Sigma values, table

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

Values of a0-, ar and sigma values into inductive and resonance components, to which reference will be made later in this chapter66. 

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. 

TABLE 3. Sigma values for SO Me and S02Me based on the ionization of benzoic acids and related processes... 

TABLE 7. Sigma values for fluoroalkylsulflnyl and fluoroalkylsulfonyl groups... 

TABLE 11. Sigma values from measurements in the gas-phase and in aqueous solution130... 

To obtain an idea of the size of the machine needed the sigma value can be calculated using the efficiency value from Table 10.6. 

Convert the DPMO into a Six Sigma value using Appendix Table 17C.2. 10 Draw conclusions using Appendix Table 17C.3. 

It should be noted that one is not restricted to log D and pKa combinations. By using log D and a parameters for example, the a-term will represent only the electronic component related to mechanism. On the subject of sigma values, care should be taken to use ortho values for o-substituents [see Norrington et a (14)] when acidity is a property of interest. Both StockdaTF and Selwyn (13 ) and Motais et al (15) for the phenols of Table V used, for example, 0.23 instead of 0.68 for o-chloro 0.80 instead of 1.24 for o-nitro. 

The fastest and easiest way to determine the pKa is to look it up in tables of pKa values stored within the CHESS system. If no value is found there, it can be calculated using LFER1s such as the Hammett equation. Hammett rho and sigma values are stored in tables inside the CHESS system. Situations that deviate from linear additivity are noted in an exceptions table to prevent erroneous calculations. If a calculation is not possible, the user would be asked to determine the value experimentally. 

Figure 6.16 Correlation of sigma values with rate of oxidation of phenolate anions by singlet oxygen. Open symbols indicate phenols with meta substituents and filled symbols, phenols with para substituents. Numbers indicate phenols listed in Table 6.13. [Reproduced with permission from P. G. Tratnyek and J. Hoigne, Environ. Set Technol. 25, 1596 (1991). Copyright 1991, American Chemical Society.]...

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. 

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). 

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). 

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

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