Hammet function

Similarly, if interaction is with a Lewis acid site (A), the Hammet function is expressed as... 

Due to the different aeid-base properties of the wide variety of components whieh are able to form DESs (ionie salts, earboxylic alcohols, polyols, amides, aminoacids...) the aeid or basie strength of the resulting eutectic mixtures could be tunable. The Hammet function has been used to evaluate the aeidity or alkalinity of different DESs by determining the ionization ratio of indieators in a system. When weak acids (HI) are chosen as indieators, the Hammet function (H ) follows the equation ... 

Deep eutectic solvents have been extensively used as an alternative green and biorenewable solvents to traditional VOCs in organic synthesis. Moreover, the addition of water to the reaction crade allows the easy separation (layer separation or precipitation) of the desired organic product. In most of these reactions, DESs were not only solvents for reactants, but also catalysts. In this sense, Shankarling and co-workers reported the reaction rate improvement of the electrophilic substitution of l-aminoanthra-9,10-quinone derivatives in the eutectic mixture lChCl/2Urea as compared wifli conventional VOCs solvents such as methanol or chloroform (see reaction (a) in Scheme 20.3.1). This rate enhancement could be related to the aforementioned basic nature of this eutectic mixture (Hammet function (H ) value 10.86). At the end of the reaction, die addition of water precipitates the dibrominated product (84-95% yield). The system was easily recycled (up to five consecutive times) by evaporation of water without appreciable lost in activity. 

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

In the recent past the potential of zeolites in the manufacture of fine chemicals has received considerable attention. High-Si zeolites can have Hammet s acidity function Ho of -12.8 which is close to those for superacids. MCM (Mobil Catalytic Materials) have opened up new vistas due to larger pore sizes. 

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp > sp spl... 

Possible methods of determining the extent of protonation include absorption spectroscopy at a wavelength for which species with n and fir-1 protons have different extinction coefficients, freezing point depression, and electrical conductivity (15). Of these, we have utilized only spectroscopy, which has the disadvantage that only the equilibrium constants for the most highly protonated states are accessible if, as is usual, the species with low protonation are insoluble. In this method, the extinction coefficient c of the compound is determined as a function of the H2SO4 content in the sulfuric acid solvent and correlated with the Hammet acidity function H0 (18) to give the pKB value of the protonated species,... 

Hammet L.P., Deyrup A.J. The acidity functions of mixtures of sulfuric and perchloric acids witti water. J. Am. Chem. Soc. 1932 54 2721-29. 

The electrochemical behaviour of the ribbed-functionalized iron(II) [65, 68] and ruthenium(II) [78] clathrochelates with alkylamine, thioaryl, thioalkyl, phenoxyl and crown ether substituents in a-dioximate fragments was characterized by E1/2 values for Fe3+/2+ and Ru3+/2+ couples (Table 37). The Tomes criterion values of most complexes exhibited reversible or quasi-reversible anodic processes. Moreover, the quasi-reversible oxidation processes are accompanied by the formation of insoluble products followed by passivation of the working electrode. The Ev2 values depend on the electron-donating properties of the substituents in the ribbed fragments. The correlations of E1/2 values for Ru3" 2+ and Fe + 2+ couples with these substituents Hammet s Opara constants were observed in Refs. 65, 68, and 78. These correlations are rather qualitative, but they enable one to conclude that ruthenium complexes are less sensitive to the change of substituents in dioximate fragments. There was no correlation between the Em values and the inductive Taft s (cr,) constants for substituents in dioximate fragments. 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

Figure 11. Correlation of triboelectric charging with electronegativity, as measured by Hammet sigma function. (Reproduced from Ref. 66. Copyright 1975, American Chemical Society.)...

In these medium the acidity cannot be measured by pH. The Hammet-Deyrup acidity function , Ho (ref. 6), which shows the facility of protonation of a weak base by a superacid, allows to class the superacids (ref. 7) (Fig. 1). 

A. Hammett Substitution Constant (o). It is a measure of either the electron-withdrawing or electron-donating capability of a substituent i.e., the functional moiety). Hammet substitution constant may be determined conveniently by actual measurement of the dissociation of a series of benzoic acid substituted derivatives vis-a-vis the dissociation of pure benzoic acid itself. 

Examination of O chemical shifts of several 2-substituted and 2,5-disubstituted furans <85MRC985> has demonstrated the additivity of substituent effects by plotting the O chemical shifts of 2,5-disubstituted furans versus those of the 2-substituted analogues. Failure of the electronic character of substituents in determining O chemical shifts is found, because they do not correlate with Hammet constants. Such a lack of correlation is known for C ortho chemical shifts in aromatic systems. A good correlation is obtained by plotting O chemical shifts versus a parameter Q of the substituent—a parameter which accounts for the paramagnetic contributions and is a function of the polarizability and ionization potential of the substituent. The cyano- and methylfurans, however, do not follow the Q relationship. 

In ion-pair chromatography, Tomlinson and Riley [9] have estimated the ion retention factors using functional group values, t, related to the Hammet constants. This shows that the hydrophobic part of the ionic solutes is responsible for file retention as well as its ionic part. The later part may be pH and ionic strength dependent. 

B] and [BH ] are, the concentrations of the neutral base and its conjugated acid. If the pKa (= —log Ka) value of the indicator is, say +3.3, then the amount of base added is equivalent to the amount of acid sites having the Hammet acidity function Ho < +3.3, where ... 

The first to have introduced such a relationship is Hammet, through what is called the Hammet acidity function. 

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