Inductive effect parameter

A quantitative description of the reactivity of monosubstituted benzenes to electrophilic substitution based on considerations of inductive effect parameters and con-jugative effect parameters from the 13 C chemical shifts of the aromatic compounds has been proposed.3 MO calculations on the proton migration in the ipso adducts formed in the reaction of CH3+ and SiH3+ with benzene have been described.4 With SiH3+ the ipso adduct is the most stable of possible isomers, whereas for CH3+ the >ara-protonated isomer is the most stable. 

Although cTi estimates by different methods or from different data sets may disagree, it is generally held that the inductive effect of a substituent is essentially independent of the nature of the reaction. It is otherwise with the resonance effect, and Ehrenson et al. have defined four different ctr values for a substituent, depending upon the electronic nature of the reaction site. An alternative approach is to add a third term, sometimes interpreted as a polarizability factor, and to estimate the inductive and resonance contribution statistically with the added parameter the resonance effect appears to be substantially independent of reaction site. " " ... 

Wheland and Pauling (1959) tried to explain the inductive effect in terms of ar-electron theory by varying the ax and ySxY parameters for nearest-neighbour atoms, then for next-nearest-neighbour atoms and so on. But, as many authors have also pointed out, it is always easy to introduce yet more parameters into a simple model, obtain agreement with an experimental finding and then claim that the model represents some kind of absolute truth. 

The error in Hiickel s treatment lies not in the quantum mechanical calculations themselves, which are correct as far as they go, but in the oversimplification of the problem and in the incorrect interpretation of the results. Consequently it has seemed desirable to us to make the necessary extensions and corrections in order to see if the theory can lead to a consistent picture. In the following discussion we have found it necessary to consider all of the different factors mentioned heretofore the resonance effect, the inductive effect, and the effect of polarization by the attacking group. The inclusion of these several effects in the theory has led to the introduction of a number of more or less arbitrary parameters, and has thus tended to remove significance from the agreement with experiment which is achieved. We feel, however, that the effects included are all justified empirically and must be considered in any satisfactory theory, and that the values used for the arbitrary parameters are reasonable. The results communicated in this paper show that the quantum mechanical theory of the structure of aromatic molecules can account for the phenomenon of directed substitution in a reasonable way. 

Marriott and Topsom have recently developed theoretical scales of substituent field and resonance parameters. The former correspond to the traditional inductive parameters but these authors are firm believers in the field model of the so-called inductive effect and use the symbol The theoretical substituent field effect scale is based on ab initio molecular orbital calculations of energies or electron populations of simple molecular systems. The results of the calculations are well correlated with Op values for a small number of substituents whose Op values on the various experimental scales (gas-phase, non-polar solvents, polar solvents) are concordant, and the regression equations are the basis for theoretical Op values of about 50 substituents. These include SOMe and S02Me at 0.37 and 0.60 respectively, which agree well with inherent best values in the literature of 0.36 and 0.58. However, it should be noted that a, for SOMe is given as 0.50 by Ehrenson and coworkers . 

The proton affinities (PA) of two restricted subsets of amines were correlated directly with inductive and polarizability effect parameters, respectively (Figs. 19 and 22). These can be combined with data on other hetero-substituted amines to give a set of 80 amines of different skeletal and substitution types (e.g. Fig. 24). In this and all other systems (below), a residual electronegativity value, %l2, (Eq. 5) derived from those of the atoms of the first, 1, and second, 2, sphere neighbors of the nitrogen atom is preferred as a measure of the inductive effect49). 

The signs of the coefficients in this equation are consistent with the interpretation of the two factors the negative sign of the coefficient for the x12 parameter indicates that an increase in the inductive effect destabilizes the corresponding ammonium ion and thereby leads to a decrease in the proton affinity. On the other hand, an increase in the effective polarizability, ad, stabilizes the ammonium ion and therefore... 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

Xrx is a parameter characterizing the homologous series RX. The values of /j,r are direct measures of the polar inductive effects of alkyl groups relative to that of methyl and correlate well with Taft s a values. Substituent-induced IP shifts can thus be handled by linear free energy relationships (LFER) of the Hammett pcr-type. 

