Hammett substitution coefficient

Thus, the scale of activating, neutral, and deactivating substituents is in line with the corresponding Hammett substitution coefficients txf. In the case of the electron-transfer mechanism, this empirical activity scale could be based on a simple theoretical equation (eq. (20)) which can be used to estimate relative or even absolute rate constants for the primary step of the oxidation reaction (see eq. (21)) [6a, 11, 14s, 31] ... 

Oxidative metabolism of phenyl rings is a common problem, and fluorine substitution, usually at the 4-position, has become a wide-spread practice to increase stability in various substance classes. A plot of Hammett c coefficients against... 

White et al.1A have obtained similar kinetic results for the acid-catalysed rearrangement of N-nitro-N-methylaniline, i.e. a first-order dependence on the nitroamine with a linear H0 plot of slope 1.19 for phosphoric acid, and a deuterium solvent isotope effect of about three, although the results have only been presented in preliminary form. Further, an excellent Hammett a+ correlation was claimed for thirteen para substituted nitroamines which gave a p value of —3.9. Since it is expected that the rate coefficients would correlate with a (rather than different basicities of the amines, the a+ correlation implies that the amino nitrogen is electron-deficient in the transition state,... 

According to this, there is a linear relationship between the logarithm of the partition coefficients, log (cah+/ca)> the Hammett acid function Ho- Plattner et al. (1949) were able to confirm this relationship for a substantial number of methyl-substituted azulenes. However, the further analysis of these measurements with a view to obtaining pAg proved difficult since the partition coefficient Pa of the free azulene between the two phases, which enters into equation (12c), is unknown. For this reason, the authors took the for which Cah+ = Ca as a charac-... 

A similar system, (CH3)2C=CH X, was studied by Endrysova and Kraus (55) in the gas phase in order to eliminate the possible leveling influence of a solvent. The rate data were separated in the contribution of the rate constant and of the adsorption coefficient, but both parameters showed no influence of the X substituents (series 61). A definitive answer to the problem has been published by Kieboom and van Bekum (59), who measured the hydrogenation rate of substituted 2-phenyl-3-methyl-2-butenes and substituted 3,4-dihydro-1,2-dimethylnaphtalenes on palladium in basic, neutral, and acidic media (series 62 and 63). These compounds enabled them to correlate the rate data by means of the Hammett equation and thus eliminate the troublesome steric effects. Using a series of substituents with large differences in polarity, they found relatively small electronic effects on both the rate constant and adsorption coefficient. 

Five new pyridazinones were synthesized with substitutions in the two-position of the phenyl ring as given in Table II. Using the Hansch approach, correlations were made between the experimentally determined 18 2/18 3 ratios and the values and a,a values taken out of the data collection of Hansch and Leo (15) (Table II and Equations 1-3). The hydrophobic parameter is derived from the 1-octanol/water partition coefficient and o is the Hammett electronic parameter. 

The pKa s of protonated benzoic acid and its derivatives are readily measured by conventional spectrophotometric methods. Data for over forty compounds have been obtained by Stewart and Granger34 and Stewart and Yates35 and the results have been summarized by Arnett36. Substituted benzoic acids are well-behaved Hammett bases, with slopes of the logarithm of the ionization ratio plots close to 1.0. But, as explained by Arnett36, this is only true, for benzoic itself t any rate, in its region of protonation. In more dilute acid a sudden sharp change in activity coefficient occurs, which means that the observed... 

Substituent effects on the A,u I reaction have been studied by Bender and Chen55. These authors measured the rates of hydrolysis of a series of 4-substituted 2,6-dimethylbenzoates in 9.70 M sulphuric acid at 25°C, and found that the values for the first-order coefficients with 4-methoxy, 4-methyl, 4-unsubstituted and 4-bromo-compounds (5.0, 0.37, 0.033 and 0.01 x I0 4 sec-1, respectively) are satisfactorily correlated by the Hammett equation, following cr+ with a slope p = —3.22. Since the esters are not fully protonated in 9.70 M H2SOj, part of this factor is due to the effects of the 4-substituent on the protonation equilibrium, p for the protonation of substituted benzoic acids is about — l35, but is likely to be considerably smaller for di-ortho-substituted compounds, since the conjugative interaction of the p-substituents with the protonated carboxyl group requires coplanarity with the ring. 

