The 7r-Inductive Effect

The last five words of Rule 1 involve a necessary proviso because heteroatoms at inactive positions in odd AH ions do exert stabilizing and destabilizing effects. While these are smaller than the first-order effects due to heteroatoms at active positions, they are nevertheless very significant. Thus while jS-picoline is a weaker acid than y-picoline, it is considerably stronger than toluene [see equations (4.95)-(4.97)]. Evidently heteroatoms can exert some additional influence on conjugated systems over and above that implied in equation (4.94). 

The 71-inductive effect can be included in equation (4.94). Since this represents the effect of a first-order perturbation, and since first-order perturbations are additive, we can write a generalized form of the equation as follows  

Here the individual terms represent the effect of simultaneous changes in electronegativity at any number of positions in a conjugated system, the changes being due either to replacement of carbon atoms by heteroatoms or to the TT-inductive effect. 

This is true of the usual heteroatoms, i.e., nitrogen, oxygen, sulfur, and the halogens. Boron and silicon are less electronegative than carbon and for them the conclusions of Rule 1 (Section 4.6) would be inverted. However, these elements play a relatively small role in conjugated systems. 

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The problem of electrophilic substitution into the anilinium ion has been examined by the methods of m.o. theory. Attempts to simulate the 7r-inductive effect in Hiickel m.o. theory by varying the Coulomb integral of C(d (the carbon atom to which the NH3+ group is attached) remove 7r-electrons from the o- and -positions and add them to the... 

Ingold introduces the terms substrate field effect and reagent field effect to describe those aspects of the direct field effect numbered (2) and (3) in 9.1.2. His description of the substituent effect of the trimethylammonio group is thus given substantially in terms of the substrate field effect and the 7r-inductive effect, i.e. it is an isolated molecule description. The reagent field effect is seen to be significant in nitration and to produce qualitatively the same... 

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. 

Three aspects of the inductive effect have to be considered 1,3,76,140 the (r-inductive effect, the inducto-electromeric or 7r-inductive effect, and the direct field effect. The first of these is the one most frequently... 

In nonconjugated dienes, the negative inductive effect of the extra double bond reduces the nucleophilicity of the other 7r-system (Fig. 8) [148], There is no evidence for anchimeric assistance by the additional double bond, as observed in certain solvolytic reactions [10,151-153]. When the two double bonds are separated by four methylene groups, they behave as isolated double bonds, and their reactivity equals that of an analogous monoene. 

The second 7r-inductive effect is the field effect, ittf, which arises through polarization of the whole 7r-system due to the electric dipole of (CH2)n Y, as shown in 1. 

The foregoing resonance structures describe the influence of the substituents on the 7r-electron distribution in the thiophene ring. Besides this effect the inductive effect of the substituents on the a-electi on system must be considered when discussing physical and chemical properties of thiophenes. 

An even better fit is displayed in the increasing shielding of the porphyrin protons in the meso positions according to the sequence Cl < C = CPh < Ph < Me for X (Table 4). Obviously, the a-donor effect of X enhances the Rh (P) 7r-bonding inductively ... 

In the two systems so far discussed it is impossible to obtain a quantitative idea of the relative importance of the inductive and resonance effects because it is impossible to achieve the operation of one of the effects without the other. When nitrogen is the basic centre, this becomes possible by steric fixation of the nitrogen lone pair orbital in the plane of the benzene ring, which virtually eliminates its overlap with the 7r-electron orbital of the ring carbon and hence also the mesomerism. So the enhanced acidity of the anilinium ion (pAT = 4-62) as compared with methylammonium (pAfg = 10-67) has been shown (Wepster, 1952) to be half inductive and half mesomeric in origin by a consideration of the following systems ([10]-[12]) ... 

A free radical is stabilized by an X substituent (Figure 13a) through a two-orbital, three-electron 7r-type interaction. The nucleophilicity of the radical is greatly increased. The X -substituted free radicals are more easily oxidized. The n effects of X substitution are somewhat augmented by the inductive effect of the electronegative X in stabilizing the radical. 

The actual structure is somewhere in between, i.e. (21 ab) a hybrid of which (21a) and (216) are the canonical forms. There will also be an inductive effect, as shown in (21a6) but this will be much smaller than the mesomeric effect as a electrons are much less polarisable, and hence less readily shifted, than 7r electrons. 

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