Substituent constants, Hammett

The electronic property of an aromatic substituent is expressed by the Hammett substituent constant, cr. This constant for a given substituent is arrived at by measuring its effect on the dissociation of benzoic acid. Benzoic acid itself is a weak acid and it ionizes into benzoate (CftH5C02 ) and proton only partially in water. The relative proportions of the ionized and nonionized species at equilibrium 

The constants am and crp for a host of substituents are collected in Table 1. It is to be noted that both crm and ap for alkyl substituents such as methyl, ethyl, isopropyl, tert-butyl, and CH2SiMe3 are negative as they exert only the positive 

The equilibrium ionization constants KA for selected p-substituted benzoic acids including benzoic acid, their pKA values, and Hammett s constants are collected in Table 2. The plots of KA x 10s and also pKA against a for these acids are shown in Fig. 4. The data in Table 2 and the plots in Fig. 4 demonstrate their linear relationships. As K,a increases, pK decreases, and a increases. 

The a values are always those determined from the ionization of substituted benzoic acids, but the p values are different for each substrate type and also for each reaction type because they reflect the sensitivity of the substrate to the electronic effect of the substituent. With p 1, the ionization or reaction rate is more sensitive to substituent s electronic effects than is the ionization of benzoic acids. With p 1, electron-withdrawing groups will increase the equilibrium constant or rate, but less than in benzoic acid dissociation. A negative p indicates that electron-donating groups increase the reaction constant. In consideration of reaction rates, a small r often indicates that the reaction involves either radical intermediates or some other pathway that requires little charge separation. 

If the free energy changes in the ionizations of a substituted and unsubstituted benzoic acids are represented by (AG°)R and (AG°)h, respectively, we obtain the relationship (AG°)R - (AG°)H = -2.303/ 7ctr on account of the above derivation. Substituting this into the above equation, we get, 

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The fundamental assumption of SAR and QSAR (Structure-Activity Relationships and Quantitative Structure-Activity Relationships) is that the activity of a compound is related to its structural and/or physicochemical properties. In a classic article Corwin Hansch formulated Eq. (15) as a linear frcc-cncrgy related model for the biological activity (e.g.. toxicity) of a group of congeneric chemicals [37, in which the inverse of C, the concentration effect of the toxicant, is related to a hy-drophobidty term, FI, an electronic term, a (the Hammett substituent constant). Stcric terms can be added to this equation (typically Taft s steric parameter, E,). 

Table 12.2 A list of some of the more common descriptors. Details of some of these descriptors can be found elsewhere as indicated. This table is restricted to those descriptors which can be computed it therefore excludes certain classes (such as the Hammett substituent constants) which are derived from experimental studies (sec Section 12.12).

The reference compound is benzoic acid. The substituent parameter, which is called the Hammett substituent constant cr, is defined by... 

Table 7-1. Hammett Substituent Constants Based on Ionization of Benzoie Aeids ...

We take the view of McDaniel and Brown that the Hammett substituent constants should be defined by Eq. (7-22). Table 7-1 lists many of these constants based on the ionization of meta- and para-substituted benzoic acids. 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

Hammett substituent constant i refers to species i, reaction i, or reactor... 

Fig. 5. Graph of the logarithm of the acceleration of the rate of phenol release due to 0.01 M cycloamylose against the Hammett substituent constant,

The Hammett substituent constants are special cases of these parameters. 

Table 5 reports values of the Hammett substituent constants. 

Experimentally determined pA, values have been compared with Hammett substituent constants (Figure 1) and an excellent correlation was found. This result provided a further support - in accord with some nuclear magnetic resonance (NMR) measurements - for existence of one particular tautomeric form (Figure 1, structure a) among the three possible ones (Figure 1, a, b, c). 

The x-band in malachite green arises from an NBMO—>n transition, so that 3- and 4-substituents affect the energy of the excited state only and bring about spectral shifts of the first absorption band which vary linearly with the appropriate Hammett substituent constants. Thus, electron-withdrawing groups cause bathochromic shifts of the x-band whereas donor substituents cause hypsochromic shifts (Table 6.6) [64,67]. The 3-band arises from a n—>n transition [68] so that substituent effects are less predictable. As the donor strength of the 4-substituent increases, however, the 3-band moves bathochromically and eventually coalesces with the x-band - at 589 nm in the case of crystal violet (6.164), which possesses two NBMOs that are necessarily degenerate [69]. 

Attempts have been made to correlate the effects of acceptor groups on the C=C barriers by Hammett substituent constants. Since the important interaction is concerned with the delocalization of a negative charge, the cr scale should be more appropriate than the normal a scale (136). However, in the ctp scale, CN is more efficient than COMe, which is contrary to experience, and therefore Shvo et al. preferred the ctr = ctp - o, scale (78 see also Sect. II-C-5). Similarly, in a series of ketene mercaptals, a reasonable correlation was found... 

The Hammett substituent constant o is a measure of the electron-with-drawing capacity of substituents directly conjugated to the reaction center [84], Since the values of this parameter were obtained from the rate constants of base-catalyzed hydrolysis of other aryl esters, it can be concluded from Eqn. 8.5 that the same electronic factors are involved in both cases. 

Hammett substituent constant, effect of cyclo-amyloses on, in hydrolysis of phenyl esters, 23 222... 

As a rule, H-acid catalysis lowers stereospeciflcity of the Diels-Alder reactions. Besides, kinetic parameters of these reactions correlates to the Hammett substituent constants. In the case of the cation-radical mechanism, correlation of kinetic parameters to the Hammett-Brown substituent constants gives the best accordance. 

A monoparametric equation results when a composite electrical effect parameter is obtained by combining all three pure electrical effect parameters with fixed values of both Pd and t]. The Hammett substituent constants are of this type. The Hammett equation is given by equation 5 ... 

Effects of structural variation on organolithium compounds TABLE 3. Values of Hammett substituent constants... 

Containing 0.1 M TBAF and 0.5 equivalent 9,10-anthraquinone as an internal standard. Hammett substituent constants taken from Hansch et al. ... 

Hansch C., P.P. Maloney, T. Fujita, and R.M. Muir (1962). Correlation of biological activity of phenoxyacetic acids with Hammett substituent constants and partition coefficients. Nature (London) 194 178-180. 

Linear correlations between l(L) and Hammett substituent constants for substituted jr-cyclopentadienyl and 7r-arene ligands [69], for other substituted Kgands,... 

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