Substituent effects separation

In general, the dissection of substituertt effects need not be limited to resonance and polar components, vdiich are of special prominence in reactions of aromatic compounds.. ny type of substituent interaction with a reaction center could be characterized by a substituent constant characteristic of the particular type of interaction and a reaction parameter indicating the sensitivity of the reaction series to that particular type of interactioa For example, it has been suggested that electronegativity and polarizability can be treated as substituent effects separate from polar and resonance effects. This gives rise to the equation... 

N-Inversion in azetidine and azetidin-2-one is rapid, even at —77 and -40 °C, respectively (B-73NMR144). Again, halo substituents on nitrogen drastically slow the inversion rate, so that Af-chloro-2-methylazetidine can be separated into two diastereomers (b-77SH(1)54). Substituent effects on N-inversion are much the same as in the aziridines Af-aryl and N- acyl... 

It is always important to keep in mind the relative nature of substituent effects. Thus, the effect of the chlorine atoms in the case of trichloroacetic acid is primarily to stabilize the dissociated anion. The acid is more highly dissociated than in the unsubstituted case because there is a more favorable energy difference between the parent acid and the anion. It is the energy differences, not the absolute energies, that determine the equilibrium constant for ionization. As we will discuss more fully in Chapter 4, there are other mechanisms by which substituents affect the energy of reactants and products. The detailed understanding of substituent effects will require that we separate polar effects fiom these other factors. 

The net effect is, therefore, a balance of these two components. Some of the results of the separation may be surprising, such as the relatively small contribution that resonance makes to the overall substituent effect of p-NOj. Another interesting result is the predominant resonance contribution in the alkyl series this (if real) must be attributed to hyperconjugation. 

The general chemical reasonableness of the results in Table 7-7 is gratifying, but this does not constitute a demonstration that the separation of substituent effects into inductive effects and resonance effects is quantitatively possible, for these effects may interact so as to be nonadditive. Ritchie and Sager express reservations about the approach in general, and other authors " have criticized results based on Eqs. (7-33) and (7-34). 

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... 

It can be concluded that mBr depends on the magnitude of the charge at the transition state and also on its delocalization either by the substituents or by the solvent. It therefore seems difficult to separate these effects, since R, the measure of solvent assistance, depends also on the same factors. The idea that transition-state shifts contribute to m-variations is supported by substituent effects. Consequently, it would be useful to obtain p-m correlations to compare the influence of the solvent and the substituents in determining the position of the bromination transition state. 

One point of debate in defining the magnitude of the captodative effect has been the separation of substituent effects on the radical itself as compared to that on the closed shell reference system. This is, as stated before, a general problem for all definitions of radical stability based on isodesmic reactions such as Eq. 1 [7,74,76], but becomes particularly important in multiply substituted cases. This problem can be approached either through estimating the substituent effects for the closed shell parents separately [77,78], or through the use of isodesmic reactions such as Eq. 5, in which only open shell species are present ... 

The range of SCS values (P-CH3O to p-NC>2) for nitrostyrenes is about 13 ppm. The correlation of the 170 chemical shift data for nitrostyrenes with that for nitrobenzenes57 gives a slope of 0.58 (Figure 3), which indicates that a comparable reduction in substituent effects results when the nitro function is separated from a p-substituted phenyl group by a carbon-carbon double bond. 

The status of Exner s revised <7/ and or values has been debated for almost thirty years. A number of prominent workers in the field are rather critical of Exner s approach. For a fairly recent appraisal of the situation, see an article by the present author76. Exner has continued to propagate his view on this matter in his book published in 198877. Some of his papers in the past few years indicate that he is developing further criticisms of aspects of the traditional separation of inductive and resonance effects and of the ways in which correlation analysis of substituent effects is generally carried out138,240 -243. 

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). 

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

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