Taft sigma values

Taft sigma values cr perform a similar function with respect to aliphatic and alicyclic systems. Values of cr are listed in Table 9.1. 

R.W. Taft, Sigma Values from Reactivities, J. Phys. Chem., 1960, 64, 1805. 

Table and discussion Electronic effects in molecules (Hammett and Taft sigma values). 644... 

TABLES AND DISCUSSION ELECTRONIC EFFECTS IN MOLECULES (HAMMETT AND TAFT 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. 

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). 

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-... 

The alternatives to mathematical descriptors derived from molecular graphs or molecular geometry are the traditional QSAR (quantitative structure-activity relationship) descriptors and quantum chemically computed parameters. The former include the partition coefficient for oil/water (often octanol/water) (log P), the Hammet sigma value (electronic parameter that measures the electron withdrawal from and the electron release to the aromatic ring by a substituent, the Taft s parameters for the electronic effects of substituents in aliphatic compounds (a ), and a steric parameter for the proximity of substituents on reaction sites (Es)- Also selected molecular properties, such as molar refractivity (MR), polarizability (a), molecular weight (MW), and density (d), have been used. 

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

The most popular of these approaches is multiple regression analysis, introduced by Hansch in 1968. In this method, the most favoured variables are (i) partition coefficients (P), from the system octanol/water (ii) the sigma (cr) and rho (p) values derived from Hammett s Linear Free Energy Equation (Section 17.2) and (iii) Taft s steric factors E ) which are used to determine what space for substituents exists between a nominated small region of the lead molecule and its receptor (Section 16.2). These variables are correlated in the following equation ... 

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. 

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