molten media. This parameter is similar to the well-known protic acidity functions, H0 (Hammett function) and pA (Izmailov s acidity function) considered in Part 1. In terms of the primary medium effects, the function ft may be represented as the sum of the primary medium effect of O2- for the given medium (—logy0O2 ) and the instrumentally measured pO (pOl) in this medium ... 

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. 

When the equilibrium constants were plotted again st the Hammett function for the substituent X, a U-shaped curve resulted, showing all substituents to enhance the stability relative to styrene. This indicated a double bond to present between the metal and olefin, with both the 7r and a components contributing equally. Equilibrium constants were of the order 0.03 -0.05. The relative stabilities of cis- and trans- olefin complexes with platinum(II) were found from a similar study 73>, following the reaction... 

Retcofsky and Griffin have outlined a method for determining Hammett substituent constants from chemical shift values in phenylphosphorus compounds (Table X). The values of the Hammett functions [Pg.175]

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