And the resonance effect

As an example, experimental kinetic data on the hydrolysis of amides under basic conditions as well as under acid catalysis were correlated with quantitative data on charge distribution and the resonance effect [13]. Thus, the values on the free energy of activation, AG , for the acid catalyzed hydrolysis of amides could be modeled quite well by Eq. (5)... 

In nitrobenzene the nitro group has a large electron affinity, and accordingly draws electrons away from the ring. The resonance effect works in the same direction, and, as a result, all positions have a deficiency of electrons. The meta positions are least affected, and the substitution takes place there with difficulty. In aniline, the inductive effect and the resonance effect oppose each other, but the latter wins out, and very easy o-p substitution takes place. 

A further difficulty in the case of fluoro-, chloro- and bromobenzenes is that with them apparently no choice of the 8 values seems to be reconcilable with the observed order of ease of substitution at the various positions unsubstituted benzene > para > ortho > meta. Both the inductive and the resonance effects are seen to leave the charge on the w-position practically unchanged, and approximately equal to 1.00c, while the observed order demands a considerably smaller value. As in the case of naphthalene, however, we shall find later that this discrepancy can apparently be explained by taking into account the polarization of the molecule by the attacking group. 

Taft and his coworkers developed a diparametric model that separates the electrical effect into contributions from the inductive (actually the field) effect and the resonance effect. This separation depends on the difference in the extent of electron delocahzation when a substituent is bonded to an sp hybridized carbon atom in one reference system and to an sp hybridized carbon atom in another. As the first case represents minimal delocalization and the second extensive delocalization, we have referred to the two effects as the locahzed and delocahzed electrical effects. This diparametric electrical effect model can be written as equation 5 ... 

These place a positive charge on the oxygen atom of the unionized molecule, and so cause it to repel the proton. On analysis of the experimental values for Ka at 25°C it is found that the inductive effect of a nitro group increases Ka by a factor of about 45, and the resonance effect in the ortho and para positions gives another factor of about 22. The acid constant of a nitrophenoi can be found approximately by multiplying that for phenol, l.t X 10 10, by the factor 45 for every meta nitro group and 1000 for every ortho or para nitro group in the molecule. The comparison of the values calculated in this way with those found by experiment is shown in Table 8-1. ... 

The Hammett cr-values contain contributions from both inductive/field effects and the resonance effect. The cr-constant can be separated quantitatively into a resonance component R, which operates mainly in the para position, and an inductive component F, which is assumed to be equal in the meta and para positions. Hansch, Leo and Taft11 have calculated the F and R values of Me3Si to be 0.01 and —0.08, respectively, as quoted in Table 1. These values seem somewhat at odds with experimentally determined values12-18 for the inductive and resonance parameters, which give mean values of —0.08 and 0.06, respectively. These values confirm the generally accepted view that MesSi is electron-supplying by inductive effects and electron-withdrawing by resonance effects. 

The higher polarity of the C=N bond and the resonance effect are probably the reason for the higher reactivity of pyridine k= 1-0 x 10 ... 

In most experiments reported, 195Au sources embedded in platinum matrices were used. For these sources, the recoilless fraction, even at liquid helium temperatures, is small, and the resonance effects are of the order of a few per cent at best. Buym and Grodzins (5) searched for... 

Let us analyse the basicity of the vinylamine at its / -carbon atom. Based on fitting of the experimental data, an amino group would increase the basicity of ethylene by 57 kcalmol-1, which is quite consistent with the experimental value of 56 kcalmol-1. What is the origin of the increased basicity The polarizability would increase it by 3.9 kcal mol-1, the field/inductive effect would decrease it by 4.1 kcalmol-1 and the resonance effect would increase it by 56.2 kcal mol-1. Consequently, the significant... 

To predict whether a substituted benzene is more or less electron rich than benzene itself, we must consider the net balance of both the inductive and the resonance effects. Alkyl groups, for instance, donate electrons by an inductive effect, but they have no resonance effect because they lack nonbonded electron pairs or n bonds. Thus, any alkyl-substituted benzene is more electron rich than benzene itself. 

Just how does a substituent exert its electronic effect Despite the vast amount of work that has been done—and is still being done—on this problem, there is no general agreement, except that at least two factors must be at work. We shall consider electron withdrawal and release to result from the operation of two factors the inductive effect and the resonance effect. 

It is generally known that the substituent effect on the aromatic ring is related to both the inductive effect caused by the substituent (electron-localization factor) and the resonance effect (electron-delocalization factor) [42]. These effects are highly correlated to Hammett s constants [43]. In terms of the... 

Recent AMI MO studies agree with earlier results (based on pK determinations and rates of solvolysis of imidazolyl-2-carboxylates) that 5-substituent effects follow and 4-substituent effects follow (Tm". The substituent effects are not readily transmitted through the —NH— system and the. resonance effects of a 5-substituent are more important than those of the same group at C-4 <93JOC7336, 94UP 302-02>. 

The Hammett a-values contain a contribution from both inductive/field effects and the resonance effect. The quantitative separation of -constants into resonance and inductive effects was first suggested by Taft34,35 who proposed the following relationship ... 

In Figure 11 various single bonds of acetaldol are shown in a reactivity space spanned by the polarity in the sigma electron distribution, the bond dissociation energy, BDE, and the resonance effect, R. 

Exner and Carsky suggested that conclusions about the relative importance of resonance/delocalization versus Coulombic/inductive effects on the acidity of carboxylic acids in the gas phase should not be generalized implicitly to discussions of acidities in solution. They reported calculations indicating that both the inductive effect and the resonance effect of the carbonyl group are attenuated in aqueous solution but that the inductive effect is reduced to a greater extent. ° As Exner and Carsky noted, however. 

For aliphatic systems, an equation proposed by Tafr is often appHed that involves the polar substituent constant <7, the steric substituent constant and the resonance effect. 

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