Inductive and field effects

It is generally recognised that the inductive effect should be subdivided into a polarisation effect on the o-bond framework and also on the rr-electrons, and it has been indicated that a major effect of strongly electron-withdrawing groups like perfluoroalkyl is by a through-space polarisation of the aromatic rr-electrons (direct field effect). 

This kind of description allows for a similar effect to be produced by other perfluoroalkyl groups (see Table 4.4), as well as the other fluorinated groups listed. Similar conclusions to those drawn here are described in a much more detailed review and analysis [30] of results available. 

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The combined inductive and field effects of these poles do not produce strong discrimination between the m- and /i-positions in nitration m p for. NMe3+, and smaller for the protonated poles). This situation is in marked contrast to that produced by, say, the nitro group ( 9.1.3), and suggests that the —M effect is more discriminating between m- and -positions than is the — I effect. 

The first three of a series of papers by Ridd and co-workers on Inductive and Field effects in Aromatic Substitution have appeared. Results of studies of the nitration of 4-phenylp5nidine and of 4-benzylpyridine in aqueous sulphuric acid were reported and use of the usual criteria (para 8.2) showed that in each case the conjugate acid was the species undergoing nitration. The values of where fm refers to the corresponding homocyclic compound (biphenyl or diphenylmethane) when plotted against r, the distance between the... 

The leaving group also affects the amount of internal versus terminal alkene that is formed. The poorer the leaving group, the more El cb-like is the transition state. This trend is illustrated for the case of the 2-butyl system by the data in Table 6.6. Positively charged leaving groups, such as in dimethylsulfonium and trimethylammonium salts, may favor a more El cb-like transition state because their inductive and field effects increase the acidity of the p protons. 

Inductive and field effects result in the polarisation of ir-electrons (—lu) (Figure 1.13). 

Other factors contribute, but often to a much lesser degree. Inductive and field effects from electronegative atoms can also stabilize the carbanion somewhat. Hydrogen bonding is very rare for carbanions. Aromaticity greatly stabilizes carbanions, but the only common example is cyclopentadienyl anion C5H5". 

Inductive o values for substituents may be obtained by fitting Hammett Equations to a standard series of reactions which are considered to possess no resonance interaction the average values of a from these correlations provides a set of coefficients (a") " which correspond to purely inductive transmission of the polar effect. The coefficients derived from the ionisation of XC6H4CH2CO2H, XC6H4CH2CH2CO2H and the alkaline hydrolysis of XC6H4CH2CO2C2H5 and XC6H4CH2CH2CO2C2H5 (a°) are regarded as purely inductive and field effect constants. 

In general, the electron-withdrawing effect of the multiple hydroxyl groups of carbohydrates makes hydroxyl, carboxyl and ammonium groups more acidic than the same groups attached to simple alkyl residues. Other factors, such as hindrance to solvation and the balance between inductive and field effects, also influence values in detail. 

The inductive and field effects have been called localized effects by Charton14, and the hyperconjugative and (p-d)fl bonding effects discussed next have been called delocalized effects14. 

Attempts to distinguish inductive and field effects have been frustrated by the lack of a molecule that provides an unambiguous answer. Such a molecule would require that the low dielectric cavity of the field effect model 2 be occupied by the chemical bonds important to the inductive effect model. If a small attenuation factor is adopted for through-bond transmission of the polar effect, both models predict similar results. 2 For example, o-chlorophenylpropionic acid is weaker than expected by the inductive model 3 and is unsuitable. 

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