Reynolds and coworkers156 based a similar operation on 13C substituent chemical shifts of meta- and para-substituted styrenes. Iterative multiple regression was used for the redefinition of the 07 and crjj scales. The authors also took the opportunity to replace the symbol 07 by op, having become convinced that the so-called inductive effect was entirely a field effect (see the present author s discussion of this matter76). The authors presented an extensive table in which their values of the substituent parameters are compared with those obtained by other authors. Their cr value of 0.144 for NO2 essentially confirmed the various values of about 0.15 already mentioned and their 07 value of 0.651 essentially confirmed the various values of about 0.65. 

This is essentially a method of providing an alternative set of 07 and or parameters for use in the DSP equation or EH equation. In the mid-1960 s Exner94 found evidence that the inductive effect from the para position of benzoic acid was stronger than that from the meta position by a factor of 1.14. He also suggested that 07 values current at that time and based on alicyclic and aliphatic reactivities were out of scale with am and ap by a factor of 1.10, and should be multiplied by this to introduce the 7r-inductive component. This led Exner to a revised analysis of am and ap in terms of inductive and resonance components. He calculated revised 07 values by multiplying the alicyclic/aliphatic values by 1.10, and then multiplying these further by 1.14 before subtracting from ap values to obtain revised values of or. 

Topsom, 1976) and to treat them separately. In this review we will be concerned solely with polar or electronic substituent effects. Although it is possible to define a number of different electronic effects (field effects, CT-inductive effects, jt-inductive effects, Jt-field effects, resonance effects), it is customary to use a dual substituent parameter scale, in which one parameter describes the polarity of a substituent and the other the charge transfer (resonance) (Topsom, 1976). In terms of molecular orbital theory, particularly in the form of perturbation theory, this corresponds to a separate evaluation of charge (inductive) and overlap (resonance) effects. This is reflected in the Klopman-Salem theory (Devaquet and Salem, 1969 Klop-man, 1968 Salem, 1968) and in our theory (Sustmann and Binsch, 1971, 1972 Sustmann and Vahrenholt, 1973). A related treatment of substituent effects has been proposed by Godfrey (Duerden and Godfrey, 1980). 

Despite these difficulties, it appears that the most potent substituent scales are those where a DSP set is used (Topsom, 1976) instead of a single a-value for the electronic effect. While one cr-inductive (cr,) parameter is used in all molecular situations, it seems preferable to apply several o-resonance (cir) parameter sets. Here, the system to which the substituent is attached is taken into account. This corresponds to the fine tuning above mentioned. Some values for common substituents are given in Table 1 (Topsom 1976). 

Some literature data is available on (CHs)2Se interacting with l2, A1(CH3)3 and Ga(CH3)3. Tentative E and C parameters of 0.881 and 13.3 respectively can be obtained for the latter acid from its interaction with (CH3)3N and (CH3)3P. This leads to E and C numbers for (CH3)2Se of 0.217 and 8.33. The C/E ratio of this donor is greater than that of (CHs) 2S as one might expect. From the E and C parameters of HNCS and (CH3)2Se, we predict an enthalpy of 3.1 kcal mole i. This is to be compared to reported values of 2.5 and 3.7 respectively for the HNCS adducts with di-n-propyl and di-n-butylselenide. The inductive effects attributed to these substituent changes is inside experimental error. 

Thus we could use the charges given by any LCAO basis of our choice to create an arbitrary set of scahng parameters (as a replacement for the cr constants) and proceed with Eqs. (5.1) and (5.2) without invoking Taft s inductive effects. For convenience, however, we shall go along with the a constants of Table 5.1. 

The effects of both alkyl and aryl substituents can be observed in the two-component tautomeric equilibria of 3-alkyl-l-aryl-2,3-dihydro-177-naphth[l,2-r ][l,3]oxazines containing C-3-epimeric naphthoxazines 52B-58B and 52G-58C (Scheme 7). The influence of the Meyer parameters (V ) of the alkyl substituents on the epimerization constants (K d ( r= [B]/[G]) can be characterized by Equation (3). Multiple linear regression analysis of log A)r according to Equation (4) leads to the conclusion that these equilibria are also influenced significantly by the inductive effect of substituent Y 0.48) <2004JOC3645>. 

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