Hammett’s a-values

In the benzene series, an approximately linear relationship has been obtained between the chemical shifts of the para-hydrogen in substituted benzenes and Hammett s a-values of the substituents. Attempts have been made, especially by Taft, ° to use the chemical shifts as a quantitative characteristic of the substituent. It is more difficult to correlate the chemical shifts of thiophenes with chemical reactivity data since few quantitative chemical data are available (cf. Section VI,A). Comparing the chemical shifts of the 5-hydrogen in 2-substituted thiophenes and the parahydrogens in substituted benzenes, it is evident that although —I—M-substituents cause similar shifts, large differences are obtained for -j-M-substituents indicating that such substituents may have different effects on the reactivity of the two aromatic systems in question. Differences also... 

Bis-pyridine complexes exist in equilibrium with five- and four-coofdinate species in solution.242 The four-coordinate complexes form polymers, which become predominant above a concentration of 5 x 10 3 M in chloroform. The association constants X, and K2 for 3- or 4- substituted pyridine complexes are well correlated to their pK, (BH+) values and Hammett s a values by equation (33) though deviations are rather large for 3-acetyl- and 2-methylpyridines. In general, K2 is smaller than Ki by a factor of 102-103. 7... 

Dorn and Knackmuss et al. (1978) correlated the maximum reaction rate for ring scission of a series of substituted catechols with Hammett s a value... 

Vaishnav et al. (1987) used log P (K ,) and Hammett s a value and BOD data for several alcohols, ketones, and related aliphatic compounds to correlate structure with extent of oxygen use at 5 days. 

Soga and co-workers examined the relation between the Hammett s a value of each substituent and the reactivities in copolymerization (Figure 17.8). It is observed that the monomer reactivity is enhanced by electron-releasing substituents in the aromatic ring. Even />-ter/-butylstyrene with a substituent of large steric hindrance shows a high reactivity. This indicates that there is a strong polar effect of the substituent on the rate of addition [28]. Similar effects were observed by Ishihara et al. in the homopolymerization of these monomers [27]. 

The energies of activation increase from about 21.5 to 34 kcal.mole and the heats of reaction vary from —11.84 kcal.mole" to about +2.20 kcal.mole" For aryl substituted amino-1,2,3-triazoles there is a good correlation between the logarithm of the rates of isomerization and Hammett s a value for substituents. 

As shown in Figure 10, a very good correlation exists between the relative rate constants and Hammett s a-values. 

The C=0 absorption bands of 2-carbonyl substituted thiophenes such as 2-thiopheneaIdehyde and 2-acetylthiophene, always occur at lower frequencies than those of the corresponding 3-isomers, because of the greater single-bond character of the C=0 bond in 2-substituted compounds. This follows from the increased conjugation of the substituent in the 2-position over that in the 3-position as is evident from the resonance structures in Section II,A.. The absorption shifts in 5-substituted 2-acetylthiophenes are in linear relation with Hammett s a-values for the substituents. The difference in the absorption frequencies of 2,5-dimethyl-3-acetylthiophene (1672 cm ) and 2,5-di-f-butyl-3-acetylthiophene (1684 cm ) may be ascribed to steric inhibition of resonance. The Raman spectrum of 2-thiophene-aldehyde shows two CJ=0 lines at 1658 and 1675 cm . The characteristic absorption frequencies of the nitro group in the six isomeric bromonitrothiophenes occur at 6.52-6.63 and at 7.45-7.58 The IR spectrum of thiophene in the region between 0.87-1.2 ix has been obtained. ... 

Table 6. Efficiency of the anodic methoxylation and Hammett s a and

The van der Waals volume can be related to the hydrophobicity of the solutes, and retention of molecular compounds can be predicted from their van der Waals volumes, 7r-energy, and hydrogen-bonding energy effects [72-74], It should be noted that the isomeric effect of substituents cannot be predicted with good precision because this is not simply related to Hammett s a or Taft s other hand, the hydrophobicity is related to enthalpy [75], Retention times of non-ionizable compounds were measured in 70 and 80% acetonitrile/water mixtures on an octadecyl-bonded silica gel at 25-60°C and the enthalpy values obtained from these measurements. 

