Hammett’s a constant

A further ring nitrogen atom in the 5-position in 2,4-diamino-1,3,5-triazine leads to a considerable reduction in basic strength (pATj = 3 91) and the site of protonation remains N-1 (or the equivalent N-5) (Roth and Strelitz, 1969). The effect of 6-substituents on pAf, correlates with Hammett s a -constants, and it is claimed that calculations of TT-electron densities and nmr spectra indicate protonation at N-3, rather than at N-1 as in the unsubstituted compound (Marimoto, 1966). 

Substituent effects on redox potentials can be rationalized by free energy relationships in many cases Although 1 can be related to aromatic compounds, a linear correlation with Hammett s a-constants fails. As can be seen from the substituents -N(CH3)2 Ik) and -OCH3 11) the electron attracting effect is the dominating one and not the resonance effect. Therefore Taft s a "-constants describe the substituent effects more correctly. According to (1)... 

Effects on the redox potential of substituents at the reducible ligand have also been investigated and linear correlations recognized with the Hammett s a constant [66, 67]. 

The results in Table V support this ground state electron distribution effect. The data in italics show that the relative quantum yields are CN < Cl < CH3 with respect to the electron-donating substituent X, and CN > Cl > CH3 with respect to electron-withdrawing substituent Y. Unfortunately, the different quantities in which the quantum yields are expressed and the different reaction media do not allow a direct comparison of the results. Kobsa s data29 on ortAo-hydroxyphenyl ketone formation in the first column can be correlated with Hammett s a constants (Eq. 1, the fraction is proportional to the quantum yield). 

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). 

Correlation between rate constant for reaction with ozone and Hammett s a constant for substituted phenols. (From Gurol, M.D. and Nekoulnalni, S., Indust. Eng. Chem. Fundam., 23, 1-54, 1984. With permission.)... 

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... 

Many other substituent parameters have been developed to improve correlations for specific types of reactions. Brown and Okamoto (1958) developed substituent constants (electrophilic reactions based on hydrolysis rates of meta- and para-substituted 2-chloro-2-phenylpropanes (CPP), which react by electrophilic carbonium ion intermediates. Formation of these intermediates is facilitated by high electron density at the reactive carbon (i.e., by meta- or para-electron donors). The parameter meta substituents), k0 is the hydrolysis rate constant for unsubstituted CPP, and k is the rate constant for a substituted CPP. 

From H NMR data correlations (107) between Jc-h and Hammett s a constants have been observed for phenyl-germanium derivatives. 

As model compounds, we used isopropyl p-nitrobenzoate for poly(vinyl p-nitrobenzoate), p-nitrophenyl iaobutyrate for poly(p-nitrophenyl acrylate), p-nitrocumene for poly(p-nitrostyrene), and p-nitrophenyl isopropyl ether for poly(p-nitrophenyl vinyl ether). The structure and Hammett s a constants of these model compounds and the chain transfer constants of polystyrene radicals to these compounds are given in Table 5 (3). 

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. 

In all cases where a sufficiently large number of substituents has been tested, a linear fi ee-energy correlation is found with Hammett s a-constants. The reaction constants are fairly high, the values rang-... 

The hydrophobic constant r is a measure of the contribution of a substituent X to the lipophilidty of compound R-X compared with R-H. The constant representing the solvent/solvent system, analogously to Hammett s p constant for the reaction type, was arbitrarily set to 1 for octanol/water and thus does not appear in Eq. (7). The lipophilidty constant ti allows the estimation of log P values for congeneric series of compounds with various substituents (see Eq. (8)). 

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

In Scheme 7-1 kx and kn refer to the rate constants for a benzene derivative, in our case the benzenediazonium ion, bearing a substituent X in the 3- or 4-position, and the corresponding unsubstituted benzene derivative respectively. The term p is Hammett s reaction constant for the reaction, and o is Hammett s substituent constant which is, at least in principle, independent of the nature of the reaction but different for the 3- and 4-positions. A plot of log kx (or log kx - log kH) versus o should give a straight line. Its slope (positive or negative) corresponds to the reaction constant p. Equilibrium relationships are treated analogously. 

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. 

Nevertheless, such a difficulty should not be exaggerated since one should take into account that even the conventional Hammett s Up constant, of a widely recognized significance for the comparison of the electron donor/acceptor character of substituents, has required corrections in... 

Alkyllithium-transition metal halide catalysis is completely different from the sodium ketyl and alfin catalysis. Natta, Danusso, Scanesi and Macchi (36) have found that the polymerization of styrene and substituted styrenes by titanium tetrachloride-triethyl aluminum catalysts was different from the above anionic systems. A plot of the log of the rate of the polymerization against Hammett s sigma constant produced a straight line with a rho value of —1.0. Electron releasing groups facilitated this polymerization. 

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... 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

In view of the above results, in photocatalytic oxidation of a series of 4-substituted diphenylethylenes, an increase in reactivity with decreasing Hammett s sigma constants (31) seems to arise not only from the lowering of the oxidation potentials of the olefins in this sequence but also from the general trend of the increase in the reactivity of olefins toward peroxyl radicals with increasing the electron donating ability of olefins (33). 

The nucleophilicity parameters for carbanions of nitronates and malonic acid derivatives have been investigated.143 The nucleophilic reactivities do not correlate with the acidity constants of the conjugate CH acids, and from the poor correlation of the reactivities of the substituted a-nitrobenzyl anions with Hammett s ex-constants it can be inferred that the nucleophilic reactivities are strongly controlled by solvation. 

The rate constants for the reactions of eaq with aromatic compounds depend very strongly on the substituent. Benzene itself has a relatively low reactivity, k = T4 x 107 M -1 s 1, phenol and aniline are even less reactive, but substituents such as CN, Br, and N02 increase the rate constant almost to the diffusion controlled limit. The rate constants for several series of mono- and disubstituted benzenes were correlated with Hammett s substituent constants a and showed good linear relationships. 

The effect of substituents on the rate constant for reaction of H with benzene was also examined. Correlation with Hammett s substituent constants was possible and showed that the hydrogen atom behaves as a slightly electrophilic reactant (Neta, 1972a, and references therein see also Brett and Gold, 1971, 1973 and photochemical studies by Pryor et al., 1973). 

Addition of OH to olefinic and aromatic compounds is also very rapid usually k = 109-1010 M -1 s"1. Despite the high rates, a reasonable correlation with Hammett s substitutent constants was obtained for the series of substituted benzenes and demonstrated the electrophilic behaviour of OH in its addition to aromatic compounds (Neta and Dorfman, 1968). 

Hammett s reaction constants using a+ values, unless otherwise indicated. 6 By use of the Yukawa and Tsuno equation (see text). 

It is perhaps worth mentioning that substitution of other groups for the C-methyl group in amidinium ions has an effect on their p/fa -values, which is best correlated with Hammett s am -constants (p = —1T98), and the same is true of C-substituted N-phenylami-dinium ions (p = —12-08) (Charton, 1965). This is in line with the suggestion made on page 286, that am -constants are a measure of the inductive and mesomeric interactions of groups with a carbon in the sp2 state of hybridization. 

The quantitative correlation of the effects of substituents upon reactivity holds a venerable place in physical-organic chemistry. Substituent constants, such as Hammett s a, measure the perturbation which a substituent makes on the electron distribution in a molecule and the reaction constant p measures the consequence of this for a reactivity of interest. The correlation of ESR parameters by substituent constants holds promise, for it compares the empirical parameters with a real function of the very property they measure. Various endeavors have been made in this direction which are discussed in later Sections. 

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