Correlations Hammett constants

C-nmr data have been recorded and assigned for a great number of hydantoin derivatives (24). As in the case of H-nmr, useful correlations between chemical shifts and electronic parameters have been found. For example, Hammett constants of substituents in the aromatic portion of the molecule correlate weU to chemical shifts of C-5 and C-a in 5-arylmethylenehydantoins (23). Comparison between C-nmr spectra of hydantoins and those of their conjugate bases has been used for the calculation of their piC values (12,25). N-nmr spectra of hydantoins and their thio analogues have been studied (26). The N -nmr chemical shifts show a linear correlation with the frequencies of the N—H stretching vibrations in the infrared spectra. 

Linear Free Energy—Linear Solvation Energy Relationships. Linear free energy (LFER) and linear solvation energy (LSER) relationships are used to develop correlations between selected properties of similar compounds. These are fundamentally a collection of techniques whereby properties can be predicted from other properties for which linear dependency has been observed. Linear relationships include not only simple y = rax + b relationships, but also more compHcated expressions such as the Hammett equation (254) which correlates equiUbrium constants for ben2enes,... 

C Hansch, RM Muir, T Fujita, PP Maloney, E Geiger, M Streich. The correlation of biological activity of plant growth regulators and chloromycetm derivatives with Hammett constants and partition coefficients. J Am Chem Soc 85 2817-2824, 1963. 

Ionization constants of czs-3-substituted acrylic acids have been correlated with the Hammett equation by Hogeveen (58) and by Charton (60). Charton has correlated ionization constants for a number of other c/s-vinylene sets with the Hammett equation (60). Charton and Charton have correlated some cw-vinylene sets with the extended Hammett equation [eq. (2)] (73). Sufficient data are available for twelve sets of cis-vinylene equilibria, of which four sets represent ionization constants of hydroxy compounds (sets 12-1 to 12-4) and eight sets represent ionization constants of carboxylic acids (sets 12-5 to 12-12). All sets have been correlated with eq. (24) and eq. (2). Sets studied are reported in Table XII. Results of the correlations are reported in Table XIII. Sets designated A were correlated with eq. (24), sets designated B were correlated with eq. (2). In the case of the second ionization constant of 2,3,5,6-tetrahydroxy-l,4-benzoquinone (set 12-3), it is uncertain which hydroxyl group ionizes therefore, the value for X = OH was excluded from the correlation. All of the sets 12-1 to 12-4 gave significant correlations with both eq. (24) and eq. (2),... 

Correlation of Relative Sweetness of 2-Substituted 5-Nltroanilines with Hammett Constant (tr) and Hydrophobicity (ir)... 

Although this reaction occurs too rapidly for direct observation, the relative rate constants for pairs of phosphines were determined by standard kinetic competition techniques. The rate constant ratios for P(CeH4R)3 relative to PPh3 correlate well with the Hammett constant 3a, giving p = -0.70. 

Para carbon shieldings, however, clearly follow the pattern described by the canonical formulae. They may be correlated well with the total charge densities22 and with the Hammett constants. 

These data show that 4-nitrosodimethylaniline can deplete GSH in the liver by multiple reactions that are not exclusively related to the nitroso function, and it appears that the slow reactivity towards thiols correlates with the strongly negative Hammett constant... 

Modifications of the arylamino moiety profoundly influence the rate of hydrolysis. Thus, the hydrolysis of N- ace ty 1 -4-am i no be n zo i c acid was ca. 1000-fold slower than that of acetanilide [66]. Sorci and Macalady [67] investigated the influence of ring substitution on the hydrolysis of para-substituted acetanilides (4.104) in alkaline solution and in soil bacteria. No correlation was found between alkaline and biotic hydrolysis, which appeared to be controlled by different physicochemical properties. Bacterial hydrolysis was best correlated with the Van der Waals radius of the substituent, whereas chemical hydrolysis was correlated with the Hammett constant characterizing the electron-withdrawing capacity of the substituent. Other studies confirmed that a correspondence between bacterial and mammalian esterases... 

