Hammett sigma values

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. 

Chemical shifts (S 13C NMR shifts related to Cl) of a large number of 3- or 4-substituted anilines are correlated with infrared N—H frequency data and with Hammett sigma values. An intermolecular hydrogen bond, as depicted in 62 and 63 (Scheme 10), was indicated to be present, on the basis of the plots of <>13C1 versus Vasym of NH2 group frequencies in chloroform100. 

Halophenols, photooxidation of 1084 Hammett constants 71 Hammett parameters 350 Hammett plots 999 Hammett sigma values 522 Hammett-type stmcture-acidity correlations 506... 

Fig. 11. Plot of pK for aromatic acids (ArC02H2 ArCOjH + H ) versus sigma-plus. Data from K. Yates (1960) J. Am. Chem. Soc. 82, 4059. Open and filled circles represent Hammett sigma values, open circles sigma-plus parameters.

The data shown in Table 4 summarize not only kinetic results but also voltammetric oxidation peak potentials, Hammett sigma values, and clinical doses. As might be expected, the rate of the cation radical reaction for the first three compounds in Table 4 increases with increase in the electron-withdrawing properties of the C(2)-position substituent as indicated by the (Tp values. The voltammetric peak potentials corresponding to oxidation of the parent compound to the cation radical follow the trend of the Hammett o-p values. 

Turning to a series of larvicides, the Dutch workers obtained the best correlation by combining partition and Hammett-sigma values with two sterimol terms (Z) and (- 4). In subsequent work, the Verloop method was subtly refined to increase predictive powers a new table of substituent values was issued, and the arithmetical correspondence between B and was noted (Verloop, 1981, 1983). 

The relative, reactivities of all. a-methylstyrenes toward the MA-type radical fails to relate to the Hammett sigma values for the substituents, contributing support to the thought that special resonance structures in the reaction contribute to the driving force for the alternating copolymerization. 

Halogenation, maleic acid derivatives, 60 Hammett equation, 138, 139 Hammett sigma values, 372 Hantzsch ester hyrogen donor, 43 MA reduction, 43 Heat of copolymerization MA-allyl chloride pair, 405 MA-yV-butyl vinyl ether pair, 405 MA-isopropenyl acetate pair, 333 MA-a-methylstyrene pair, 404, 405 MA-styrene pair, 404, 405 MA-vinyl acetate pair, 333, 405 polyester-styrene mixtures, 487 vinyl acetate with maleates and fumarates, 333 Heat of polymerization MA-epoxide reactions, 482 MA monomer, 244... 

Most of the information available concerns SOMe and S02Me. Bordwell and his colleagues were the leading pioneers in determining sigma values for these groups. The results of the main determinations of the ordinary Hammett o constant and of the exalted constant o are shown in Tables 3 and 4 respectively. 

A good correlation with ordinary Hammett a values was based on 16 well-behaved substituents, and p-SOMe conformed well to this. Various other substituents showed deviations which were attributed to enhanced + R effects. These included p-SPh and this was explained in terms of 7t(pd) bonding, which was thus taken to play no part in the effect of p-SOMe on the methyl hyperfine splitting. More recently several 4-substituted benzyl radicals of the type RSO C6H4CH2 (n — 0,1 or 2 R = Me, Ph, Tol, COMe or OMe) have been examined by ESR spectroscopy249. The ability to delocalize spin density onto the substituent decreases in general as n increases and the effect of R depends on the oxidation state of sulfur. These authors have devised a new scale of substituent effects (sigma dot... 

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. 

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. 

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 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. 

Substituent effects on the hydrolysis rate of diethyl benzal in 50 % dioxane—water at 30 °C follow the Hammett relationship with p = —3.35 if log (k/kQ) is plotted versus the sigma values of meta substituents [163]. Data for electron-releasing para substituents exhibit positive deviations. The deviations are negative, however, if the data are plotted versus a + rather than a. A good linear relationship is obtained by application of the Yukawa—Tsuno equation [185]... 

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