Substituent constants, table

The relative strength of a substituted benzoic acid and hence the value of cr depends on the nature and position of the substituent in the ring. For this reason, cr is called the substituent constant. Table 26-6 lists several substituent constants and these will be seen to correspond to the polar character of the respective substituents. Thus the more electron-attracting a substituent is, by either resonance or induction, the more acid-strengthening it is, and the more positive is its cr value (relative to H as 0.000). Conversely, the more strongly a substituent donates electrons by resonance or induction, the more... 

Table 3-1 gives substituent constants for several groups. 

Table 3-1. Substituent constants for various groups to be used in Eqs. (1) and (2) (

Table 12.2 A list of some of the more common descriptors. Details of some of these descriptors can be found elsewhere as indicated. This table is restricted to those descriptors which can be computed it therefore excludes certain classes (such as the Hammett substituent constants) which are derived from experimental studies (sec Section 12.12).

Herein is the rate constant for a dienophile with substituent x ko is the corresponding rate constant for unsubstituted 2,4c Ox is the substituent constant for substituent x and p is the reaction constant, defined as the slope of the plot of log (k / ko) versus Ox. The parameter p is a measure of the sensitivity of the reactions towards introduction of substituents. Figure 2.3 and Table 2.4 show the results of correlating the kinetic data for the reaction of 2.4a-e with 2.5 with a. ... 

Good to excellent Hammett plots were obtained using substituent constants (see Figure 2.6). Surprisingly, literature examples of good Hammett correlations of stability constants are rare The p-values are shown in Table 2.7. 

For the nine substituents m- andp-methyl, p-fluoro, m- and p-chloro, m- and p-bromo, and m- and p-iodo, using the results for nitration carried out at 25 °C in nitromethane or acetic anhydride - (see tables 9.1, 9.5), a plot of logjoA/ j against cr+ produced a substituent constant p = —6-53 with a standard deviation from the regression line i = 0-335, 2 correlation coefficient c = 0-975. Inclusion of... 

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

TABLE 9.1 Hammett and Taft Substituent Constants Continued)... 

The Hammett equation in the form of Eq. (4.14) or Eq. (4.15) is free of complications due to steric effects, since it is applied only to meta and para substituents. The geometry of the benzene ring ensures that groups in these positions cannot interact stoically with the site of reaction. Tables of a values for many substituents have been collected some values are given in Table 4.5, but substituent constants are available for a much wider range of... 

Given in Table 4.5 in addition to the Hammett equation are ct and substituent constant sets which reflect a recognition that the extent of resonance participation can vary for different reactions. The values are used for reactions in which there is direct resonance interaction between an electron-donor substituent and a cationic reaction center, hereas the a set pertains to reactions in which there is a direct resonance interaction between the substitutent and an electron-rich reaction site. These are cases in which the resonance conqionent of the... 

Table 7-1. Hammett Substituent Constants Based on Ionization of Benzoie Aeids ...

We take the view of McDaniel and Brown that the Hammett substituent constants should be defined by Eq. (7-22). Table 7-1 lists many of these constants based on the ionization of meta- and para-substituted benzoic acids. 

The nonspecialist reading Table 7-7 will probably be impressed by the substantial consistency among cti values evaluated by different methods, but the specialist tends to concentrate on the differences. There is one very interesting difference in Table 7-7, that for cti of alkyl groups based on Eq. (7-33) compared with cti based on the ionization of 3, the latter values showing practically no effect of inductive electron release and certainly no trend with increased branching. (The uncertainties associated with these substituent constants can be found in the original literature.) Swain... 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

Table 7-9. Polar Substituent Constants, a, for Aliphatic Substrates X-R...

It is obvious, fipom some of the series of substituents in Table 7-11, that the steric constant reflects intuitive notions of group size, and Taft " pointed out close parallels with van der Waals radii. For asymmetrical groups a single van der Waals radius cannot be defined, and the situation is complicated by conformational flexibility. Nevertheless, empirical relationships can be established, and Kutter and Hansch gave Eq. (7-55),... 

In the same series of compounds,i.e. the 4-chloroquinolines, rates of methoxydechlorination are treated in a similar manner, yielding a further set of 6-1 (epi-) and 7-1 (cata-) substituent constants, which are listed in Table VII. The necessary p-value was obtained by plotting the rate data against the pA data and using the slope of the... 

Table 4 Rate Coefficients, Efficiency q, and Steric Substituent Constant...

In our opinion the reliability of the basis of the Hammett equation and of the parameters F, R, R+, and R is now clearly sufficient for practically all physical organic purposes. A compilation of all substituent constants and the four parameters F, R, R +, and R was published by Hansch et al. (1991). In the main table of that paper the constants om and ap as well as the parameters F and R are listed for no less than 530 substituents ... 

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. ... 

Table 7-4. Substituent constants and F- and F-values for (F)-addition products of arenediazonium ions and for related compounds. (Data from Hansch et al., 1991, except for Ref. f).)...

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

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