Hammett sigma constant

Hansch-Leo hydrophobic substituent parameter Hammett sigma constants Modtried Swam Lupton constants... 

It was found that certain substituents render nitrobenzene inert to reduction by potassium borohydride, whereas other substituents activate the reduction to azoxy compounds. The substituents which favored this reduction all had positive Hammett sigma constants (e.g., p-C 1, p-Br, p-I, p-COOH, m-C 1, m-Br, m-1, m-CHO reduced to m-azoxybenzyl alcohol and m-OC2H5). Among the by-products of the reaction of a p-halogen compound, when carried out in ethanol solution, was p-nitrophenetole. The reduction ofp-fluoronitrobenzene afforded only p-nitrophenetole [48]. 

Substituted aromatic nitro compounds having a substituent with a positive Hammett sigma constant can be reduced with potassium borohydride to the azoxy stage. 

Values of Hammett sigma constants are listed in Table 7.1. Taft sigma (tr ) values may be used similarly with respect to aliphatic and alicyclic systems. Values of a constants are also listed in Table 7.1. 

TABLE 7.4 Special Hammett Sigma Constants ([Pg.785]

Note Log P is the partition coefficient para positions, respectively F and R are polar and resonance constants, respectively, proposed by Swain and Lupton (1968). 

For a given reaction under a given set of conditions, p, R, T, and AGg are all constant therefore o is linear with AG. A typical Hammett plot is shown in a plot of log k/k0 for a series of substituted ethyl benzoates vs. the Hammett sigma constant of substituents in Figure 5.5. 

Ertl, R, Simple quantum chemical parameters as an alternative to the Hammett sigma constants in QSAR studies, QSAR, 16(5), 377-382, 1997. 

For all regression analyses, herbicidal activity data were taken from the appropriate column in Table VII. Corresponding tt, tt or tt" values were selected from Table VI. Hammett sigma constants (a) were taken from the compilation of Jaffe (13) and correspond to those in Table IV. Since it was assumed throughout that <7 would be relatively unaffected by the presence of surfactant, the <7 values in Table IV were used to correlate data obtained in the presence and absence of Tween 80 for all three weed types. This assumption is reasonable since the surfactant was used at a low 0.1% level in all herbicidal and partitioning tests. Furthermore, surfactant effects would be expected to manifest themselves primarily in the partitioning behavior (tt values) of the TFMS compounds... 

The Hansch equation mathematically describes two events controlling the biological activity of a compound. The first involves cell or membrane penetration (influenced by the partition coefficient), and the second involves interaction of the active molecule at a receptor site (modified by variations in the Hammett sigma constant). 

Hammett Sigma Constant. Hammett sigma constants, which are a measure of the electron-donating and withdrawing capability of aromatic substituents, were taken from tabulations of Jaffe (8). 

The electronegativity of the substituent (as reflected by the value of its Hammett sigma constant). 

Herbicidal Activity Correlations. Tables I and II give pre-emergence herbicidal activity and partition coefficient data gathered in the presence of 0.1% Tween 80 for the 15 TFMS compound evaluated in this study. For reasons discussed previously (6), in the correlations which follow, the Hammett sigma constant was assumed to be relatively unaffected by the presence of the surfactant, so that the o-values listed in Tables I and II could be used to correlate data obtained both in the presence and absence of surfactant. Pertinent herbicidal activity data for the TFMS compounds acting on Foxtail grass are presented in Table I. Similar data for the same compounds acting on the broadleaf Wild Mustard are tabulated in Table II. 

Ertl, P. (1997). Simple Quantum-Chemical Parameters as an Alternative to the Hammett Sigma-Constant in OSAR Studies. Quant.Struct.-Act.Relat., 16,377-382. 

The term is also used as a collective description for related electronic substituent constants based on other standard reaction series, of which o+, cr, and o° are typical it is also used for constants that represent dissected electronic effects, such as Oi and Or. For this purpose it might be better always to spell out the term in full, i.e., as Hammett sigma constant, and restrict o-constants to the scale of substituent constants, based on benzoic acid. 

Figure 2.7. A plot of log k/kg for a series of phenyl substituted ethyl benzoates versus the Hammett sigma constant of the phenyl substituent groups. From Tinsley (1979). Reprinted by permission of John Wiley and Sons.

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