Hammett equation compounds

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,... 

The chlorination of a series of compounds having electron-withdrawing substituents has been studied. The relative rates of chlorination and the isomer distributions are known. The data give a satisfactory correlation with the Hammett equation using (t, but no rate measurement for benzene under precisely comparable conditions is possible. How could you estimate f,f y and fp for chlorination from the available data ... 

The classic example, and still the most useful one, of a LFER is the Hammett equation, which correlates rates and equilibria of many side-chain reactions of meta- and para-substituted aromatic compounds. The standard reaction is the aqueous ionization equilibrium at 25°C of meta- and para-substituted benzoic acids. 

Hydroxypyridine 1-oxide is insoluble in chloroform and other suitable solvents, and, although the solid-state infrared spectrum indicates that strong intermolecular hydrogen bonding occurs, no additional structural conclusions could be reached. Jaffe has attempted to deduce the structure of 4-hydroxypyridine 1-oxide using the Hammett equation and molecular orbital calculations. This tautomeric compound reacts with diazomethane to give both the 1- and 4-methoxy derivatives, " and the relation of its structure to other chemical reactions has been discussed by Hayashi. ... 

Earlier studies of 4-aminopyridine 1-oxide were less conclusive. The solid-state infrared spectrum could be interpreted to indicate the existence of both the imino structure and/or, more probably, the amino structure. Comparison of the actual pKa value of 4-aminopyridine 1-oxide wdth the value calculated using the Hammett equation was considered to indicate that the compound existed as such or as an equilibrium mixture with l-hydroxypyrid-4-onimine, the latter possibility being considered the less likely on the basis of resonance and bond energies/ Resonance energy and ultraviolet spectral considerations have been advanced to support the 4-aminopyridine 1-oxide structure/ The presence of an infrared absorption band at... 

Ever since its original formulation, the Hammett equation has been one of the most widely discussed and applied relations between structure and reactivity of organic compounds, and it has been... 

The use of the Hammett equation has also been extended to several new types of applications. Since these are not germane to the subject matter of the present chapter, we wiU simply mention work on applications to ethylenic and acetylenic compounds the various applications to physical properties, such as infrared frequencies and intensities, ultraviolet spectra, polarographic half-wave potentials, dipole moments,NMR and NQR spectra,and solubility data and applications to preparative data and biological activity. 

Similar, although less well documented, is Bryson s observation that a constant term must occasionally be included in the Hammett equation, i.e. A K = ap +a, which means that the intercept is significantly different from the equilibrium constant for the parent compound. 

Chapman and co-workers have investigated the nucleophilic displacement of chlorine in various chloronitropyridines by three pyridines. In each of these series of three compounds, an excellent correlation is observed, but, again, longer series would be extremely desirable. Similarly, the nucleophilic attack of a series of four pyridines on propylene oxide follows the Hammett equation with high pre-... 

Pyridine bases are well known as ligands in complexes of transition metals, and it might well be anticipated that the equilibrium constants for the formation of such complexes, which are likely to be closely related to the base strength, would follow the Hammett equation. Surprisingly, only very few quantitative studies of such equilibria seem to have been reported, and these only for very short series of compounds. Thus, Murmann and Basolo have reported the formation constants, in aqueous solution at 25°, of the silver(I) complexes... 

The application of the Hammett equation to bicyclic aromatic compounds of the quinoline and isoquinoline type may be envisaged in several ways. In quinoline, e.g., the homocyclic ring A in 9) may be... 

The first reaction series to be considered are the basicities of the various quinolines. Baciocchi and Illuminati have demonstrated that the pK values of quinolines substituted in the B-ring follow the Hammett equation well if ApK, i.e., the difference between the pK values of substituted and unsubstituted compounds, is plotted against a, the quinoline points fall on the same line as the pyridine points, as shown in Fig. 5, so that the p-values for the two series are identical. 

Finally, the Hammett equation has been applied in a few instances to heterocycles of the indole-benzofuran type. The double p method of Eq. (3) was first designed for this type of system and was here applied. When this approach was originally proposed, the only truly hetero-cychc system to which it was apphed was the substituted phthahds 14, and pertinent data on the hydrolysis of these compounds are included in Table IX. 

It seems reasonable to assume that in regard to their formation and decomposition the derivatives of phenylpentazole correspond roughly to the unsubstituted compound (see Table V). The rate data for m-and p-substituted phenylpentazoles conform to the Hammett equation with p= -f-1.01. Finally, the high rate of decomposition and... 

The third volume of this series covers three specific groups of compounds the carbolines (reviewed by R. A. Abramovitch and I. D. Spenser), the thiatriazoles (K. A. Jensen and C. Pedersen), and the pentazoles (I. Ugi). The remaining four chapters deal with topics of general chemical interest from the heterocyclic viewpoint the quaternization of heterocyclics (G. F. Duffin), carbene reactions (C. W. Rees and C. E. Smithen), applications of the Hammett equation (H. H. Jaffe and H. Lloyd Jones), and some aspects of the nucleophilic substitution of heterocyclic azines (G. Rluminati). 

DSP treatments allow one to separate the field and mesomeric effects of substituents on chemical reactivities and physical properties (electronic and NMR spectra, etc.) of organic compounds. In Section 8.3 we will discuss heterolytic dediazoniation of substituted benzenediazonium ions. For this series of reactions the classical Hammett equation completely fails to give useful results (see Fig. 8-1), but the DSP treatment yields a good and mechanistically very meaningful correlation. 

The Hammett equation is not successful for reactions of aliphatic compounds if the normal cr constants are used. A new scale of substituent constants, labeled cr, was invented to allow the extension of the method to such systems. The difference in the rates of hydrolysis of esters in basic versus acidic solutions is used to define the scale. The transition states are... 

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

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),... 

The ability of compounds with double bonds to act both as electron donors and as electron acceptors in charge transfer complex formation is well known (81,82). Hammond (83) has studied the correlations of association constants and of the energy of the charge transfer absorption of 2-substituted-l,4-benzoquinones complexed with hexamethylbenzene with the Hammett equation. Charton (84) has studied the correlation with eq. (2) of association constants of 1-substituted propenes with Ag. ... 

In eq. (32), pi and p2 denote the percent of the product containing boron at C and, respectively. Applying the extended Hammett equation to the partial rate constants ki and k2 for the compound bearing the X substituent gives... 

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