One of the characteristics of the acid-catalyzed hydrolysis of esters, that is shared by ester formation also, is that substituent effects on the rate coefficients are small, and not simply related to a values (see below, p. 131). The data in Table 14 show that this is also true for the, sO-exchange reaction of substituted benzoic acids. This is borne out by the relative constancy of the ratio khyJkexch for the different substituted acids it was not possible to obtain a meaningful p value from the data of Table 14, because of the small number of points and the large amount of scatter evident on the Hammett plot. Mesitoic acid is highly unreactive, compared with the m- and p-substi-tuted esters used, as is its methyl ester towards alkaline hydrolysis138, and presumably reacts by the seriously hindered Aac2 route. 

A plot of log, k/k0 vs. o gives a Hammett p of +0.43 with a correlation coefficient of 0.96 (Fig. 8) for the alcoholysis reaction, where k is the rate constant for the phenyl-substituted material and A , is for the reference phenyl compound. 

The slope and the pA BH+ values obtained by application of equation 35 to spectral data for iV-benzoylthioLirca and six met a - and para-substituted V-benzoylthioureas are given in Table 24. A plot of these pA BH+ values vs a yields the straight line shown in Figure 17, the slope (Hammett p value) of which equals —0.42 (correlation coefficient 0.961). 

Relative rate-coefficients for the substitution of a number of benzylboronic esters are given in Table 3(b) a Hammett plot yields a value of +0.93+0.08 for p. If... 

Hammett correlations were developed from experimental data for substituted phenols studied under the UV/Ti02 process (D Oliviera et al., 1993). The mechanism for this reaction is understood to proceed via the hydroxyl radical. Experimental data from the study of dichlorophenols and trichlo-rophenols under UV/Ti02 were used for QSAR analysis (D Oliviera et al., 1993). Figure 9.13 demonstrates the QSAR model for substituted phenols formulated from experimental data. The QSAR model developed for substituted phenols shows a goodness of fit of 0.9766. A good correlation was also established for substituted phenols using Hammett s constant, o the correlation coefficient is 0.987 (D Oliviera et al., 1993). Similar correlation coefficients for the constants o and ores demonstrate that the descriptor ores can be used to accurately predict kinetic rate constants for substituted phenols. 

Examples for compounds are given in Figure 8.1, and the regression analysis equation is provided below for the QSAR of triazine derivatives in photosynthesis (Draber, 1992). The inhibitory potency expressed as a pl50 value is equal to a lipophilicity parameter tt (log of the partition coefficient P), an electronic substitution parameter a (the Hammett constant) and to a lesser degree to a steric component Es (the Taft constant). 

The values of k2 for ring-substituted phenylacetylenes are well correlated by the Hammett relationship, using a+ constants, and p is very large, — 5-2. The p value compares well with those obtained in the acid-catalysed hydration of arylacetylene derivatives (see Table 1) and is entirely consistent with the formation of a vinyl cation 45 in which the positive charge is at the carbon next to the phenyl ring and is largely shared by it. Interestingly, the p value (by use of a+) based on k2 rate coefficients for the bromination of styrene in acetic acid is — 4-5. 

Hansch constant — A measure of the capability of a solute for hydrophobic (lipophilic) interaction (see - hydrophobic effect) based on the partition coefficient, P, for distribution of the solute between octan-l-ol and water. The most general way of applying P in correlation analysis, etc. is as logP, but the behavior of substituted benzene derivatives maybe quantified by a substituent constant scale, n, which is defined in a way analogous to the Hammett a scale. There are various n scales, depending on the substrate series used as reference. See also - hy-drophilicity. 

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