Xrx is a parameter characterizing the homologous series RX. The values of /j,r are direct measures of the polar inductive effects of alkyl groups relative to that of methyl and correlate well with Taft s a values. Substituent-induced IP shifts can thus be handled by linear free energy relationships (LFER) of the Hammett pcr-type. 

The values presented in the second column show the quantum yields of the aryl ester conversion and can be correlated with other values in Table V, only because in ethanol at 20°, 70-80% of the ester is converted to ortho-rearranged product.59 It was shown that the given data for para-substituted esters could be also correlated with Hammett s a constants (Eq. 2). 

The reactivity of phenols in the Cu-complex-catalyzed oxidation was studied by measuring the oxidation rate, the rate constant ke, and the redox potentials of the Cu complex and of the phenol (Fig. 29)158). The logarithm of the oxidation rate is proportional to log ke, which supports the assumption that the electron-transfer step is rate-determining. A linear relationship is observed between log ke and Hammett s o value of the phenol, which is proportional to the oxidation potential of the phenol. 

Of five possible mechanisms, the authors are in favour of the one proceeding through the initial rate-determining formation of a charge-transfer complex, followed by formation of a zwitterionic peroxide, prototropy and loss of water. This choice was made on the basis of the observed sign of the Hammett s p values for the substituents X and Y at the aromatic rings, as compared with the predicted effect of the substituents in each of the mechanisms considered. 

Butter and Beachell (11) utilized a somewhat different method of estimating the electron density apportioned to the unsubstituted ring. A plot of shielding values of eight monosubstituted ferrocenes vs. Hammett s a constants was obtained for both the substituted and unsubstituted ring protons. From the slopes of these plots the authors obtained p values of... 

The kinetics for the Christmas rearrangement of 9-benzyl- and of variously 4-substituted 9-phenyl-8-azapurine-6-thiones were found to be first order, and the rate constants and equilibrium constants were recorded. A plot of log forward component versus Hammett s sigma values for the 4-phenyl substituents was rectilinear. The 8-azapurine isomer was disfavored by electron-withdrawing substituents and by increased temperatures. In dimethyl sulfoxide, the equilibrium favored the 8-azapurine isomer more than in alcohols. Ultraviolet monitoring of the reaction, in both directions, detected no buildup of any acyclic intermediate." ... 

An inductive polar substituent constant (a, ) based on the dissociation of 4-substituted quinuclidines (Equations 19 and 20) was introduced by Grob no scaling factor is used in this analysis to relate the values to Hammett s a. 

Several two-parameter equations have been suggested based on the additivity of I and R effects, each being the combination of various components as described earlier. Taft originally introduced this approach for the electronic effects of benzene and defined Or values employing values of Oj (from the aliphatic series - see Chapter 2) and Hammett s a for para substituents (Equation 18). 

Comparison of relative reactivities at constant steric effects in terms of a few selected Hammett s p values is shown in Table VII for ArS- and in Table VIII for anilines. Although some of the data in Table VII are based only on two points, clearly the p values are structure-dependent. Moreover, these values show no clear trend. The reactivity of P,P-dihalovinyl sulfones are similar to those of the P-halovinyl sulfones, but their p values are much lower. The addition reactions with ArS- show higher p values than for most of the substitutions, but for the structurally similar vinyl sulfones, the p for the substitution is higher. In general, the p values for the anilines are significantly higher, but this fact does not necessarily mean an earlier transition state for the anionic nucleophiles because the p values for the equilibrium acidities of the anilinium ions are higher than those for the thiols. 

In Figure 2, the logarithmic chain transfer constant is plotted against Hammett s a constant (3). The chain transfer constant increases as the electron attracting property of the substituents increases. Although the experimental data are very limited, almost linear relationship is observed, and the p value of +1.4 is obtained from the slope. 

Japanese workers [26, 27] proposed Equation (24), where Ef is the resonance contribution of the substituent to the transition state and y its reaction constant. When the resonance contribution is very small the expression reduces to Hammett s equation. The hydrogen abstraction reaction of nuclear substituted cumenes toward the polystyryl radical was taken as the model reaction and given the arbitrary value of 7= 1.0 and r (cumene) = 0.0. Cammarata, Yau, Collett and Martin [28] related to Hammett s a by equation (25). 

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