Hammett sigma

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

Equation (3) has good quantitative predictive power and is a successful extra-thermodynamic relationship like the Hammett sigma function. No other approach to modeling complex formation equilibria, including HSAB itself, can predict log values for unidentate ligands so accurately. 

Among the most extensive studies of monomer reactivity have been those involving the copolymerization of various meta- and para-substituted styrenes with other styrene monomers (styrene, a-methylstyrene, and p-chlorostyrene) as the reference monomer [Kennedy and Marechal, 1983], The relative reactivities of the various substituted styrenes have been correlated by the Hammett sigma-rho relationship ... 

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 Hammett sigma constants at the meta and 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. 

Note that the field effect, F, and Hammett sigma values can only be related to the loss of hydrogen bond acceptor strength, e(5. As noted by Taft (1996) the relationship between ea and aqueous pKa is poor, but log P-octanol is unaffected by solute ea. 

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... 

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