Substituent constant determination

The constant o° is another normal substituent constant determined by choosing only reaction series in which at least one methylene group insulates the reaction site from the aromatic ring. The resonance parameter, o°R, is determined from Equation 2.15 and is the resonance contribution of a substituent when it is not directly conjugated with the reaction site.15 Table 2.4 lists a number of an and u° values. Note the close correspondence between the two. 

These data were plotted according to the Hammett equation to give the plot in Figure 5.17. The first thing to note is that the Hammett plot is linear. The linearity of die plot implies that the substituent constants determined for the ionization of... 

The numerical values of the terms a and p are defined by specifying the ionization of benzoic acids as the standard reaction to which the reaction constant p = 1 is assigned. The substituent constant, a, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The a values so defined are used in the correlation of other reaction series, and the p values of the reactions are thus determined. The relationship between Eqs. (4.12) and (4.14) is evident when the Hammett equation is expressed in terms of fiee energy. For the standard reaction, o%K/Kq = ap. Thus,... 

Substituent constants calculated in this way are in good agreement with empirical Of values. The same system was used to calculate values by determining charge accumulation or depletion on the a and p carbons of substituted ethylenes using the 4-3IG method. 

Research on the nature of substituent constants continues, with results that can bewilder the nonspecialist. The dominant approach is a statistical one, and the main goal is to dissect substituent effects into separate electronic causes. This has led to a proliferation of terms, symbols, and conclusions. A central issue is (here we change terminology somewhat from our earlier usage) to determine the balance of field and inductive effects contributing to the observed polar electronic effect. In... 

In connection with normal substituent constants it must finally be mentioned that S02Me was included in an elaborate statistical analysis of a large body of data relating to substituent effects in aromatic systems87. What are claimed to be normal substituent constants emerge as am° = 0.69 and ap° = 0.73, not greatly different from those determined by Yukawa and coworkers84 or in the case of ap° from the ordinary ap value. 

Yukawa, Tsuno and their colleagues121 have made studies more specifically related to determining substituent constants, which have included both methylsulfinyl and methylsulfonyl groups. Hydroxyl chemical shifts were determined for a large number of m-and p-substituted phenols in DMF and in DMSO. The results were treated by the LArSR relationship121 (usually called the Yukawa-Tsuno equation, see Section II.B) ... 

Values of Pr for the substituent constants commonly used with eq. (1) are given in Table I. When correlations are made with eq. (1), the substituent constant used determines the composition of the electrical effect. 

The conformation of nonsulfated L-idosyluronic acid residues is a controversial issue, especially on account of a possible role of the 2-O-sulfo substituent in determining the relative orientation of biologically relevant groups. However, the interproton coupling-constants observed for (nonsulfated) methyl a-D-idopyranosiduronic acid131 and dermatan sulfate350 are essentially the same as those of the sulfated L-iduronate resi-... 

It appeared to the author some years ago that, irrespective of the mechanism of the toxic action of DDT, there might be a correlation of structure and toxicity in analogous compounds. Hammett (13) has shown that the rate and equilibrium constants of over 50 side-chain reactions of meta and para substituted aromatic compounds may be correlated with the so-called substituent constant a, according to the equation log k — log k0 = pa, where k and k0 are rate (or equilibrium) constants for substituted and unsubstituted compounds, respectively, p is the reaction constant giving the slope of the linear relationship, and a is the substituent constant, which is determined by the nature and... 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

The reaction of 3,4-bis(benzenesulfonyl)furoxan with alcohols and thiols in basic media affords a variety of alkoxy-and alkylthio-substituted (benzenesulfonyl)furoxans. For these derivatives a paramount problem is to determine the position (3- or 4-) of the substitution in the furoxan ring. The structures of these derivatives were assigned on the basis of both chemical and NMR evidence. In particular, 13C NMR substituent constants were obtained by NMR study of suitable furoxan models. By assuming a complete additivity of the substituent effects at the furoxan ring, these values were used for structural determination <1997FES405>. 

These methods of modelling steric effects will be discussed in detail in Section III of this work. With the exception of the correlation of a for the Si(OAk)MePh groups with equation 22, all of the results were significant. The data sets were small but the overall trend seems clear there is generally a dependence on steric effects for these substituent constants. We have therefore generally excluded the values determined by this method from the tables of substituent constants given in this work. Exceptions have been made in a few cases for groups for which values have not been determined by any other methods. 

Experimentally determined pA, values have been compared with Hammett substituent constants (Figure 1) and an excellent correlation was found. This result provided a further support - in accord with some nuclear magnetic resonance (NMR) measurements - for existence of one particular tautomeric form (Figure 1, structure a) among the three possible ones (Figure 1, a, b, c). 

These effects of electron withdrawal from and release to the ring also find quantitative expression in the <7+ substituent constants (Brown and Okamoto, 1967), and linear correlations between cr+ values and ionization potentials have been reported (Crable and Kearns, 1962). More recently, the very strongly electrophilic reagent CF3 has been shown to attack the benzene ring at a rate, k, determined by an activation energy linearly related to the ionization potential of the benzene electrons in... 

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