Substituent constants estimation

A clearer indication of the absolute and relative contributions of field, resonance, and polarizability effects to the acidity of the various compounds can be obtained by calculating the individual PpOF, pRoR, and pj a terms for each acid rather than just focusing on the p , pj, and p° values, respectively. These terms are summarized in Table 18 for the compounds with Y-groups with unknown substituent constants (Y = C=CH, CH=NH, and CH=S), these terms were calculated based on approximate substituent constants estimated as described in reference 118. 

Some values for the same substituent constant estimated by different methods are found to be rather scattered (see Table I). The reason for the wide scatter in a constants for pyridyl groups is that they were obtained via correlations of low sensitivity or statistical unreliability and that the measurements were made with various solvents having strong solvation effects (83KGS66). 

In obtaining the best correlation equations it has frequently been necessary to exclude members of a set. In many cases the range of variation in azw is limited. For these reasons the percent of the data accounted for by the correlation equations is not as great as we would wish. We must emphasize again that the equations in Table 45 are intended for the estimation of otherwise unavailable substituent constants. Unless the number of data points in the set is large, lOOr is >99.00%, and the new constant to be estimated can be found by interpolation rather than by extrapolation, all substituent constants estimated by means of... 

Hammett and Taft substituent constants and, in particular. Tables 9.1 through 9.4 may also prove useful for estimating values. 

These constants, K toK/, may be estimated by use of the Hammett equation. Estimation of 1 and K 4 involves application of the methods outlined in Section II, A, i.e., application of substituent constants for and N+H to the Hammett equation for the acid-base equilibria of benzoic acids. Estimation of A2 and involves application of the method used in Section III,A, i.e., the p-value for the basicity of substituted pyridines, with cr-values for COOH and COO . Provided the necessary a- and p-values are known, this procedure permits the calculation of four independent, or virtually independent, estimates of Krp. A check on the method is available from the relationships shown in Eq. (16) which is readily obtained by multiplication of Eq. (12) and (14) and of Eq. (13) and (15). 

The changes in the substituent constants and in the parameters F and R on going from the diazonium ion to various addition products provide a useful probe for understanding the mechanism(s) of addition of these nucleophiles to arenediazonium ions. Such constants and parameters are listed in Table 7-4. All values are taken from the relevant tables in the paper by Hansch et al. (1991). With the exception of the last three entries, which we shall discuss later, the products of nucleophile additions are arranged in a sequence of decreasing electron-withdrawing capability, as estimated from the values of <7m and op for the substituent corresponding to the nucleophile added. ... 

Calculated from um and op values estimated as in Assumed equal to substituent constants for CO,Et. 

In the kinetic analysis of the experimental data with an autoclave, the non-linear least square method was used to estimate the rate constants under nonisothermal conditions. The simulation of liquefaction calculated by substituing the estimated values into the rate equations showed good agreement with experimental values. 

Garst, J.E. (1984) Accurate, wide-range, automated, high-performance chromatographic method for the estimation of octanol/water partition coefficients. II Equilibrium in partition coefficient measurements, additivity of substituent constants, and correlation of biological data. J. Pharm. Sci. 73, 1623-1629. 

The values in italics are based on estimated substituent constants. 

Methods have been presented, with examples, for obtaining quantitative structure-property relationships for alternating conjugated and cross-conjugated dienes and polyenes, and for adjacent dienes and polyenes. The examples include chemical reactivities, chemical properties and physical properties. A method of estimating electrical effect substituent constants for dienyl and polyenyl substituents has been described. The nature of these substituents has been discussed, but unfortunately the discussion is very largely based on estimated values. A full understanding of structural effects on dienyl and polyenyl systems awaits much further experimental study. It would be particularly useful to have more chemical reactivity studies on their substituent effects, and it would be especially helpful if chemical reactivity studies on the transmission of electrical effects in adjacent multiply doubly bonded systems were available. Only further experimental work will show how valid our estimates and predictions are. 

Values of electrical effect substituent constants for oxime and hydroxylamino groups have been reported ° their values are set forth in Tables la-c. No value for the acid group, CO(NHOH), was available. We have estimated values for it they are given in Tables la-c. Also in Tables la-c are values for some other types of substituents either for purposes of comparison or because they were used in correlations in this work. 

Entirely electron donor (ED) (Ped = 100). Example P = PNMe2 The values in italics are based on estimated substituent constants. 

In a manner analogous to that used previously to predict chemical shifts in aromatic systems (Box 4.6), we can use substituent constants to estimate the chemical shifts of a range of compounds, e.g. substituted aromatic compounds (Box 4.16). 

The Hammett treatment provides a correlation of much experimental data. Tables 26-6 and 26-7 contain 38 substituent constants and 16 reaction constants. This means that we can calculate relative k or K values for 608 individual reactions. To illustrate, let us suppose that we need to estimate the relative rates of Reaction 16 of Table 26-7 for the para-substituents R = OCH3 and R = CF3. According to the p value of 4.92 for this reaction and the cr values of p-OCH3 and p-CF3 in Table 26-6, we may write... 

Mitigating this situation to some extent are the correlation equations for estimating kHO (water) from kHO (air) or from the Hammett relation (see below), making it possible to tap into the abundant database of measured values for kHO (air) (Atkinson, 1989), the excellent SARs for estimating kHO (air) values (Atkinson, 1987), and the utility of the Hammett equation with its large database of substituent constants (Exner, 1978 Perrin et al., 1981). 

Table. Estimated Polar Substituent Constants Solvent CDCI3...

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