Taft sigma constants

Hammett-Taft sigma constants Electron density TT-Bond reactivity Electron polarizability Dielectric constant Dipole moments Ionization potential Electron affinity... 

Quantitative structure-activity relationships for fathead minnow Pimephales promelas) LC50 data were developed. As others have observed, log toxicity and log octanol/water partition coefficent (log P) are linearly related for alcohols (n = 20), alkyl ketones (n = 14), alkyl esters (n = 16), and alkyl nitriles (n = 8). When the sums of Taft sigma constants for the substituents of the most anionic carbon in each chemical structure were included in the regression of log LC50 versus log P, the coefficient of determination (r ) improved from 0.84 (for log P only) to 0.92 for alcohols, from 0.82 to 0.92 for ketones, from 0.77 to 0.91 for esters, and from 0.15 to 0.94 for nitriles. The r for the sum of Taft sigma constants versus LC50 for each class was less than 0.17. When the sum of a, was used instead of Taft sigma constants similar results were obtained. 

Quantitative structure-activity relationships, fathead minnow, Pimephales promelas, Taft sigma constants, octanol/water partition coefficients, ketones, nitriles, alcohols, esters. 

The Taft sigma constant and a, constants are inductive or polar substituent constants (Hansch and Leo 1979) derived from hydrolysis rates of substituted esters (Taft 1956) and ionization of substituted acetic acids (Charton 1981), respectively. Either constant could be useful in describing anionic potential. Therefore, they were evaluated as descriptors for the above mentioned data sets to determine if the outliers could be predicted. The following data indicate that either constant is a good descriptor. 

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. 

Hammett s sigma and rho values (also Taft s sigma values for aliphatic compounds) are discussed in Section 17.2. The most frequently needed values, experimentally determined, are in Table 17.4 (p. 647) and a great many others will be found in Appendix A.l of Perrin, Dempsey and Serjeant s book (1981), in which will also be found Appendix A.6, a useful list of sigma constants for heteroatoms in heterocyclic rings (note also the help available on p. 87 for obtaining a rho constant when the nucleus is not benzene). Sigma and rho constants are also listed by Hansch and Leo (1979). 

TAFT SIGMA AND SIGMA, CONSTANTS IMPROVE LOG OCTANOL/WATER PARTITION COEFFICIENT BASED OSAR FOR FATHEAD MINNOW TOXICITY... 

Log P (log of the octanol/water partition coefficient) values were obtained from Basak et al. (1984), Hunter et al. (1985), and Veith et al. (1983). Taft substituent constants a ) were obtained from Dean (1985). The substituent constants were obtained from Charton (1981). For the analysis, the values of the substituent constants for the carbon atom alpha to the cyano, alcohol, or carbonyl carbon were summed. If there was more than one alpha carbon, the highest surh was used. When a sigma constant was not listed in the references, a value was estimated from values for similar structures. This was done only for alkyl groups or substituents on alkyl chains where a trend in sigma constant values versus alkyl chain length could be determined. 

Linear Free Energy Relationships The observation that familiar polar effects influence the orientation ratios for radical addition to olefins prompts the application of the well established free-energy relationships. The orientation data can be accommodated in a Hammett plot by assigning a polar constant, a, to each radical (14). This is defined as the sum of the Hammett sigma constants for the groups attached to the radical centre. In practice, Taft a constants have been used (45). 

See also rho (p) value sigma (a) constant Taft equation Yukawa-Tsuno equation. 

See also Hammett equation sigma (a) constant STABILITY CONSTANT TAFT EQUATION. 

The sigma values of or fc-substituents present problems similar to those of />am-substituents plus, very often, those introduced by steric distortion and by hydrogen-bonding. The most determined attack on this multifactorial problem was made by Fujita and Nishioka (1975) who set (Tq as equal to dp, plus a fraction of the Swain and Lupton field constant (to deal with the greater proximity), plus a fraction of Taft s steric factor see p. 627). When hydro-... 

---