UV-vis spectra containing a well-resolved peak at ca 530 nm with two shoulders between 380 and 590 nm. The IR v(C=C) bands are between 2031 and 2045 cm considerably lower than the bis-adducts (ca 2100 cm ), and decrease with increasing electron-accepting power of the substituents (linear correlation with Hammett constants). Changes may occur by a donor effect, d j —> r (C=C) back-donation, or r(C=C) -> d r donation. In contrast with CO and cyano complexes, where d -> 7r (CX) back-donation occurs, here the changes in y(C=C) result from electron donation from the alkynyl group to the Ru2 center. 

Only electrical effects should be of major importance. The equilibrium constants were correlated with the Hammett equation (equation 5). This was necessary, due to the small size of the data set which included both meta and para substituents. The application of the Hammett equation to data sets including both meta and para substituents is most successful when the geometry of the system resembles that of the benzoic acids from which the Hammett constants were obtained. That is not the case in this reaction. The regression equation is equation 33 ... 

It must be noted that all these ap values have no significant correlation with the corresponding Taft or Hammett constants the precise interpretation of these crp values remains open to doubt. 

R.W. Fuller, M.M. Marsh, J. Mills, Inhibition of monoamine oxidase by A/-(phenox-yethyl)cyclopropylamines. Correlation of inhibition with Hammett constants and partition coefficients, J. Med. Chem. 11 (1968) 397-398. 

The decomposition of arenediazonium tetrafluoroborates RC6H4N2 + BF4 (R = Me, OMe, Cl, Br, N02, C02R, OPh) in aqueous solutions (pH 1-7) which gives phenols and phenyl-diazenylphenols is first order with respect to the concentration of the diazonium salt.246 At lower pH, the rates (k) are constant but above a certain pH value, k increases with pH linearly. Correlations with the Hammett constants er have been proposed. 

Structural Effects and Solvent. The effect of solvent on the equilibrium of Reaction 4 can be first discussed in terms of effects on the susceptibility to substituent effects. The values of pK2, characterizing this equilibrium, are a satisfactorily linear function of the Hammett constants correlation coefficient r (Table VI). The values of reaction constant p are practically independent of the ethanol concentration (Table VI), as was already indicated by the almost constant value of the difference (A) between pK2(H20) and p 2 (mixed solvent) for a given composition of the mixed solvent (Table I). The same situation is indicated for DMSO mixtures (Table II) by the small variations in A for any given solvent composition. In this case, the number of accessible p 2 values was too small to allow a meaningful determination of reaction constants p. The structural dependence for various water-ethanol mixtures is thus represented by a set of parallel lines. The shifts between these lines are given by the differences between the pK2H values (p 2 of Reaction 4 for the unsubstituted benzaldehyde) in the different solvent mixtures. 

Examination of nO chemical shifts of several 2-substituted and 2,5-disubstituted furans (85MRC985) has demonstrated the additivity of substituent effects by plotting the nO chemical shifts of 2,5-disubstituted furans versus those of the 2-substituted analogues. The electronic character of substituents alone does not determine l70 chemical shifts, because they do not correlate with Hammett constants. 

A good linear correlation between the equilibrium constant log(/f) and the Hammett constant back-bonding fragment W(CO)5 was substituted for Cr(CO)5 in [M(CO)5 Se = C(Ph)H ] the equilibrium... 

The Hammett correlation for substituted carboxylic acids is demonstrated in Figure 5.26. The Hammett constant om fails for aliphatic compounds, and the derived constant o must be used to predict accurate Hammett correlations. The least-substituted carboxylic acid, formic acid, was used as the reference compound. The Hammett correlation for substituted carboxylic acids (CAs) demonstrates that the CAs substituted by electron-withdrawing substituents, such as Cl, oxidize the fastest. CAs substituted by electron-donating groups, such as CH3 and NH2, oxidize more slowly than those substituted by electron-withdrawing substituents. The reaction pathway for substituted carboxylic acids is shown in Figure 5.27. These trends are different for phenols and alkanes, because the reaction site is at the election pair located at the oxygen atom. 

A Hammett correlation for substituted ketones was also made using the derived Hammett constant cf. The least-substituted ketone, acetone, was used as the reference compound in the correlation. Only three ketones were used for the correlation therefore, the model may not be as robust as the other QSAR models. 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

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